Ext4 Disk Layout

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This document attempts to describe the on-disk format for ext4 filesystems. The same general ideas should apply to ext2/3 filesystems as well, though they do not support all the features that ext4 supports, and the fields will be shorter.

NOTE: This is a work in progress, based on notes that the author (djwong) made while picking apart a filesystem by hand. The data structure definitions were pulled out of fs/ext4/ext4.h in 2.6.38. He welcomes all comments and corrections, since there is undoubtedly plenty of lore that doesn't necessarily show up on freshly created demonstration filesystems.



ext4 divides a storage device into an array of logical blocks both to reduce bookkeeping overhead and to increase throughput by forcing larger transfer sizes. Generally, the block size will be 4KiB (coincidentally, the same size as pages on x86 and the block layer's default block size), though the actual size is calculated as 2 ^ (10 + sb.s_log_block_size) bytes. Throughout this document, disk locations are given in terms of these logical blocks, not raw LBAs, and not 1024-byte blocks. For the sake of convenience, the logical block size will be referred to as $block_size throughout the rest of the document.

When referenced in preformatted text blocks, sb refers to fields in the super block, and inode refers to fields in an inode table entry.


An ext4 file system is split into a series of block groups. To reduce performance difficulties due to fragmentation, the block allocator tries very hard to keep each file's blocks within the same group, thereby reducing seek times. The size of a block group is specified in sb.s_blocks_per_group blocks, though it can also calculated as 8 * block_size_in_bytes. With the default block size of 4KiB, each group will contain 32,768 blocks, for a length of 128MiB. The number of block groups is the size of the device divided by the size of a block group.


The layout of a standard block group is approximately as follows (each of these fields is discussed in a separate section below):

Group 0 Padding ext4 Super Block Group Descriptors Reserved GDT Blocks Data Block Bitmap inode Bitmap inode Table Data Blocks
1024 bytes 1 block many blocks many blocks 1 block 1 block many blocks many more blocks

For the special case of block group 0, the first 1024 bytes are unused, to allow for the installation of x86 boot sectors and other oddities. The superblock will start at offset 1024 bytes, whichever block that happens to be (usually 0). However, if for some reason the block size = 1024, then block 0 is marked in use and the superblock goes in block 1. For all other block groups, there is no padding.

The ext4 driver primarily works with the superblock and the group descriptors that are found in block group 0. Redundant copies of the superblock and group descriptors are written to some of the block groups across the disk in case the beginning of the disk gets trashed, though not all block groups necessarily host a redundant copy (see following paragraph for more details). If the group does not have a redundant copy, the block group begins with the data block bitmap. Note also that when the filesystem is freshly formatted, mkfs will allocate "reserve GDT block" space after the block group descriptors and before the start of the block bitmaps to allow for future expansion of the filesystem. By default, a filesystem is allowed to increase in size by a factor of 1024x over the original filesystem size.

The location of the inode table is given by grp.bg_inode_table_*. It is continuous range of blocks large enough to contain sb.s_inodes_per_group * sb.s_inode_size bytes.

As for the ordering of items in a block group, it is generally established that the super block and the group descriptor table, if present, will be at the beginning of the block group. The bitmaps and the inode table can be anywhere, and it is quite possible for the bitmaps to come after the inode table, or for both to be in different groups (flex_bg). Leftover space is used for file data blocks, indirect block maps, extent tree blocks, and extended attributes.

Flexible Block Groups

Starting in ext4, there is a new feature called flexible block groups (flex_bg). In a flex_bg, several block groups are tied together as one logical block group; the bitmap spaces and the inode table space in the first block group of the flex_bg are expanded to include the bitmaps and inode tables of all other block groups in the flex_bg. For example, if the flex_bg size is 4, then group 0 will contain (in order) the superblock, group descriptors, data block bitmaps for groups 0-3, inode bitmaps for groups 0-3, inode tables for groups 0-3, and the remaining space in group 0 is for file data. The effect of this is to group the block metadata close together for faster loading, and to enable large files to be continuous on disk. Backup copies of the superblock and group descriptors are always at the beginning of block groups, even if flex_bg is enabled. The number of block groups that make up a flex_bg is given by 2 ^ sb.s_log_groups_per_flex.

Meta Block Groups

Normally, a complete copy of the entire block group descriptor table is recorded after every copy of the superblock. Assuming the default group size of 2^27 bytes (128MiB) and 64-byte group descriptors, this imposes a limitation of 2^21 block groups, or 256TiB. With the meta block group feature enabled, each block group contains redundant copies of the block group descriptor for that group, thereby enabling the creation of the full 2^32 block groups, for a total size of 512EiB.

Lazy Block Group Initialization

New also for ext4, the inode bitmap and inode tables in a group are uninitialized if the corresponding flag is set in the group descriptor. This is to reduce mkfs time considerably. If the group descriptor checksum feature is enabled, then even the group descriptors can be uninitialized.

Special inodes

ext4 reserves some inode for special features, as follows:

inode Purpose
0 Doesn't exist; there is no inode 0.
1 List of defective blocks.
2 Root directory.
3 ACL index.
4 ACL data.
5 Boot loader.
6 Undelete directory.
7 Reserved group descriptors inode.
8 Journal inode.
11 First non-reserved inode. Usually this is the lost+found directory.

The Super Block

The superblock records various information about the enclosing filesystem, such as block counts, inode counts, supported features, maintenance information, and more.

If the sparse_super feature flag is set, redundant copies of the superblock and group descriptors are kept only in the groups whose group number is either 0 or a power of 3, 5, or 7. If the flag is not set, redundant copies are kept in all groups.

