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iakovlev.org

The Second Extended File System

Internal Layout

Dave Poirier

      instinc@users.sf.net
     

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license can be acquired electronically from http://www.fsf.org/licenses/fdl.html or by writing to 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA


Table of Contents
About this book
1. Disk Organisation
1.1. superblock
1.1.1. s_inodes_count
1.1.2. s_blocks_count
1.1.3. s_r_blocks_count
1.1.4. s_free_blocks_count
1.1.5. s_free_inodes_count
1.1.6. s_first_data_block
1.1.7. s_log_block_size
1.1.8. s_log_frag_size
1.1.9. s_blocks_per_group
1.1.10. s_frags_per_group
1.1.11. s_inodes_per_group
1.1.12. s_mtime
1.1.13. s_wtime
1.1.14. s_mnt_count
1.1.15. s_max_mnt_count
1.1.16. s_magic
1.1.17. s_state
1.1.18. s_errors
1.1.19. s_minor_rev_level
1.1.20. s_lastcheck
1.1.21. s_checkinterval
1.1.22. s_creator_os
1.1.23. s_rev_level
1.1.24. s_def_resuid
1.1.25. s_def_resgid
1.1.26. s_first_ino
1.1.27. s_inode_size
1.1.28. s_block_group_nr
1.1.29. s_feature_compat
1.1.30. s_feature_incompat
1.1.31. s_feature_ro_compat
1.1.32. s_uuid
1.1.33. s_volume_name
1.1.34. s_last_mounted
1.1.35. s_algo_bitmap
1.2. Group Descriptor
1.2.1. bg_block_bitmap
1.2.2. bg_inode_bitmap
1.2.3. bg_inode_table
1.2.4. bg_free_blocks_count
1.2.5. bg_free_inodes_count
1.2.6. bg_used_dirs_count
1.2.7. bg_pad
1.2.8. bg_reserved
1.3. Block Bitmap
1.4. Inode Bitmap
1.5. Inode Table
1.5.1. i_mode
1.5.2. i_uid
1.5.3. i_size
1.5.4. i_atime
1.5.5. i_ctime
1.5.6. i_mtime
1.5.7. i_dtime
1.5.8. i_gid
1.5.9. i_links_count
1.5.10. i_blocks
1.5.11. i_flags
1.5.12. i_osd1
1.5.13. i_block
1.5.14. i_generation
1.5.15. i_file_acl
1.5.16. i_dir_acl
1.5.17. i_faddr
1.5.18. i_osd2
1.6. Data Blocks
2. Directory Structure
2.1. Directory File Format
2.1.1. inode
2.1.2. rec_len
2.1.3. name_len
2.1.4. file_type
2.1.5. name
2.2. Sample Directory
2.3. Indexed Directory Format
2.3.1. Index Structure
2.3.2. Lookup Algorithm
2.3.3. Insert Algorithm
2.3.4. Splitting
2.3.5. Key Collisions
2.3.6. Hash Function
2.3.7. Performance
3. Inodes, file identifiers
3.1. Inode Number
3.2. Locating the Inode structure
3.3. Locating the Inode Table
4. File Attributes
4.1. Standard Attributes
4.1.1. SUID, SGID and -rwxrwxrwx
4.1.2. File Size
4.1.3. Owner and Group
4.2. Extended Attributes
4.2.1. Attribute Block Header
4.2.2. Attribute Entry Header
4.3. Behaviour Control Flags
4.3.1. EXT2_SECRM_FL - Secure Deletion
4.3.2. EXT2_UNRM_FL - Record for Undelete
4.3.3. EXT2_COMPR_FL - Compressed File
4.3.4. EXT2_SYNC_FL - Synchronous Updates
4.3.5. EXT2_IMMUTABLE_FL - Immutable File
4.3.6. EXT2_APPEND_FL - Append Only
4.3.7. EXT2_NODUMP_FL - Do No Dump/Delete
4.3.8. EXT2_NOATIME_FL - Do Not Update .i_atime
4.3.9. EXT2_DIRTY_FL - Dirty
4.3.10. EXT2_COMPRBLK_FL - Compressed Blocks
4.3.11. EXT2_NOCOMPR_FL - Access Raw Compressed Data
4.3.12. EXT2_ECOMPR_FL - Compression Error
4.3.13. EXT2_BTREE_FL - B-Tree Format Directory
4.3.14. EXT2_INDEX_FL - Hash Indexed Directory
4.3.15. EXT2_IMAGIC_FL -
4.3.16. EXT2_JOURNAL_DATA_FL - Journal File Data
4.3.17. EXT2_RESERVED_FL - Reserved
A. Credits

About this book

The latest version of this document may be downloaded from http://www.freesoftware.fsf.org/ext2-doc/

This book is intended as an introduction and guide to the Second Extended File System, also known as Ext2. The reader should have a good understanding of the purpose of a file system as well as the associated vocabulary (file, directory, partition, etc).

Trying to implement ext2 drivers isn't always an easy task, the most difficult issue is unfortunately the documentation available. It seems like most of the documentation on the net about the internal layout of Ext2 was written to complement the Linux sources rather than be a complete document by themselves.

Hopefully this document will fix this problem, may it be of help to as many of you as possible.

Unless otherwise stated, all values are stored in little endian byte order.


Chapter 1. Disk Organisation

The first aspect of using the Second Extended File System one has to grasp is that all the meta-data structures size are based on a "block" size rather than a "sector" size. This block size is variable depending on the size of the file system. On a floppy disk for example, it is 1KB (2 sectors), while on a 10GB partition, the block size is normally 4KB or 8KB (8 and 16 sectors respectively).

Each block is further sub-divided into "fragments", but I have yet to see a file system which fragment size doesn't match block size. Although my guts tells me that there must be some folks out there using different sizes for fragments and blocks.

Except for the superblock, all meta-data structures are resized to fit into blocks. This is something to remember when trying to mount any other file system than one on a floppy. The "Inode Table Block" for example will contain more entries in a 4KB block than in a 1KB block, so one will have to take that into account when accessing this particular structure.

