3. Global Structures

The filesystem is sharded into a number of block groups, each of which have static metadata at fixed locations.

3.1. 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 superblock checksum is calculated against the superblock structure, which includes the FS UUID.

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 This number of blocks can only be allocated by the super-user.
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. This must be at least 1 for 1k-block filesystems and is typically 0 for all other block sizes.
0x18 __le32 s_log_block_size Block size is 2 ^ (10 + s_log_block_size).
0x1C __le32 s_log_cluster_size Cluster size is 2 ^ (10 + s_log_cluster_size) blocks if bigalloc is enabled. Otherwise s_log_cluster_size must equal s_log_block_size.
0x20 __le32 s_blocks_per_group Blocks per group.
0x24 __le32 s_clusters_per_group Clusters per group, if bigalloc is enabled. Otherwise s_clusters_per_group must equal s_blocks_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. See super_state for more info.
0x3C __le16 s_errors Behaviour when detecting errors. See super_errors for more info.
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 Creator OS. See the table super_creator for more info.
0x4C __le32 s_rev_level Revision level. See the table super_revision for more info.
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. See the super_compat table for more info.
0x60 __le32 s_feature_incompat Incompatible feature set. If the kernel or fsck doesn't understand one of these bits, it should stop. See the super_incompat table for more info.
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. See the super_rocompat table for more info.
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 (Not used in e2fsprogs/Linux)
      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? (Not used in e2fsprogs/Linux)
0xCD __u8 s_prealloc_dir_blocks #. of blocks to preallocate for directories. (Not used in e2fsprogs/Linux)
0xCE __le16 s_reserved_gdt_blocks Number of reserved GDT entries for future filesystem expansion.
      Journalling support is valid only if EXT4_FEATURE_COMPAT_HAS_JOURNAL is 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. See super_def_hash for more info.
0xFD __u8 s_jnl_backup_type If this value is 0 or EXT3_JNL_BACKUP_BLOCKS (1), then the s_jnl_blocks field contains a duplicate copy of the inode's i_block[] array and i_size.
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. See the super_mountopts table for more info.
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 journal inode's i_block[] array in the first 15 elements and i_size_high and i_size in the 16th and 17th elements, respectively.
      64bit support is valid only if EXT4_FEATURE_COMPAT_64BIT is set.
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. See the super_flags table for more info.
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_checksum_type Metadata checksum algorithm type. The only valid value is 1 (crc32c).
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. (Not used in e2fsprogs/Linux.)
0x184 __le32 s_snapshot_id Sequential ID of active snapshot. (Not used in e2fsprogs/Linux.)
0x188 __le64 s_snapshot_r_blocks_count Number of blocks reserved for active snapshot's future use. (Not used in e2fsprogs/Linux.)
0x190 __le32 s_snapshot_list inode number of the head of the on-disk snapshot list. (Not used in e2fsprogs/Linux.)
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_usr_quota_inum Inode number of user quota file.
0x244 __le32 s_grp_quota_inum Inode number of group quota file.
0x248 __le32 s_overhead_blocks Overhead blocks/clusters in fs. (Huh? This field is always zero, which means that the kernel calculates it dynamically.)
0x24C __le32 s_backup_bgs[2] Block groups containing superblock backups (if sparse_super2)
0x254 __u8 s_encrypt_algos[4] Encryption algorithms in use. There can be up to four algorithms in use at any time; valid algorithm codes are given in the super_encrypt table below.
0x258 __u8 s_encrypt_pw_salt[16] Salt for the string2key algorithm for encryption.
0x268 __le32 s_lpf_ino Inode number of lost+found
0x26C __le32 s_prj_quota_inum Inode that tracks project quotas.
0x270 __le32 s_checksum_seed Checksum seed used for metadata_csum calculations. This value is crc32c(~0, $orig_fs_uuid).
0x274 __u8 s_wtime_hi Upper 8 bits of the s_wtime field.
0x275 __u8 s_mtime_hi Upper 8 bits of the s_mtime field.
0x276 __u8 s_mkfs_time_hi Upper 8 bits of the s_mkfs_time field.
0x277 __u8 s_lastcheck_hi Upper 8 bits of the s_lastcheck_hi field.
0x278 __u8 s_first_error_time_hi Upper 8 bits of the s_first_error_time_hi field.
0x279 __u8 s_last_error_time_hi Upper 8 bits of the s_last_error_time_hi field.
0x27A __u8 s_pad[2] Zero padding.
0x27C __le16 s_encoding Filename charset encoding.
0x27E __le16 s_encoding_flags Filename charset encoding flags.
0x280 __le32 s_reserved[95] Padding to the end of the block.
0x3FC __le32 s_checksum Superblock checksum.