The ext4 superblock is laid out as follows in struct ext4_super_block:

Offset Size Name Description
0x0 __le32 s_inodes_count Total inode count.
0x4 __le32 s_blocks_count_lo Total block count.
0x8 __le32 s_r_blocks_count_lo Reserved block count.
0xC __le32 s_free_blocks_count_lo Free block count.
0x10 __le32 s_free_inodes_count Free inode count.
0x14 __le32 s_first_data_block First data block.
0x18 __le32 s_log_block_size Block size is 2 ^ (10 + s_log_block_size).
0x1C __le32 s_obso_log_frag_size (Obsolete) fragment size.
0x20 __le32 s_blocks_per_group Blocks per group.
0x24 __le32 s_obso_frags_per_group (Obsolete) fragments per group.
0x28 __le32 s_inodes_per_group Inodes per group.
0x2C __le32 s_mtime Mount time, in seconds since the epoch.
0x30 __le32 s_wtime Write time, in seconds since the epoch.
0x34 __le16 s_mnt_count Number of mounts since the last fsck.
0x36 __le16 s_max_mnt_count Number of mounts beyond which a fsck is needed.
0x38 __le16 s_magic Magic signature, 0xEF53
0x3A __le16 s_state File system state. Valid values are:
0x0001 Cleanly umounted
0x0002 Errors detected
0x0004 Orphans being recovered
0x3C __le16 s_errors Behaviour when detecting errors. One of:
1 Continue
2 Remount read-only
3 Panic
0x3E __le16 s_minor_rev_level Minor revision level.
0x40 __le32 s_lastcheck Time of last check, in seconds since the epoch.
0x44 __le32 s_checkinterval Maximum time between checks, in seconds.
0x48 __le32 s_creator_os OS. One of:
0 Linux
1 Hurd
2 Masix
3 FreeBSD
4 Lites
0x4C __le32 s_rev_level Revision level. One of:
0 Original format
1 v2 format w/ dynamic inode sizes
0x50 __le16 s_def_resuid Default uid for reserved blocks.
0x52 __le16 s_def_resgid Default gid for reserved blocks.
These fields are for EXT4_DYNAMIC_REV superblocks only.

Note: the difference between the compatible feature set and the incompatible feature set is that if there is a bit set in the incompatible feature set that the kernel doesn't know about, it should refuse to mount the filesystem.

e2fsck's requirements are more strict; if it doesn't know about a feature in either the compatible or incompatible feature set, it must abort and not try to meddle with things it doesn't understand...