The next major aspect is that the file system is split into "block groups". While a floppy would contain only one block group holding all the blocks of the file system, a hard disk of 10GB could easily be split into 30 of such block groups; each holding a certain quantity of blocks.

At the start of each block group are various meta-data structures detailing the location of the other, more informative, meta-data structures defining the current file system state. Here's the organisation of an ext2 file system on a floppy:

Figure 1-1. floppy disk meta-data layout

offset   # of blocks description
 -------- ----------- -----------
        0           1 boot record
          -- block group 0 --
 (1024 bytes)       1 superblock
        2           1 group descriptors
        3           1 block bitmap
        4           1 inode bitmap
        5          23 inode table
       28        1412 data blocks
   

And here's the organisation of a 20MB ext2 file system:

Figure 1-2. 20mb partition meta-data layout

offset   # of blocks description
 -------- ----------- -----------
        0           1 boot record
          -- block group 0 --
 (1024 bytes)       1 superblock
        2           1 group descriptors
        3           1 block bitmap
        4           1 inode bitmap
        5         214 inode table
      219        7974 data blocks
          -- block group 1 --
     8193           1 superblock backup
     8194           1 group descriptors backup
     8195           1 block bitmap
     8196           1 inode bitmap
     8197         214 inode table
     8408        7974 data blocks
          -- block group 2 --
    16385           1 block bitmap
    16386           1 inode bitmap
    16387         214 inode table
    16601        3879 data blocks
   

The layout on disk is very predictable as long as you know a few basic information; block size, blocks per group, inodes per group. This information is all located in, or can be computed from, the superblock structure.

Without the superblock information, the disk is useless; therefore as soon as enough space is available, one or more superblock backups will be created on the disk.

The block bitmap and inode bitmap are used to identify which blocks and which inode entries are free to use. The data blocks is where the various files will be stored. Note that a directory is also seen as a file under Ext2, we will go in more detail about that later on.

While all ext2 implementations try to be compatible, some fields in the various structures have been customized to fit the requirements of a specific operating system. Where such differences are known, they will be indicated in proper time.


1.1. superblock

The superblock is the structure on an ext2 disk containing the very basic information about the file system properties. It is layed out in the following form:

Figure 1-3. superblock structure

offset  size    description
 ------- ------- -----------
       0       4 s_inodes_count
       4       4 s_blocks_count
       8       4 s_r_blocks_count
      12       4 s_free_blocks_count
      16       4 s_free_inodes_count
      20       4 s_first_data_block
      24       4 s_log_block_size
      28       4 s_log_frag_size
      32       4 s_blocks_per_group
      36       4 s_frags_per_group
      40       4 s_inodes_per_group
      44       4 s_mtime
      48       4 s_wtime
      52       2 s_mnt_count
      54       2 s_max_mnt_count
      56       2 s_magic
      58       2 s_state
      60       2 s_errors
      62       2 s_minor_rev_level
      64       4 s_lastcheck
      68       4 s_checkinterval
      72       4 s_creator_os
      76       4 s_rev_level
      80       2 s_def_resuid
      82       2 s_def_resgid
    -- EXT2_DYNAMIC_REV Specific --
      84       4 s_first_ino
      88       2 s_inode_size
      90       2 s_block_group_nr
      92       4 s_feature_compat
      96       4 s_feature_incompat
     100       4 s_feature_ro_compat
     104      16 s_uuid
     120      16 s_volume_name
     136      64 s_last_mounted
     200       4 s_algo_bitmap
    -- Performance Hints         --
     204       1 s_prealloc_blocks
     205       1 s_prealloc_dir_blocks
     206       2 - (alignment)
    -- Journaling Support        --
     208      16 s_journal_uuid
     224       4 s_journal_inum
     228       4 s_journal_dev
     232       4 s_last_orphan
    -- Unused                    --
     236     788 - (padding)
    

1.1.1. s_inodes_count

32bit value indicating the total number of inodes, both used and free, in the file system.


1.1.2. s_blocks_count

32bit value indicating the total number of blocks, both used and free, in the file system.


1.1.3. s_r_blocks_count

32bit value indicating the total number of blocks reserved for the usage of the super user. This is most useful if for some reason a user, maliciously or not, fill the file system to capacity; the super user will have this specified amount of free blocks at his disposal so he can edit and save configuration files.


1.1.4. s_free_blocks_count

32bit value indicating the total number of free blocks, including the number of reserved blocks (see s_r_blocks_count). This is a sum of all free blocks of all the block groups.


1.1.5. s_free_inodes_count

32bit value indicating the total number of free inodes. This is a sum of all free inodes of all the block groups.


1.1.6. s_first_data_block

32bit value identifying the first data block, in other word the id of the block containing the superblock structure.

Note that this value is always 0 for file systems with a block size larger than 1KB, and always 1 for file systems with a block size of 1KB. The superblock is always starting at the 1024th byte of the disk, which normally happens to be the first byte of the 3rd sector.


1.1.7. s_log_block_size

The block size is computed using this 32bit value as the number of bits to shift left the value 1024. This value may only be positive.

block size = 1024 << s_log_block_size;
     

1.1.8. s_log_frag_size

The fragment size is computed using this 32bit value as the number of bits to shift left the value 1024. Note that a negative value would shift the bit right rather than left.

if( positive )
   fragmnet size = 1024 << s_log_frag_size;
 else
   framgnet size = 1024 >> -s_log_frag_size;
     

1.1.9. s_blocks_per_group

32bit value indicating the total number of blocks per group. This value in combination with s_first_data_block can be used to determine the block groups boundaries.


1.1.10. s_frags_per_group

32bit value indicating the total number of fragments per group. It is also used to determine the size of the block bitmap of each block group.


1.1.11. s_inodes_per_group

32bit value indicating the total number of inodes per group. This is also used to determine the size of the inode bitmap of each block group.


1.1.12. s_mtime

Unix time, as defined by POSIX, of the last time the file system was mounted.


1.1.13. s_wtime

Unix time, as defined by POSIX, of the last write access to the file system.


1.1.14. s_mnt_count

32bit value indicating how many time the file system was mounted since the last time it was fully verified.


1.1.15. s_max_mnt_count

32bit value indicating the maximum number of times that the file system may be mounted before a full check is performed.


1.1.16. s_magic

16bit value identifying the file system as Ext2. The value is currently fixed to 0xEF53.


1.1.17. s_state

16bit value indicating the file system state. When the file system is mounted, this state is set to EXT2_ERROR_FS. When the file system is not yet mounted, this value may be either EXT2_VALID_FS or EXT2_ERROR_FS in the event the file system was not cleanly unmounted.


1.1.18. s_errors

16bit value indicating what the file system driver should do when an error is detected. The following values have been defined:

Table 1-1. EXT2_ERRORS values

EXT2_ERRORS_CONTINUE 1 continue as if nothing happened
EXT2_ERRORS_RO 2 remount read-only
EXT2_ERRORS_PANIC 3 cause a kernel panic
EXT2_ERRORS_DEFAULT varies as of revision 0.5, this is the same as EXT2_ERRORS_CONTINUE

1.1.19. s_minor_rev_level

16bit value identifying the minor revision level within its revision level.


1.1.20. s_lastcheck

Unix time, as defined by POSIX, of the last file system check.


1.1.21. s_checkinterval

Maximum Unix time interval, as defined by POSIX, allowed between file system checks.


1.1.22. s_creator_os

32bit identifier of the os that created the file system. Defined values are:

Table 1-2. EXT2_OS values

EXT2_OS_LINUX 0 Linux
EXT2_OS_HURD 1 Hurd
EXT2_OS_MASIX 2 MASIX
EXT2_OS_FREEBSD 3 FreeBSD
EXT2_OS_LITES4 4 Lites

1.1.23. s_rev_level

32bit revision level value. There are currently only 2 values defined:

Table 1-3. EXT2 revisions

EXT2_GOOD_OLD_REV 0 original format
EXT2_DYNAMIC_REV 1 V2 format with dynamic inode sizes

1.1.24. s_def_resuid

16bit value used as the default user id for reserved blocks.


1.1.25. s_def_resgid

16bit value used as the default group id for reserved blocks.


1.1.26. s_first_ino

32bit value used as index to the first inode useable for standard files. In the non-dynamic file system revisions, the first non-reserved inode was fixed to 11. With the introduction the dynamic revision of the file system it is now possible to modify this value.


1.1.27. s_inode_size

16bit value indicating the size of the inode structure. In non-dynamic file system revisions this value is assumed to be 128.


1.1.28. s_block_group_nr

16bit value used to indicate the block group number hosting this superblock structure. This can be used to rebuild the file system from any superblock backup.


1.1.29. s_feature_compat

32bit bitmask of compatible features. The file system implementation is free to support them or not without risk of damaging the meta-data. (more information will be added soon)


1.1.30. s_feature_incompat

32bit bitmask of incompatible features. The file system implementation should refuse to mount the file system if any of the indicated feature is unsupported. (more information will be added soon)


1.1.31. s_feature_ro_compat

32bit bitmask of "read-only" features. The file system implementation should mount as read-only if any of the indicated feature is unsupported. (more information will be added soon)


1.1.32. s_uuid

128bit value used as the volume id. This should, as much as possible, be unique for each file system formatted.


1.1.33. s_volume_name

16 bytes volume name, mostly unusued. A valid volume name would consist of only ISO-Latin-1 characters and be 0 terminated.


1.1.34. s_last_mounted

64 bytes directory path where the file system was last mounted. While not normally used, it could serve for auto-finding the mountpoint when not indicated on the command line. Again the path should be zero terminated for compatibility reasons. Valid path is constructed from ISO-Latin-1 characters.


1.1.35. s_algo_bitmap

32bit value used by compression algorithms to determine the methods used. (I do not have any more detail about this field, if you do please do send me all the information you have, thanks).


1.2. Group Descriptor

The group descriptors is an array of the group_desc structure, each describing a "block group", giving the location of its inode table, blocks and inodes bitmaps, and some other useful informations.

The group descriptors are located on the first block following the block containing the superblock structure. Here's what one of the group descriptor looks like:

Figure 1-4. group_desc structure

offset  size    description
 ------- ------- -----------
       0       4 bg_block_bitmap
       4       4 bg_inode_bitmap
       8       4 bg_inode_table
      12       2 bg_free_blocks_count
      14       2 bg_free_inodes_count
      16       2 bg_used_dirs_count
      18       2 bg_pad
      20      12 bg_reserved
    

For each group in the file system, such a group_desc is created. Each represent a single "block group" within the file system and the information within any one of them is pertinent only to the group it is describing. Every "Group Descriptor Table" contains all the information about all the groups.

All indicated "block id" are absolute.


1.2.1. bg_block_bitmap

32bit block id of the first block of the "block bitmap" for the group represented.


1.2.2. bg_inode_bitmap

32bit block id of the first block of the "inode bitmap" for the group represented.


1.2.3. bg_inode_table

32bit block id of the first block of the "inode table" for the group represented.


1.2.4. bg_free_blocks_count

16bit value indicating the total number of free blocks for the represented group.


1.2.5. bg_free_inodes_count

16bit value indicating the total number of free inodes for the represented group.


1.2.6. bg_used_dirs_count

16bit value indicating the number of inodes allocated to directories for the represented group.


1.2.7. bg_pad

16bit value used for padding the structure on a 32bit boundary.


1.2.8. bg_reserved

3 successive 32bit values reserved for future implementations.


1.3. Block Bitmap

The "Block Bitmap" is normally located at the first block, or second block if a superblock backup is present, of the block group. Its official location can be determined by reading the "bg_block_bitmap" in its associated group descriptor.

Each bit represent the current state of a block within that group, where 1 means "used" and 0 "free/available". The first block of this block group is represented by bit 0 of byte 0, the second by bit 1 of byte 0. The 8th block is represented by bit 7 (most significant bit) of byte 0 while the 9th block is represented by bit 0 (least significant bit) of byte 1.


1.4. Inode Bitmap

The "Inode Bitmap" works in a similar way as the "Block Bitmap", difference being in each bit representing an inode in the "Inode Table" rather than a block.

There is one inode bitmap per group and its location may be determined by reading the "bg_inode_bitmap" in its associated group descriptor.