0x54 __le32 s_first_ino First non-reserved inode.
0x58 __le16 s_inode_size Size of inode structure, in bytes.
0x5A __le16 s_block_group_nr Block group # of this superblock.
0x5C __le32 s_feature_compat Compatible feature set flags. Kernel can still read/write this fs even if it doesn't understand a flag; fsck should not do that. Any of:
0x1 Directory preallocation.
0x2 "imagic inodes". Not clear from the code what this does.
0x4 Has a journal.
0x8 Supports extended attributes.
0x10 Has reserved GDT blocks for filesystem expansion.
0x20 Has directory indices.
0x40 "Lazy BG". Not in 2.6.38, seems to have been for uninitialized block groups?
0x80 "Exclude inode". Not documented or used outside of e2fsprogs.
0x60 __le32 s_feature_incompat Incompatible feature set. If the kernel or fsck doesn't understand one of these bits, it should stop. Any of:
0x1 Compression.
0x2 Directory entries record the file type. See ext4_dir_entry_2 below.
0x4 Filesystem needs recovery.
0x8 Filesystem has a separate journal device.
0x10 Meta block groups. See the earlier discussion of this feature.
0x40 Files in this filesystem use extents.
0x80 Enable a filesystem size of 2^64 blocks.
0x100 Multiple mount protection. Not implemented.
0x200 Flexible block groups. See the earlier discussion of this feature.
0x400 Inodes can be used for large extended attributes. (Not implemented?)
0x1000 Data in directory entry. (Not implemented?)
0x64 __le32 s_feature_ro_compat Readonly-compatible feature set. If the kernel doesn't understand one of these bits, it can still mount read-only. Any of:
0x1 Sparse superblocks. See the earlier discussion of this feature.
0x2 This filesystem has been used to store a file greater than 2GiB.
0x8 This filesystem has files whose sizes are represented in units of logical blocks, not 512-byte sectors. This implies a very large file indeed!
0x10 Group descriptors have checksums. In addition to detecting corruption, this is useful for lazy formatting with uninitialized groups.
0x20 Indicates that the old ext3 32,000 subdirectory limit no longer applies.
0x40 Indicates that large inodes exist on this filesystem.
0x80 This filesystem has a snapshot.
0x68 __u8 s_uuid[16] 128-bit UUID for volume.
0x78 char s_volume_name[16] Volume label.
0x88 char s_last_mounted[64] Directory where filesystem was last mounted.
0xC8 __le32 s_algorithm_usage_bitmap For compression
Performance hints. Directory preallocation should only happen if the EXT4_FEATURE_COMPAT_DIR_PREALLOC flag is on.
0xCC __u8 s_prealloc_blocks # of blocks to try to preallocate for ... files?
0xCD __u8 s_prealloc_dir_blocks # of blocks to preallocate for directories.
0xCE __le16 s_reserved_gdt_blocks Number of reserved GDT entries for future filesystem expansion.
Journaling support valid if EXT4_FEATURE_COMPAT_HAS_JOURNAL set.
0xD0 __u8 s_journal_uuid[16] UUID of journal superblock
0xE0 __le32 s_journal_inum inode number of journal file.
0xE4 __le32 s_journal_dev Device number of journal file, if the external journal feature flag is set.
0xE8 __le32 s_last_orphan Start of list of orphaned inodes to delete.
0xEC __le32 s_hash_seed[4] HTREE hash seed.
0xFC __u8 s_def_hash_version Default hash algorithm to use for directory hashes. One of:
0x0 Legacy.
0x1 Half MD4.
0x2 Tea.
0x3 Legacy, unsigned.
0x4 Half MD4, unsigned.
0x5 Tea, unsigned.
0xFD __u8 s_jnl_backup_type ?
0xFE __le16 s_desc_size Size of group descriptors, in bytes, if the 64bit incompat feature flag is set.
0x100 __le32 s_default_mount_opts Default mount options. Any of:
0x0001 Print debugging info upon (re)mount.
0x0002 New files take the gid of the containing directory (instead of the fsgid of the current process).
0x0004 Support userspace-provided extended attributes.
0x0008 Support POSIX access control lists (ACLs).
0x0010 Do not support 32-bit UIDs.
0x0020 All data and metadata are commited to the journal.
0x0040 All data are flushed to the disk before metadata are committed to the journal.
0x0060 Data ordering is not preserved; data may be written after the metadata has been written.
0x0100 Disable write flushes.
0x0200 Track which blocks in a filesystem are metadata and therefore should not be used as data blocks.
0x0400 Enable DISCARD support, where the storage device is told about blocks becoming unused.
0x0800 Disable delayed allocation.
0x104 __le32 s_first_meta_bg First metablock block group, if the meta_bg feature is enabled.
0x108 __le32 s_mkfs_time When the filesystem was created, in seconds since the epoch.
0x10C __le32 s_jnl_blocks[17] Backup copy of the first 68 bytes of the journal inode.
64bit support valid if EXT4_FEATURE_COMPAT_64BIT
0x150 __le32 s_blocks_count_hi High 32-bits of the block count.
0x154 __le32 s_r_blocks_count_hi High 32-bits of the reserved block count.
0x158 __le32 s_free_blocks_count_hi High 32-bits of the free block count.
0x15C __le16 s_min_extra_isize All inodes have at least # bytes.
0x15E __le16 s_want_extra_isize New inodes should reserve # bytes.
0x160 __le32 s_flags Miscellaneous flags. Any of:
0x0001 Signed directory hash in use.
0x0002 Unsigned directory hash in use.
0x0004 To test development code.
0x164 __le16 s_raid_stride RAID stride. This is the number of logical blocks read from or written to the disk before moving to the next disk. This affects the placement of filesystem metadata, which will hopefully make RAID storage faster.
0x166 __le16 s_mmp_interval # seconds to wait in multi-mount prevention (MMP) checking. In theory, MMP is a mechanism to record in the superblock which host and device have mounted the filesystem, in order to prevent multiple mounts. This feature does not seem to be implemented...
0x168 __le64 s_mmp_block Block # for multi-mount protection data.
0x170 __le32 s_raid_stripe_width RAID stripe width. This is the number of logical blocks read from or written to the disk before coming back to the current disk. This is used by the block allocator to try to reduce the number of read-modify-write operations in a RAID5/6.
0x174 __u8 s_log_groups_per_flex Size of a flexible block group is 2 ^ s_log_groups_per_flex.
0x175 __u8 s_reserved_char_pad
0x176 __le16 s_reserved_pad
0x178 __le64 s_kbytes_written Number of KiB written to this filesystem over its lifetime.
0x180 __le32 s_snapshot_inum inode number of active snapshot.
0x184 __le32 s_snapshot_id Sequential ID of active snapshot.
0x188 __le64 s_snapshot_r_blocks_count Number of blocks reserved for active snapshot's future use.
0x190 __le32 s_snapshot_list inode number of the head of the on-disk snapshot list.
0x194 __le32 s_error_count Number of errors seen.
0x198 __le32 s_first_error_time First time an error happened, in seconds since the epoch.
0x19C __le32 s_first_error_ino inode involved in first error.
0x1A0 __le64 s_first_error_block Number of block involved of first error.
0x1A8 __u8 s_first_error_func[32] Name of function where the error happened.
0x1C8 __le32 s_first_error_line Line number where error happened.
0x1CC __le32 s_last_error_time Time of most recent error, in seconds since the epoch.
0x1D0 __le32 s_last_error_ino inode involved in most recent error.
0x1D4 __le32 s_last_error_line Line number where most recent error happened.
0x1D8 __le64 s_last_error_block Number of block involved in most recent error.
0x1E0 __u8 s_last_error_func[32] Name of function where the most recent error happened.
0x200 __u8 s_mount_opts[64] ASCIIZ string of mount options.
0x240 __le32 s_reserved[112] Padding to the end of the block.

Total size is 1024 bytes.

Block Group Descriptors

Each block group on the filesystem has one of these descriptors associated with it. As noted in the Layout section above, the group descriptors (if present) are the second item in the block group. The standard configuration is for each block group to contain a full copy of the block group descriptor table unless the sparse_super feature flag is set.

Notice how the group descriptor records the location of both bitmaps and the inode table (i.e. they can float). This means that within a block group, the only data structures with fixed locations are the superblock and the group descriptor table. The flex_bg mechanism uses this property to group several block groups into a flex group and lay out all of the groups' bitmaps and inode tables into one long run in the first group of the flex group.

If the meta_bg feature flag is set, then several block groups are grouped together into a meta group. Note that in the meta_bg case, however, the first and last two block groups within the larger meta group contain only group descriptors for the groups inside the meta group.

flex_bg and meta_bg do not appear to be mutually exclusive features.

The block group descriptor is laid out in struct ext4_group_desc.