When the inode table is created, all the reserved inodes are marked as used. For the "Good Old Revision" this means the first 11 bits of the inode bitmap.


1.5. Inode Table

The "Inode Table" is used to keep track of every file; their location, size, type and access rights are all stored in inodes. The filename is not stored in there though, within the inode tables all files are refenced by their inode number.

There is one inode table per group and it can be located by reading the "bg_inode_table" in its associated group descriptor. There are s_inodes_per_group inodes per table.

Each inode contain the information about a single physical file on the system. A file can be a directory, a socket, a buffer, character or block device, symbolic link or a regular file. So an inode can be seen as a block of information related to an entity, describing its location on disk, its size and its owner. An inode looks like this:

Figure 1-5. inode structure

offset  size    description
 ------- ------- -----------
       0       2 i_mode
       2       2 i_uid
       4       4 i_size
       8       4 i_atime
      12       4 i_ctime
      16       4 i_mtime
      20       4 i_dtime
      24       2 i_gid
      26       2 i_links_count
      28       4 i_blocks
      32       4 i_flags
      36       4 i_osd1
      40  15 x 4 i_block
     100       4 i_generation
     104       4 i_file_acl
     108       4 i_dir_acl
     112       4 i_faddr
     116      12 i_osd2
    

The first few entries of the inode tables are reserved. In the EXT2_GOOD_OLD_REV there are 11 entries reserved while in the newer EXT2_DYNAMIC_REV the number of reserved inodes entries is specified in the s_first_ino of the superblock structure. Here's a listing of the known reserved inode entries:

Table 1-4. EXT2_*_INO values

EXT2_BAD_INO 0x01 bad blocks inode
EXT2_ROOT_INO 0x02 root directory inode
EXT2_ACL_IDX_INO 0x03 ACL index inode (deprecated?)
EXT2_ACL_DATA_INO 0x04 ACL data inode (deprecated?)
EXT2_BOOT_LOADER_INO 0x05 boot loader inode
EXT2_UNDEL_DIR_INO 0x06 undelete directory inode

1.5.1. i_mode

16bit value used to indicate the format of the described file and the access rights. Here are the possible values, which can be combined in various ways:

Table 1-5. EXT2_S_I values

-- file format --
EXT2_S_IFMT 0xF000 format mask
EXT2_S_IFSOCK 0xC000 socket
EXT2_S_IFLNK 0xA000 symbolic link
EXT2_S_IFREG 0x8000 regular file
EXT2_S_IFBLK 0x6000 block device
EXT2_S_IFDIR 0x4000 directory
EXT2_S_IFCHR 0x2000 character device
EXT2_S_IFIFO 0x1000 fifo
-- access rights --
EXT2_S_ISUID 0x0800 SUID
EXT2_S_ISGID 0x0400 SGID
EXT2_S_ISVTX 0x0200 sticky bit
EXT2_S_IRWXU 0x01C0 user access rights mask
EXT2_S_IRUSR 0x0100 read
EXT2_S_IWUSR 0x0080 write
EXT2_S_IXUSR 0x0040 execute
EXT2_S_IRWXG 0x0038 group access rights mask
EXT2_S_IRGRP 0x0020 read
EXT2_S_IWGRP 0x0010 write
EXT2_S_IXGRP 0x0008 execute
EXT2_S_IRWXO 0x0007 others access rights mask
EXT2_S_IROTH 0x0004 read
EXT2_S_IWOTH 0x0002 write
EXT2_S_IXOTH 0x0001 execute

1.5.2. i_uid

16bit user id associated with the file.


1.5.3. i_size

32bit value indicating the size of the file in bytes.


1.5.4. i_atime

32bit value representing the number of seconds since january 1st 1970 of the last time this file was accessed.


1.5.5. i_ctime

32bit value representing the number of seconds since january 1st 1970 when the file was created.


1.5.6. i_mtime

32bit value representing the number of seconds since january 1st 1970 of the last time this file was modified.


1.5.7. i_dtime

32bit value representing the number of seconds since january 1st 1970 when the file was deleted. It is important that unless the file is deleted that this value is always 0.


1.5.8. i_gid

16bit value of the group having access to this file.


1.5.9. i_links_count

16bit value indicating how many times this particular inode is linked (referred to).


1.5.10. i_blocks

32bit value indicating the amount of blocks reserved for the associated file data. This includes both currently in used and currently reserved blocks in case the file grows in size.

A point worth of note is that this value indicate the number of 512 bytes block and not the number of blocks of the size indicated in the superblock. So if a file uses only 1 file system block and is 1024 bytes big, its .i_blocks value will be 2.


1.5.11. i_flags

32bit value indicating how the ext2 implementation should behave when accessing the data for this inode. (See the Behaviour flags section.)


1.5.12. i_osd1

32bit OS dependant value.


1.5.12.1. Hurd

32bit value labeled as "translator".


1.5.12.2. Linux

32bit value currently reserved.


1.5.12.3. Masix

32bit value currently reserved.


1.5.13. i_block

Array used to locate the blocks the particular file is stored on. Each entry is a 32bit block number. The first 12 entries in this array are block numbers, which can be used to fetch the first 12 blocks associated with the file.

The 13th entry is an indirect block number. Which means that at the specified data block, you will find an array of direct block numbers.

The 14th entry is an bi-indirect block number. Which means that at the specified data block, you will find an array of indirect block number, which in turn contains an array of block numbers that can be accessed directly.

The 15th entry is an tri-indirect block number. It is a block number which contains an array of bi-indirect block number, etc.

Each indirect/bi-indirect/tri-indirect block array contains as many entries of 32bit block numbers as possible (to fill one entire block).


1.5.14. i_generation

32bit value used to indicate the file version (used by NFS).


1.5.15. i_file_acl

32bit value indicating the block number containing the extended attributes. In previous revisions this value was always 0.

A general description of ACL for Digital UNIX can be found at this url for the moment: http://www.tru64unix.compaq.com/docs/base_doc/DOCUMENTATION/HTML/AA-Q0R2D-TET1_html/sec.c27.html


1.5.16. i_dir_acl

32bit value used to indicate the "high size" of the file. In previous revisions this value was always 0.


1.5.17. i_faddr

32bit value indicating the location of the last file fragment.


1.5.18. i_osd2

96bit OS dependant structure.


1.5.18.1. Hurd

Figure 1-6. inode osd2 structure: Hurd

offset  size    description
 ------- ------- -----------
       0       1 h_i_frag
       1       1 h_i_fsize
       2       2 h_i_mode_high
       4       2 h_i_uid_high
       6       2 h_i_gid_high
       8       4 h_i_author
      

1.5.18.1.1. h_i_frag

8bit fragment number.


1.5.18.1.2. h_i_fsize

8bit fragment size.


1.5.18.2. Linux

Figure 1-7. inode osd2 structure: Linux

offset  size    description
 ------- ------- -----------
       0       1 l_i_frag
       1       1 l_i_fsize
       2       2 reserved
       4       2 l_i_uid_high
       6       2 l_i_gid_high
       8       4 reserved
      

1.5.18.2.1. l_i_frag

8bit fragment number.


1.5.18.2.2. l_i_fsize

8bit fragment size.


1.5.18.3. Masix

Figure 1-8. inode osd2 structure: Masix

offset  size    description
 ------- ------- -----------
       0       1 m_i_frag
       1       1 m_i_fsize
       2      10 reserved
      

1.5.18.3.1. m_i_frag

8bit fragment number.


1.5.18.3.2. m_i_fsize

8bit fragment size.


1.6. Data Blocks

Data blocks are used to store the various files' content, including directory listing, extended attributes, symbolic links, etc.


Chapter 2. Directory Structure

Directories are stored as files and can be identified as such by looking up the ext2_inode.i_mode file format bits for the EXT2_S_IFDIR value.

The root directory is always the second entry of the inode table (EXT2_ROOT_INO is of value 2). Any subdirectory from there can be located by looking at the content of the root directory file.


2.1. Directory File Format

Figure 2-1. directory entry

offset  size    description
 ------- ------- -----------
       0       4 inode
       4       2 rec_len
       6       1 name_len
       7       1 file_type
       8     ... name
    

Earlier implementations of Ext2 used a 16bit name_len, but since this value is stored in Intel (little-endian) byte order and most implementation restricted filenames to maximum 255 characters, allowing a byte to be recycled.


2.1.1. inode

32bit inode number of the file entry. A value of 0 indicate that the entry is not used.


2.1.2. rec_len

16bit unsigned displacement to the next directory entry from the start of the current directory entry.


2.1.3. name_len

8bit unsigned value indicating how many characters are contained in the name.


2.1.4. file_type

8bit unsigned value used to indicate file type. As noted, this value may be 0 in earlier implementations. Currently defined values are:

Table 2-1. EXT2_FT values

EXT2_FT_UNKNOWN 0  
EXT2_FT_REG_FILE 1  
EXT2_FT_DIR 2  
EXT2_FT_CHRDEV 3  
EXT2_FT_BLKDEV 4  
EXT2_FT_FIFO 5  
EXT2_FT_SOCK 6  
EXT2_FT_SYMLINK 7  
EXT2_FT_MAX 8  

2.1.5. name

Name of the entry. The allowed character set is the ISO-Latin-1.


2.2. Sample Directory

Here's a sample of the home directory of one user on my system:

$ ls -1a /home/eks
 .
 ..
 .bash_profile
 .bashrc
 mbox
 public_html
 tmp
    

For which the following data representation can be found on the storage device:

Figure 2-2. Sample Directory Data Layout

offset  size    description
 ------- ------- -----------
       0       4 inode number (783362)
       4       2 record length (9)
       6       1 name length (1)
       7       1 file type (EXT2_FT_DIR)
       8       1 name (.)
 
       9       4 inode number (1109761)
      13       2 record length (10)
      15       1 name length (2)
      16       1 file type (EXT2_FT_DIR)
      17       2 name (..)
 
      19       4 inode number (783364)
      23       2 record length (21)
      25       1 name length (13)
      26       1 file type (EXT2_FT_REG_FILE)
      27      13 name (.bash_profile)
 
      40       4 inode number (783363)
      44       2 record length (15)
      46       1 name length (7)
      47       1 file type (EXT2_FT_REG_FILE)
      48       7 name (.bashrc)
 
      55       4 inode number (783377)
      59       2 record length (12)
      61       1 name length (4)
      62       1 file type (EXT2_FT_REG_FILE)
      63       4 name (mbox)
 
      67       4 inode number (783545)
      71       2 record length (19)
      73       1 name length (11)
      74       1 file type (EXT2_FT_DIR)
      75      11 name (public_html)
 
      86       4 inode number (669354)
      90       2 record length (11)
      92       1 name length (3)
      93       1 file type (EXT2_FT_DIR)
      94       3 name (tmp)
 
      97       4 inode number (0)
      101      2 record length (3999)
      103      1 name length (0)
      104      1 file type (EXT2_FT_UNKNOWN)
      105      0 name ()
    

It should be noted that some implementation will pad directory entries to have better performances on the host processor, it is thus important to use the record length and not the name length to find the next record.


2.3. Indexed Directory Format

Using the standard linked list directory format can become very slow once the number of files starts growing. To improve performances in such a system, a hashed index was created, which allow to quickly locate the particular file searched.

Bit EXT2_INDEX_FL in the behaviour control flags is set if the indexed directory format is used.


2.3.1. Index Structure

The root of the index tree is in the 0th block of the file. Space is reserved for a second level of the index tree in blocks 1 though 511 (for 4K filesystem blocks). Directory leaf blocks are appended starting at block 512, thus the tail of the directory file looks like a normal Ext2 directory and can be processed directly by ext2_readdir. For directories with less than about 90K files there is a hole running from block 1 to block 511, so an empty directory has just two blocks in it, though its size appears to be about 2 Meg in a directory listing.

So a directory file looks like:

0: Root index block
 1: Index block/0
 2: Index block/0
 ...
 511: Index block/0
 512: Dirent block
 513: Dirent block
 ...
     

Each index block consists of 512 index entries of the form:

	hash, block
     

where hash is a 32 bit hash with a collision flag in its least significant bit, and block is the logical block number of an index of leaf block, depending on the tree level.