Offset Size Name Description
0x0 __le32 bg_block_bitmap_lo Lower 32-bits of location of block bitmap.
0x4 __le32 bg_inode_bitmap_lo Lower 32-bits of location of inode bitmap.
0x8 __le32 bg_inode_table_lo Lower 32-bits of location of inode table.
0xC __le16 bg_free_blocks_count_lo Lower 16-bits of free block count.
0xE __le16 bg_free_inodes_count_lo Lower 16-bits of free inode count.
0x10 __le16 bg_used_dirs_count_lo Lower 16-bits of directory count.
0x12 __le16 bg_flags Block group flags. Any of:
0x1 inode table and bitmap are not initialized.
0x2 block bitmap is not initialized.
0x4 inode table is zeroed.
0x14 __u32 bg_reserved[2] Likely block/inode bitmap checksum. (Huh?)
0x1C __le16 bg_itable_unused_lo Lower 16-bits of unused inode count.
0x1E __le16 bg_checksum Group descriptor checksum; crc16(sb_uuid+group+desc). Probably only calculated if the rocompat bg_checksum feature flag is set.
0x20 __le32 bg_block_bitmap_hi Upper 32-bits of location of block bitmap.
0x24 __le32 bg_inode_bitmap_hi Upper 32-bits of location of inodes bitmap.
0x28 __le32 bg_inode_table_hi Upper 32-bits of location of inodes table.
0x2C __le16 bg_free_blocks_count_hi Upper 16-bits of free block count.
0x2E __le16 bg_free_inodes_count_hi Upper 16-bits of free inode count.
0x30 __le16 bg_used_dirs_count_hi Upper 16-bits of directory count.
0x32 __le16 bg_itable_unused_hi Upper 16-bits of unused inode count.
0x34 __u32 bg_reserved2[3] Padding to 64 bytes.

Total size is 64 bytes.

Block and inode Bitmaps

The data block bitmap tracks the usage of data blocks within the block group.

The inode bitmap records which entries in the inode table are in use.

As with most bitmaps, one bit represents the usage status of one data block or inode table entry. This implies a block group size of 8 * number_of_bytes_in_a_logical_block.

Inode Table

In a regular UNIX filesystem, the inode stores all the metadata pertaining to the file (time stamps, block maps, extended attributes, etc), not the directory entry. To find the information associated with a file, one must traverse the directory files to find the directory entry associated with a file, then load the inode to find the metadata for that file. ext4 appears to cheat (for performance reasons) a little bit by storing a copy of the file type (normally stored in the inode) in the directory entry. (Compare all this to FAT, which stores all the file information directly in the directory entry, but does not support hard links and is in general more seek-happy than ext4 due to its simpler block allocator and extensive use of linked lists.)

The inode table is a linear array of struct ext4_inode. The table is sized to have enough blocks to store at least sb.s_inode_size * sb.s_inodes_per_group bytes. The number of the block group containing an inode can be calculated as (inode_number - 1) / sb.s_inodes_per_group, and the offset into the group's table is (inode_number - 1) % sb.s_inodes_per_group. There is no inode 0.

The inode table entry is laid out in struct ext4_inode.

Offset Size Name Description
0x0 __le16 i_mode File mode. Any of:
0x1 S_IXOTH (Others may execute)
0x2 S_IWOTH (Others may write)
0x4 S_IROTH (Others may read)
0x8 S_IXGRP (Group members may execute)
0x10 S_IWGRP (Group members may write)
0x20 S_IRGRP (Group members may read)
0x40 S_IXUSR (Owner may execute)
0x80 S_IWUSR (Owner may write)
0x100 S_IRUSR (Owner may read)
0x200 S_ISVTX (Sticky bit)
0x400 S_ISGID (Set GID)
0x800 S_ISUID (Set UID)
These are mutually-exclusive file types:
0x1000 S_IFIFO (FIFO)
0x2000 S_IFCHR (Character device)
0x4000 S_IFDIR (Directory)
0x6000 S_IFBLK (Block device)
0x8000 S_IFREG (Regular file)
0xA000 S_IFLNK (Symbolic link)
0xC000 S_IFSOCK (Socket)
0x2 __le16 i_uid Lower 16-bits of Owner UID.
0x4 __le32 i_size_lo Lower 32-bits of size in bytes.
0x8 __le32 i_atime Last access time, in seconds since the epoch.
0xC __le32 i_ctime Last inode change time, in seconds since the epoch.
0x10 __le32 i_mtime Last data modification time, in seconds since the epoch.
0x14 __le32 i_dtime Deletion Time, in seconds since the epoch.
0x18 __le16 i_gid Lower 16-bits of GID.
0x1A __le16 i_links_count Hard link count.
0x1C __le32 i_blocks_lo Lower 32-bits of block count.
0x20 __le32 i_flags Inode flags. Any of:
0x1 This file requires secure deletion. (not implemented)
0x2 This file should be preserved should undeletion be desired. (not implemented)
0x4 File is compressed. (not really implemented)
0x8 All writes to the file must be synchronous.
0x10 File is immutable.
0x20 File can only be appended.
0x40 The dump(1) utility should not dump this file.
0x80 Do not update access time.
0x100 Dirty compressed file. (not used)
0x200 File has one or more compressed clusters. (not used)
0x400 Do not compress file. (not used)
0x800 Compression error. (not used)
0x1000 Directory has hashed indexes.
0x2000 AFS magic directory.
0x4000 File data must always be written through the journal.
0x8000 File tail should not be merged.
0x10000 All directory entry data should be written synchronously (see dirsync).
0x20000 Top of directory hierarchy.
0x40000 This is a huge file.
0x80000 Inode uses extents.
0x200000 Inode used for a large extended attribute.
0x400000 This file has blocks allocated past EOF.
0x80000000 Reserved for ext4 library.
Aggregate flags:
0x4BDFFF User-visible flags.
0x4B80FF User-modifiable flags.
0x24 4 bytes

Union osd1:

Tag Contents
Offset Size Name Description
0x0 __le32 l_i_version Version
Offset Size Name Description
0x0 __le32 h_i_translator ??
Offset Size Name Description
0x0 __le32 m_i_reserved ??
0x28 __le32 i_block[EXT4_N_BLOCKS=15] Block map or extent tree. See the section "The Contents of i_block".
0x64 __le32 i_generation File version (for NFS).
0x68 __le32 i_file_acl_lo Lower 32-bits of extended attribute block. ACLs are of course one of many possible extended attributes; I think the name of this field is a result of the first use of extended attributes being for ACLs.
0x6C __le32 i_size_high Upper 32-bits of file size.
0x70 __le32 i_obso_faddr (Obsolete) fragment address.
0x74 12 bytes

Union osd2:

Tag Contents
Offset Size Name Description
0x0 __le16 l_i_blocks_high Upper 16-bits of the block count.
0x2 __le16 l_i_file_acl_high Upper 16-bits of the extended attribute block (historically, the file ACL location). See the Extended Attributes section below.
0x4 __le16 l_i_uid_high Upper 16-bits of the Owner UID.
0x6 __le16 l_i_gid_high Upper 16-bits of the GID.
0x8 __u32 l_i_reserved2 ??
Offset Size Name Description
0x0 __le16 h_i_reserved1 ??
0x2 __u16 h_i_mode_high Upper 16-bits of the file mode.
0x4 __le16 h_i_uid_high Upper 16-bits of the Owner UID.
0x6 __le16 h_i_gid_high Upper 16-bits of the GID.
0x8 __u32 h_i_author Author code?
Offset Size Name Description
0x0 __le16 h_i_reserved1 ??
0x2 __u16 m_i_file_acl_high Upper 16-bits of the extended attribute block (historically, the file ACL location).
0x4 __u32 m_i_reserved2[2] ??
0x80 __le16 i_extra_isize Size of this inode - 128.
0x82 __le16 i_pad1 ??
0x84 __le32 i_ctime_extra Extra change time bits. This provides sub-second precision.
0x88 __le32 i_mtime_extra Extra modification time bits. This provides sub-second precision.
0x8C __le32 i_atime_extra Extra access time bits. This provides sub-second precision.
0x90 __le32 i_crtime File creation time, in seconds since the epoch.
0x94 __le32 i_crtime_extra Extra file creation time bits. This provides sub-second precision.
0x98 __le32 i_version_hi Upper 32-bits for version number.

Note that the size of the structure is 156 bytes, though the standard inode size in ext4 is 256 bytes. It was 128 previously. I think(?) the extra space can be used for extended attributes.

The Contents of inode.i_block

Depending on the type of file an inode describes, the 60 bytes of storage in inode.i_block can be used in different ways. In general, regular files and directories will use it for file block indexing information, and special files will use it for special purposes.

Symbolic Links

The target of a symbolic link will be stored in this field if the target string is less than 60 bytes long. Otherwise, either extents or block maps will be used to allocate data blocks to store the link target.

Direct/Indirect Block Addressing

In ext2/3, file block numbers were mapped to logical block numbers by means of an (up to) three level 1-1 block map. To find the logical block that stores a particular file block, the code would navigate through this increasingly complicated structure. Notice that there is neither a magic number nor a checksum to provide any level of confidence that the block isn't full of garbage.

i.i_block Offset Where It Points
0 to 11 Direct map to file blocks 0 to 11.
12 Indirect block: (file blocks 12 to ($block_size / 4) + 11, or 12 to 1035 if 4KiB blocks)
Indirect Block Offset Where It Points
0 to ($block_size / 4) Direct map to ($block_size / 4) blocks (1024 if 4KiB blocks)
13 Double-indirect block: (file blocks $block_size/4 + 12 to ($block_size / 4) ^ 2 + ($block_size / 4) + 11, or 1036 to 1049611 if 4KiB blocks)
Double Indirect Block Offset Where It Points
0 to ($block_size / 4) Map to ($block_size / 4) indirect blocks (1024 if 4KiB blocks)
Indirect Block Offset Where It Points
0 to ($block_size / 4) Direct map to ($block_size / 4) blocks (1024 if 4KiB blocks)
14 Triple-indirect block: (file blocks ($block_size / 4) ^ 2 + ($block_size / 4) + 11 to ($block_size / 4) ^ 3 + $block_size / 4) ^ 2 + ($block_size / 4) + 12, or 1049611 to 1074791436 if 4KiB blocks)
Triple Indirect Block Offset Where It Points
0 to ($block_size / 4) Map to ($block_size / 4) double indirect blocks (1024 if 4KiB blocks)
Double Indirect Block Offset Where It Points
0 to ($block_size / 4) Map to ($block_size / 4) indirect blocks (1024 if 4KiB blocks)
Indirect Block Offset Where It Points
0 to ($block_size / 4) Direct map to ($block_size / 4) blocks (1024 if 4KiB blocks)

Note that with this block mapping scheme, it is necessary to fill out a lot of mapping data even for a large contiguous file! This inefficiency led to the creation of the extent mapping scheme, discussed below.

Notice also that a file using this mapping scheme cannot be placed higher than 2^32 blocks.

Extent Tree

In ext4, the file to logical block map has been replaced with an extent tree. Under the old scheme, allocating a contiguous run of 1,000 blocks requires an indirect block to map all 1,000 entries; with extents, the mapping is reduced to a single struct ext4_extent with ee_len = 1000. If flex_bg is enabled, it is possible to allocate very large files with a single extent, at a considerable reduction in metadata block use, and some improvement in disk efficiency. The inode must have the extents flag (0x80000) flag set for this feature to be in use.