The hash value of the 0th index entry isn't needed because it can always be obtained from the level about, so it is used to record the count of index entries in an index block. This gives a nice round branching factor of 512, the evenness being a nicety that mainly satisfies my need to seek regularity, rather than winning any real performance. (On the other hand, the largeness of the branching factor matters a great deal.)

The root index block has the same format as the other index blocks, with its first 8 bytes reserved for a small header:

1 byte header length (default: 8)
 1 byte index type (default: 0)
 1 byte hash version (default:0)
 1 byte tree depth (default: 1)
     

The treatment of the header differs slightly in the attached patch. In particular, only a single level of the index tree (the root) is implemented here. This turns out to be sufficient to handle more than 90,000 entries, so it is enough for today. When a second level is added to the tree, capacity will incease to somewhere around 50 million entries, and there is nothing preventing the use of n levels, should there ever be a reason. It's doubtfull that a third level will ever be required, but if it is, the design provides for it.


2.3.2. Lookup Algorithm

Lookup is straightforword:

- Compute a hash of the name
 - Read the index root
 - Use binary search (linear in the current code) to find the
   first index or leaf block that could contain the target hash
   (in tree order)
 - Repeat the above until the lowest tree level is reached
 - Read the leaf directory entry block and do a normal Ext2
   directory block search in it.
 - If the name is found, return its directory entry and buffer
 - Otherwise, if the collision bit of the next directory entry is
   set, continue searching in the successor block
     

Normally, two logical blocks of the file will need to be accessed, and one or two metadata index blocks. The effect of the metadata index blocks can largely be ignored in terms of disk access time since these blocks are unlikely to be evicted from cache. There is some small CPU cost that can be addressed by moving the whole directory into the page cache.


2.3.3. Insert Algorithm

Insertion of new entries into the directory is considerably more complex than lookup, due to the need to split leaf blocks when they become full, and to satisfy the conditions that allow hash key collisions to be handled reliably and efficiently. I'll just summarize here:

- Probe the index as for lookup
 - If the target leaf block is full, split it and note the block
   that will receive the new entry
 - Insert the new entry in the leaf block using the normal Ext2
   directory entry insertion code.
     

The details of splitting and hash collision handling are somewhat messy, but I will be happy to dwell on them at length if anyone is interested.


2.3.4. Splitting

In brief, when a leaf node fills up and we want to put a new entry into it the leaf has to be split, and its share of the hash space has to be partitioned. The most straightforward way to do this is to sort the entrys by hash value and split somewhere in the middle of the sorted list. This operation is log(number_of_entries_in_leaf) and is not a great cost so long as an efficient sorter is used. I used Combsort for this, although Quicksort would have been just as good in this case since average case performance is more important than worst case.

An alternative approach would be just to guess a median value for the hash key, and the partition could be done in linear time, but the resulting poorer partitioning of hash key space outweighs the small advantage of the linear partition algorithm. In any event, the number of entries needing sorting is bounded by the number that fit in a leaf.


2.3.5. Key Collisions

Some complexity is introduced by the need to handle sequences of hash key collisions. It is desireable to avoid splitting such sequences between blocks, so the split point of a block is adjusted with this in mind. But the possibility still remains that if the block fills up with identically-hashed entries, the sequence may still have to be split. This situation is flagged by placing a 1 in the low bit of the index entry that points at the sucessor block, which is naturally interpreted by the index probe as an intermediate value without any special coding. Thus, handling the collision problem imposes no real processing overhead, just come extra code and a slight reduction in the hash key space. The hash key space remains sufficient for any conceivable number of directory entries, up into the billions.


2.3.6. Hash Function

The exact properties of the hash function critically affect the performance of this indexing strategy, as I learned by trying a number of poor hash functions, at times intentionally. A poor hash function will result in many collisions or poor partitioning of the hash space. To illustrate why the latter is a problem, consider what happens when a block is split such that it covers just a few distinct hash values. The probability of later index entries hashing into the same, small hash space is very small. In practice, once a block is split, if its hash space is too small it tends to stay half full forever, an effect I observed in practice.

After some experimentation I came up with a hash function that gives reasonably good dispersal of hash keys across the entire 31 bit key space. This improved the average fullness of leaf blocks considerably, getting much closer to the theoretical average of 3/4 full.

But the current hash function is just a place holder, waiting for an better version based on some solid theory. I currently favor the idea of using crc32 as the default hash function, but I welcome suggestions.

Inevitably, no matter how good a hash function I come up with, somebody will come up with a better one later. For this reason the design allows for additional hash functiones to be added, with backward compatibility. This is accomplished simply, by including a hash function number in the index root. If a new, improved hash function is added, all the previous versions remain available, and previously created indexes remain readable.

Of course, the best strategy is to have a good hash function right from the beginning. The initial, quick hack has produced results that certainly have not been disappointing.


2.3.7. Performance

OK, if you have read this far then this is no doubt the part you've been waiting for. In short, the performance improvement over normal Ext2 has been stunning. With very small directories performance is similar to standard Ext2, but as directory size increases standard Ext2 quickly blows up quadratically, while htree-enhanced Ext2 continues to scale linearly.

Uli Luckas ran benchmarks for file creation in various sizes of directories ranging from 10,000 to 90,000 files. The results are pleasing: total file creation time stays very close to linear, versus quadratic increase with normal Ext2.

Time to create:

Figure 2-3. Performance of Indexed Directories

		Indexed		Normal
 		=======		======
 10000 Files:	0m1.350s	0m23.670s
 20000 Files:	0m2.720s	1m20.470s
 30000 Files:	0m4.330s	3m9.320s
 40000 Files:	0m5.890s	5m48.750s
 50000 Files:	0m7.040s	9m31.270s
 60000 Files:	0m8.610s	13m52.250s
 70000 Files:	0m9.980s	19m24.070s
 80000 Files:	0m12.060s	25m36.730s
 90000 Files:	0m13.400s	33m18.550s
     

A graph is posted at: http://www.innominate.org/~phillips/htree/performance.png

All of these tests are CPU-bound, which may come as a surprise. The directories fit easily in cache, and the limiting factor in the case of standard Ext2 is the looking up of directory blocks in buffer cache, and the low level scan of directory entries. In the case of htree indexing there are a number of costs to be considered, all of them pretty well bounded. Notwithstanding, there are a few obvious optimizations to be done:

- Use binary search instead of linear search in the interior index
   nodes.
 