Extents are arranged as a tree. Each node of the tree begins with a struct ext4_extent_header. If the node is an interior node (eh.eh_depth > 0), the header is followed by eh.eh_entries instances of struct ext4_extent_idx; each of these index entries points to a block containing more nodes in the extent tree. If the node is a leaf node (eh.eh_depth == 0), then the header is followed by eh.eh_entries instances of struct ext4_extent; these instances point to the file's data blocks. The root node of the extent tree is stored in inode.i_block, which allows for the first four extents to be recorded without the use of extra metadata blocks.

The extent tree header is recorded in struct ext4_extent_header, which is 12 bytes long:

Offset Size Name Description
0x0 __le16 eh_magic Magic number, 0xF30A.
0x2 __le16 eh_entries Number of valid entries following the header.
0x4 __le16 eh_max Maximum number of entries that could follow the header.
0x6 __le16 eh_depth Depth of this extent node in the extent tree. 0 = this extent node points to data blocks; otherwise, this extent node points to other extent nodes.
0x8 __le32 eh_generation Generation of the tree.

Internal nodes of the extent tree, also known as index nodes, are recorded as struct ext4_extent_idx, and are 12 bytes long:

Offset Size Name Description
0x0 __le32 ei_block This index node covers file blocks from 'block' onward.
0x4 __le32 ei_leaf_lo Lower 32-bits of the block number of the extent node that is the next level lower in the tree. The tree node pointed to can be either another internal node or a leaf node, described below.
0x8 __le16 ei_leaf_hi Upper 16-bits of the previous field.
0xA __u16 ei_unused

Leaf nodes of the extent tree are recorded as struct ext4_extent, and are also 12 bytes long:

Offset Size Name Description
0x0 __le32 ee_block First file block number that this extent covers.
0x4 __le16 ee_len Number of blocks covered by extent.
0x6 __le16 ee_start_hi Upper 16-bits of the block number to which this extent points.
0x8 __le32 ee_start_lo Lower 32-bits of the block number to which this extent points.

Directory Entries

In an ext4 filesystem, a directory is more or less a flat file that maps an arbitrary byte string (usually ASCII) to an inode number on the filesystem. There can be many directory entries across the filesystem that reference the same inode number--these are known as hard links, and that is why hard links cannot reference files on other filesystems. As such, directory entries are found by reading the data block(s) associated with a directory file for the particular directory entry that is desired.

Linear (Classic) Directories

By default, directory files contained an almost-linear array of directory entries in that directory. I write "almost" because it's not a linear array in the memory sense because directory entries are not split across filesystem blocks. Therefore, it is more accurate to say that a directory is a series of data blocks and that each block contains a linear array of directory entries. The end of each the per-block array is signified either by a record pointing to inode 0 or by reaching the end of the block. The end of the entire directory is of course signified by reaching the end of the file. By default the filesystem uses struct ext4_dir_entry_2 for directory entries unless the "filetype" feature flag is not set, in which case it uses struct ext4_dir_entry.

The original directory entry format is struct ext4_dir_entry, which is at most 263 bytes long, though on disk you'll need to reference dirent.rec_len to know for sure.

Offset Size Name Description
0x0 __le32 inode Number of the inode that this directory entry points to.
0x4 __le16 rec_len Length of this directory entry.
0x6 __le16 name_len Length of the file name.
0x8 char name[EXT4_NAME_LEN] File name.

Since file names cannot be longer than 255 bytes, the new directory entry format shortens the rec_len field and uses the space for a file type flag, probably to avoid having to load every inode during directory tree traversal. This format is ext4_dir_entry_2, which is at most 263 bytes long, though on disk you'll need to reference dirent.rec_len to know for sure.

Offset Size Name Description
0x0 __le32 inode Number of the inode that this directory entry points to.
0x4 __le16 rec_len Length of this directory entry.
0x6 __u8 name_len Length of the file name.
0x7 __u8 file_type File type code, one of:
0x0 Unknown.
0x1 Regular file.
0x2 Directory.
0x3 Character device file.
0x4 Block device file.
0x5 FIFO.
0x6 Socket.
0x7 Symbolic link.
0x8 char name[EXT4_NAME_LEN] File name.

Hash Tree Directories

A linear array of directory entries isn't great for performance, so a new feature was added to ext3 to provide a faster (but peculiar) balanced tree keyed off a hash of the directory entry name. If the EXT4_INDEX_FL (0x1000) flag is set in the inode, this directory uses a hashed btree (htree) to organize and find directory entries. For backwards read-only compatibility with ext2, this tree is actually hidden inside the directory file, masquerading as "empty" directory data blocks! It was stated previously that the end of the linear directory entry table was signified with an entry pointing to inode 0; this is (ab)used to fool the old linear-scan algorithm into thinking that the rest of the directory block is empty so that it moves on.

The root of the tree always lives in the first data block of the directory. By ext2 custom, the '.' and '..' entries must appear at the beginning of this first block, so they are put here as two struct ext4_dir_entry_2s and not stored in the tree. The rest of the root node contains metadata about the tree and finally a hash->block map to find nodes that are lower in the htree. If dx_root.info.indirect_levels is non-zero then the htree has two levels; the data block pointed to by the root node's map is an interior node, which is indexed by a minor hash. Interior nodes in this tree contains a zeroed out struct ext4_dir_entry_2 followed by a minor_hash->block map to find leafe nodes. Leaf nodes contain a linear array of all struct ext4_dir_entry_2; all of these entries (presumably) hash to the same value. If there is an overflow, the entries simply overflow into the next leaf node, and the least-significant bit of the hash (in the interior node map) that gets us to this next leaf node is set.

To traverse the directory as a htree, the code calculates the hash of the desired file name and uses it to find the corresponding block number. If the tree is flat, the block is a linear array of directory entries that can be searched; otherwise, the minor hash of the file name is computed and used against this second block to find the corresponding third block number. That third block number will be a linear array of directory entries.