 - If there is only one leaf block in a directory, bypass the index
   probe, go straight to the block.
 
 - Map the directory into the page cache instead of the buffer cache.
     

Each of these optimizations will produce a noticeable improvement in performance, but naturally it will never be anything like the big jump going from N**2 to Log512(N), ~= N. In time the optimizations will be applied and we can expect to see another doubling or so in performance.

There will be a very slight performance hit when the directory gets big enough to need a second level. Because of caching this will be very small. Traversing the directories metadata index blocks will be a bigger cost, and once again, this cost can be reduced by moving the directory blocks into the page cache.

Typically, we will traverse 3 blocks to read or write a directory entry, and that number increases to 4-5 with really huge directories. But this is really nothing compared to normal Ext2, which traverses several hundred blocks in the same situation.


Chapter 3. Inodes, file identifiers

Every file, directory, symlink, special device, or anything else really stored in a ext2 file system, is identified by an inode. If you know the inode number of the file you want to read, even if you don't know the path to the file or even the file name, you can still locate the file on disk and read it.


3.1. Inode Number

The "inode number" is an index in the inode table to an inode structure. The size of the inode table is fixed at format time, it is built to hold a maximum number of entries. Due to the normally sufficiently large amount of entries reserved, the table is quite big and thus, it was split equally among all the "block groups" (see Chapter 1 for more information).


3.2. Locating the Inode structure

The s_inodes_per_group field in the superblock structure tells us how many inodes are defined per group. Knowing that inode 1 is the first inode defined in the inode table, one can use the following formulaes:

group = (inode - 1) / s_inodes_per_group
    

to locate which blocks group holds the part of the inode table containing the searched inode entry, and:

index = (inode - 1) % s_inodes_per_group
    

to get the index within this partial inode table to the searched inode entry. Here are a couple of sample values that could be used to test your implementation:

Figure 3-1. Sample inode computations

s_inodes_per_group = 1712
 
 inode number computation
 ------------ -----------
        1     group = (1 - 1) / 1712 = 0
              index = (1 - 1) % 1712 = 0
 
        2     group = (2 - 1) / 1712 = 0
              index = (2 - 1) % 1712 = 1
 
      963     group = (963 - 1) / 1712 = 0
              index = (963 - 1) % 1712 = 962
 
     1712     group = (1712 - 1) / 1712 = 0
              index = (1712 - 1) % 1712 = 1711
 
     1713     group = (1713 - 1) / 1712 = 1
              index = (1713 - 1) % 1712 = 0
 
     3424     group = (3424 - 1) / 1712 = 1
              index = (3424 - 1) % 1712 = 1711
 
     3425     group = (3425 - 1) / 1712 = 2
              index = (3425 - 1) % 1712 = 0
    

As many of you are most likely already familiar with, an index of 0 means the first entry. The reason behind using 0 rather than 1 is that it can more easily be multiplied by the structure size to find the final offset of its location in memory or on disk.


3.3. Locating the Inode Table

As introduced in Section 3.1, the inode table is split equally among all group. If a file system was created to allow a thousand inodes, split between 5 groups, there would be 200 inodes per partial inode table. Figure 3-1 illustrates such similar distribution.

Each partial inode table can be located using the bg_inode_table field of the group_descriptor structure of its associated blocks group.


Chapter 4. File Attributes

Most of the file (also directory, symlink, device...) attributes are located in the inode associated with the file. Some other attributes are only available as extended attributes.


4.1. Standard Attributes

4.1.1. SUID, SGID and -rwxrwxrwx

There isn't much to say about those, they are located with the SGID and SUID bits in ext2_inode.i_mode.


4.1.2. File Size

The size of a file can be determined by looking at the ext2_inode.i_size field.


4.1.3. Owner and Group

Under most implementations, the owner and group are 16bit values, but on some recent Linux and Hurd implementations the owner and group id are 32bit. When 16bit values are used, only the "low" part should be used as valid, while when using 32bit value, both the "low" and "high" part should be used, the high part being shifted left 16 places then added to the low part.

The low part of owner and group are located in ext2_inode.i_uid and ext2_inode.i_gid respectively.

The high part of owner and group are located in ext2_inode.osd2.hurd.h_i_uid_high and ext2_inode.osd2.hurd.h_i_gid_high, respectively, for Hurd and located in ext2_inode.osd2.linux.l_i_uid_high and ext2_inode.osd2.linux.l_i_gid_high, respectively, for Linux.


4.2. Extended Attributes

Extended attributes are name:value pairs associated permanently with files and directories, similar to the environment strings associated with a process. An attribute may be defined or undefined. If it is defined, its value may be empty or non-empty.

Extended attributes are extensions to the normal attributes which are associated with all inodes in the system. They are often used to provide additional functionality to a filesystem - for example, additional security features such as Access Control Lists (ACLs) may be implemented using extended attributes.

Extended attributes are accessed as atomic objects. Reading retrieves the whole value of an attribute and stores it in a buffer. Writing replaces any previous value with the new value.

In each ext2 inode, we have the i_file_acl field, reserved for Access Control Lists. This field is used for storing the block number on which the extended attributes of an inode are stored instead (ACLs are stored as extended attributes).

Extended attributes are stored on `plain' disk blocks, which are not part of any files. The disk block layout is similar to the layout used for directories. After the attribute block header, entry headers follow. The size of entry headers varies with the length of the attribute name.

The attribute values are on the same block as their attribute entry descriptions, aligned to the end of the attribute block. This allows for additional attributes to be added more easily.

A list of attribute names associated with a file can be retrieved. The filesystem handler returns a string of names separated by null characters, terminated by two null characters at the end of the list.


4.2.1. Attribute Block Header

Figure 4-1. ext2_xattr_header structure

offset  size    description
 ------- ------- -----------
       0       4 h_magic
       4       4 h_refcount
       8       4 h_blocks
      12       4 h_hash
      16      16 reserved
     

4.2.1.1. h_magic

32bit magic number of identification (EXT2_XATTR_MAGIC = 0xEA020000).


4.2.1.2. h_refcount

32bit value used as reference count. This value is incremented everytime a link is created to this attribute block and decremented when a link is destroyed. Whenever this value reaches 0 the attribute block can be freed.


4.2.1.3. h_blocks

32bit value indicating how many blocks are currently used by the extended attributes.


4.2.1.4. h_hash

32bit hash value of all attributes.


4.2.2. Attribute Entry Header

Figure 4-2. ext2_xattr_header structure

offset  size    description
 ------- ------- -----------
       0       1 e_name_len
       1       1 e_name_index
       2       2 e_value_offs
       4       4 e_value_block
       8       4 e_value_size
      12       4 e_hash
      16     ... e_name
     

The total size of an attribute entry is always rounded to the next 4-bytes boundary.


4.2.2.1. e_name_len

8bit unsigned value indicating the length of the name.


4.2.2.2. e_name_index

8bit unsigned value used as attribute name index.


4.2.2.3. e_value_offs

16bit unsigned offset to the value within the value block.


4.2.2.4. e_value_block

32bit id of the block holding the value.


4.2.2.5. e_value_size

32bit unsigned value indicating the size of the attribute value.


4.2.2.6. e_hash

32bit hash of attribute name and value.


4.2.2.7. e_name

Attribute name.


4.3. Behaviour Control Flags

The i_flags value in the inode structure allows to specify how the file system should behave in regard to the file. The following bits are currently defined:

Table 4-1. Behaviour Control Flags

EXT2_SECRM_FL 0x00000001 secure deletion
EXT2_UNRM_FL 0x00000002 record for undelete
EXT2_COMPR_FL 0x00000004 compressed file
EXT2_SYNC_FL 0x00000008 synchronous updates
EXT2_IMMUTABLE_FL 0x00000010 immutable file
EXT2_APPEND_FL 0x00000020 append only
EXT2_NODUMP_FL 0x00000040 do not dump/delete file
EXT2_NOATIME_FL 0x00000080 do not update .i_atime
EXT2_DIRTY_FL 0x00000100 dirty (file is in use?)
EXT2_COMPRBLK_FL 0x00000200 compressed blocks
EXT2_NOCOMPR_FL 0x00000400 access raw compressed data
EXT2_ECOMPR_FL 0x00000800 compression error
EXT2_BTREE_FL 0x00010000 b-tree format directory
EXT2_INDEX_FL 0x00010000 Hash indexed directory
EXT2_IMAGIC_FL 0x00020000 ?
EXT3_JOURNAL_DATA_FL 0x00040000 journal file data
EXT2_RESERVED_FL 0x80000000 reserved for ext2 implementation

4.3.1. EXT2_SECRM_FL - Secure Deletion

Enabling this bit will cause random data to be written over the flie's content several time before the blocks are unlinked. Note that this is highly implementation dependant and as such, it should not be assumed to be 100% secure. Make sure to study the implementation notes before relying on this option.


4.3.2. EXT2_UNRM_FL - Record for Undelete

When supported by the implementation, setting this bit will cause the deleted data to be moved to a temporary location, where the user can restore the original file without any risk of data lost. This is most useful when using ext2 on a desktop or workstation.


4.3.3. EXT2_COMPR_FL - Compressed File

The file's content is compressed. There is no note about the particular algorithm used other than maybe the s_algo_bitmap field of the superblock structure.


4.3.4. EXT2_SYNC_FL - Synchronous Updates

The file's content in memory will be constantly synchronized with the content on disk. This is mostly used for very sensitive boot files or encryption keys that you do not want to lose in case of a crash.


4.3.5. EXT2_IMMUTABLE_FL - Immutable File

The blocks associated with the file will not be exchanged. If for any reason a file system defragmentation is launched, such files will not be moved. Mostly used for stage2 and stage1.5 boot loaders.


4.3.6. EXT2_APPEND_FL - Append Only

Writing can only be used to append content at the end of the file and not modify the current content. Example of such use could be mailboxes, where anybody could send a message to a user but not modify any already present.


4.3.7. EXT2_NODUMP_FL - Do No Dump/Delete

Setting this bit will protect the file from deletion. As long as this bit is set, even if the i_links_count is 0, the file will not be removed.


4.3.8. EXT2_NOATIME_FL - Do Not Update .i_atime

The i_atime field of the inode structure will not be modified when the file is accessed if this bit is set. The only good use I can think of that are related to security.


4.3.9. EXT2_DIRTY_FL - Dirty

I do not have information at this moment about the use of this bit.


4.3.10. EXT2_COMPRBLK_FL - Compressed Blocks

This flag is set if one or more blocks are compressed. You can have more information about compression on ext2 at http://www.netspace.net.au/~reiter/e2compr/ Note that the project has not been updated since 1999.


4.3.11. EXT2_NOCOMPR_FL - Access Raw Compressed Data

When this flag is set, the file system implementation will not uncompress the data before fowarding it to the application but will rather give it as is.


4.3.12. EXT2_ECOMPR_FL - Compression Error

This flag is set if an error was detected when trying to uncompress the file.


4.3.14. EXT2_INDEX_FL - Hash Indexed Directory

When this bit is set, the format of the directory file is hash indexed. This is covered in details in Section 2.3.


Appendix A. Credits

I would like to personally thank everybody who contributed to this document, you are numerous and in many cases I haven't kept track of all of you. Be sure that if you are not in this list, it's a mistake and do not hesitate to contact me, it will be a pleasure to add your name to the list.

Andreas Gruenbacher (a.gruenbacher@bestbits.at)
   Section 4.2
 
 Daniel Phillips (phillips@innominate.de)
   Section 2.3.1
   Section 2.3.2
   Section 2.3.3
   Section 2.3.4
   Section 2.3.5
   Section 2.3.6
   Section 2.3.7
 
 Jeremy Stanley of Access Data Inc.
   Pointed out the inversed values for EXT2_S_IFSOCK and EXT2_S_IFLNK
   
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