To traverse the directory as a linear array (such as the old code does), the code simply reads every data block in the directory. The blocks used for the htree will appear to have no entries (aside from '.' and '..') and so only the leaf nodes will appear to have any interesting content.

The root of the htree is in struct dx_root, which is the full length of a data block:

Offset Type Name Description
0x0 struct fake_dirent (8 bytes) dot Directory entry for '.'.
0x8 char dot_name[4] ".\0\0\0"
0xC struct fake_dirent (8 bytes) dotdot Directory entry for '..'.
0x14 char dotdot_name[4] "..\0\0"
0x18 __le32 struct dx_root_info.reserved_zero Zero to make the rest of this directory data block seem empty.
0x1C u8 struct dx_root_info.hash_version Hash version, one of:
0x0 Legacy.
0x1 Half MD4.
0x2 Tea.
0x3 Legacy, unsigned.
0x4 Half MD4, unsigned.
0x5 Tea, unsigned.
0x1D u8 struct dx_root_info.info_length Length of the tree information, 0x8.
0x1E u8 struct dx_root_info.indirect_levels Depth of the htree.
0x1F u8 struct dx_root_info.unused_flags
0x20 struct dx_entry entries[0] As many 8-byte struct dx_entry as fits in the rest of the data block.

Interior nodes of an htree are recorded as struct dx_node, which is also the full length of a data block:

Offset Type Name Description
0x0 struct fake_dirent (8 bytes) fake Zeroed out to make this data block seem empty of directory entries.
0x8 struct dx_entry entries[0] As many 8-byte struct dx_entry as fits in the rest of the data block.

The hash maps that exist in both struct dx_root and struct dx_node are recorded as struct dx_entry, which is 8 bytes long:

Offset Type Name Description
0x0 __le32 hash Hash code.
0x4 __le32 block Block number (within the directory file, not filesystem blocks) of the next node in the htree.

(If you think this is all quite clever and peculiar, so does the author.)

Extended Attributes

Extended attributes (xattrs) are typically stored in a separate data block on the disk and referenced from inodes via inode.i_file_acl*. The first use of extended attributes seems to have been for storing file ACLs and other security data (selinux), though with the user_xattr mount option it is possible for users to store extended attributes (so long as all attribute names begin with "user.").

It appears that ext4 is capable of associating several hundred extended attributes with a file and also capable of storing large values (up to the size of a filesystem block), though the function ext4_xattr_check_entry seems to imply that storing names and values in different blocks is not really supported. Thus it seems that only one block can be used to store all the names and values associated with a file's attributes. It is also possible for many files to point to the same extended attribute data block.

The beginning of an extended attribute block is in struct ext4_xattr_header, which is 32 bytes long:

Offset Type Name Description
0x0 __le32 h_magic Magic number for identification, 0xEA020000.
0x4 __le32 h_refcount Reference count.
0x8 __le32 h_blocks Number of disk blocks used.
0xC __le32 h_hash Hash value of all attributes.
0x10 __u32 h_reserved[4]

Following the struct ext4_xattr_header is an array of struct ext4_xattr_entry; each of these entries is at least 16 bytes long.

Offset Type Name Description
0x0 __u8 e_name_len Length of name.
0x1 __u8 e_name_index Attribute name index.
0x2 __le16 e_value_offs Location of this attribute's value on the disk block where it is stored. Multiple attributes can share the same value.
0x4 __le32 e_value_block The disk block where the value is stored. Zero indicates the value is in the same block as this entry.
0x8 __le32 e_value_size Length of attribute value.
0xC __le32 e_hash Hash value of name and value.
0x10 char e_name[e_name_len] Attribute name. Does not include trailing NULL.

Attribute values can follow the end of the entry table. There appears to be a requirement that they be aligned to 4-byte boundaries.

Journal (jbd2)

Introduced in ext3, the ext4 filesystem employs a journal to protect the filesystem against corruption in the case of a system crash. A small continuous region of disk (default 128MiB) is reserved inside the filesystem as a place to land "important" data writes on-disk as quickly as possible. Once the important data transaction is fully written to the disk and flushed from the disk write cache, a record of the data being committed is also written to the journal. At some later point in time, the journal code writes the transactions to their final locations on disk (this could involve a lot of seeking or a lot of small read-write-erases) before erasing the commit record. Should the system crash during the second slow write, the journal can be replayed all the way to the latest commit record, guaranteeing the atomicity of whatever gets written through the journal to the disk. The effect of this is to guarantee that the filesystem does not become stuck midway through a metadata update.

For performance reasons, ext4 by default only writes filesystem metadata through the journal. This means that file data blocks are /not/ guaranteed to be in any consistent state after a crash. If this default guarantee level (data=ordered) is not satisfactory, there is a mount option to control journal behavior. If data=journal, all data and metadata are written to disk through the journal. This is slower but safest. If data=writeback, dirty data blocks are not flushed to the disk before the metadata are written to disk through the journal.

The journal inode is typically inode 8. The first 68 bytes of the journal inode are replicated in the ext4 superblock. The journal itself is normal (but hidden) file within the filesystem. The file usually consumes an entire block group, though mke2fs tries to put it in the middle of the disk.

NOTE: Both ext4 and ocfs2 use jbd2.


Generally speaking, the journal has this format:

Superblock [(descriptor_block data_blocks|revocation_block) [more data or revocations] commmit_block] [more transactions...]

           |<---------------------------------- one transaction ----------------------------------->|

Notice that a transaction begins with either a descriptor and some data, or a block revocation list. A finished transaction always ends with a commit. If there is no commit record (or the checksums don't match), the transaction will be discarded during replay.

Block Header

Every block in the journal starts with a common 12-byte header struct journal_header_s:

Offset Type Name Description
0x0 __be32 h_magic jbd2 magic number, 0xC03B3998.
0x4 __be32 h_blocktype Description of what this block contains. One of:
1 Descriptor. This block precedes a series of data blocks that were written through the journal during a transaction.
2 Block commit record. This block signifies the completion of a transaction.
3 Journal superblock, v1.
4 Journal superblock, v2.
5 Block revocation records. This speeds up recovery by enabling the journal to skip writing blocks that were subsequently rewritten.
0x8 __be32 h_sequence The transaction ID that goes with this block.

Super Block

The super block for the journal is much simpler as compared to ext4's. The key data kept within are size of the journal, and where to find the start of the log of transactions.

The journal superblock is recorded as struct journal_superblock_s, which is 1024 bytes long:

Offset Type Name Description
0x0 journal_header_t (12 bytes) s_header Common header identifying this as a superblock.
Static information describing the journal.
0xC __be32 s_blocksize Journal device block size.
0x10 __be32 s_maxlen Total number of blocks in this journal.
0x14 __be32 s_first First block of log information.
Dynamic information describing the current state of the log.
0x18 __be32 s_sequence First commit ID expected in log.
0x1C __be32 s_start Block number of the start of log. If zero, the journal is clean.
0x20 __be32 s_errno Error value, as set by jbd2_journal_abort().
The remaining fields are only valid in a version 2 superblock.
0x24 __be32 s_feature_compat; Compatible feature set. Any of:
0x1 Journal maintains checksums on the data blocks.
0x28 __be32 s_feature_incompat Incompatible feature set. Any of:
0x1 Journal has block revocation records.
0x2 Journal can deal with 64-bit block numbers.
0x4 Journal commits asynchronously.
0x2C __be32 s_feature_ro_compat Read-only compatible feature set. There aren't any of these currently.
0x30 __u8 s_uuid[16] 128-bit uuid for journal. This is compared against the copy in the ext4 super block at mount time.
0x40 __be32 s_nr_users Number of file systems sharing this journal.
0x44 __be32 s_dynsuper Location of dynamic super block copy. (Not used?)
0x48 __be32 s_max_transaction Limit of journal blocks per transaction. (Not used?)
0x4C __be32 s_max_trans_data Limit of data blocks per transaction. (Not used?)
0x50 __u32 s_padding[44]
0x100 __u8 s_users[16*48] ids of all file systems sharing the log. (Not used?)

Descriptor Block

The descriptor block contains an array of journal block tags that describe the final locations of the data blocks that follow in the journal. Descriptor blocks are open-coded instead of being completely described by a data structure, but here is the block structure anyway. Descriptor blocks consume at least 36 bytes, but use a full block:

Offset Type Name Descriptor
0x0 journal_header_t (open coded) Common block header.
0xC struct journal_block_tag_s open coded array[] Enough tags either to fill up the block or to describe all the data blocks that follow this descriptor block.

Journal block tags have the following format, as recorded by struct journal_block_tag_s. They can be 8, 12, 24, or 38 bytes:

Offset Type Name Descriptor
0x0 __be32 t_blocknr Lower 32-bits of the location of where the corresponding data block should end up on disk.
0x4 __be32 t_flags Flags that go with the descriptor. Any of:
0x1 On-disk block is escaped. The first four bytes of the data block just happened to match the jbd2 magic number.
0x2 This block has the same UUID as previous, therefore the UUID field is omitted.
0x4 The data block was deleted by the transaction. (Not used?)
0x8 This is the last tag in this descriptor block.
This next field is only present if the super block indicates support for 64-bit block numbers.
0x8 __be32 t_blocknr_high Upper 32-bits of the location of where the corresponding data block should end up on disk.
This field appears to be open coded. It always comes at the end of the tag, after t_flags or t_blocknr_high. This field is not present if the "same UUID" flag is set.
0x8 or 0xC char uuid[16] A UUID to go with this tag. This field appears to be copied from a field in struct journal_s that is never set, which means that the UUID is probably all zeroes. Or perhaps it will contain garbage.

Data Block

In general, the data blocks being written to disk through the journal are written verbatim into the journal file after the descriptor block. However, if the first four bytes of the block match the jbd2 magic number then those four bytes are replaced with zeroes and the "escaped" flag is set in the descriptor block.

Revocation Block

A revocation block is used to record a list of data blocks in this transaction that supersede any older copies of those data blocks that might still be lurking in the journal. This can speed up recovery because those older copies don't have to be written out to disk.

Revocation blocks are described in struct jbd2_journal_revoke_header_s, are at least 16 bytes in length, but use a full block:

Offset Type Name Description
0x0 journal_header_t r_header Common block header.
0xC __be32 r_count Number of bytes used in this block.
0x10 __be32 or __be64 blocks[0] Blocks to revoke.

After r_count is a linear array of block numbers that are effectively revoked by this transaction. The size of each block number is 8 bytes if the superblock advertises 64-bit block number support, or 4 bytes otherwise.

Commit Block

The commit block is a sentry that indicates that a transaction has been completely written to the journal. Once this commit block reaches the journal, the data stored with this transaction can be written to their final locations on disk.

The commit block is described by struct commit_header, which is 32 bytes long (but uses a full block):

Offset Type Name Descriptor
0x0 journal_header_s (open coded) Common block header.
0xC unsigned char h_chksum_type The type of checksum to use to verify the integrity of the data blocks in the transaction. One of:
1 CRC32
2 MD5
3 SHA1
0xD unsigned char h_chksum_size The number of bytes used by the checksum. Most likely 4.
0xE unsigned char h_padding[2]
0x10 __be32 h_chksum[JBD2_CHECKSUM_BYTES] 32 bytes of space to store checksums.
0x30 __be64 h_commit_sec The time that the transaction was committed, in seconds since the epoch.
0x38 __be32 h_commit_nsec Nanoseconds component of the above timestamp.
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