The superblock state is some combination of the following:

Value Description
0x0001 Cleanly umounted
0x0002 Errors detected
0x0004 Orphans being recovered

The superblock error policy is one of the following:

Value Description
1 Continue
2 Remount read-only
3 Panic

The filesystem creator is one of the following:

Value Description
0 Linux
1 Hurd
2 Masix
3 FreeBSD
4 Lites

The superblock revision is one of the following:

Value Description
0 Original format
1 v2 format w/ dynamic inode sizes

Note that EXT4_DYNAMIC_REV refers to a revision 1 or newer filesystem.

The superblock compatible features field is a combination of any of the following:

Value Description
0x1 Directory preallocation (COMPAT_DIR_PREALLOC).
0x2 “imagic inodes”. Not clear from the code what this does (COMPAT_IMAGIC_INODES).
0x4 Has a journal (COMPAT_HAS_JOURNAL).
0x8 Supports extended attributes (COMPAT_EXT_ATTR).
0x10 Has reserved GDT blocks for filesystem expansion (COMPAT_RESIZE_INODE). Requires RO_COMPAT_SPARSE_SUPER.
0x20 Has directory indices (COMPAT_DIR_INDEX).
0x40 “Lazy BG”. Not in Linux kernel, seems to have been for uninitialized block groups? (COMPAT_LAZY_BG)
0x80 “Exclude inode”. Not used. (COMPAT_EXCLUDE_INODE).
0x100 “Exclude bitmap”. Seems to be used to indicate the presence of snapshot-related exclude bitmaps? Not defined in kernel or used in e2fsprogs (COMPAT_EXCLUDE_BITMAP).
0x200 Sparse Super Block, v2. If this flag is set, the SB field s_backup_bgs points to the two block groups that contain backup superblocks (COMPAT_SPARSE_SUPER2).

The superblock incompatible features field is a combination of any of the following:

Value Description
0x1 Compression (INCOMPAT_COMPRESSION).
0x2 Directory entries record the file type. See ext4_dir_entry_2 below (INCOMPAT_FILETYPE).
0x4 Filesystem needs recovery (INCOMPAT_RECOVER).
0x8 Filesystem has a separate journal device (INCOMPAT_JOURNAL_DEV).
0x10 Meta block groups. See the earlier discussion of this feature (INCOMPAT_META_BG).
0x40 Files in this filesystem use extents (INCOMPAT_EXTENTS).
0x80 Enable a filesystem size of 2^64 blocks (INCOMPAT_64BIT).
0x100 Multiple mount protection (INCOMPAT_MMP).
0x200 Flexible block groups. See the earlier discussion of this feature (INCOMPAT_FLEX_BG).
0x400 Inodes can be used to store large extended attribute values (INCOMPAT_EA_INODE).
0x1000 Data in directory entry (INCOMPAT_DIRDATA). (Not implemented?)
0x2000 Metadata checksum seed is stored in the superblock. This feature enables the administrator to change the UUID of a metadata_csum filesystem while the filesystem is mounted; without it, the checksum definition requires all metadata blocks to be rewritten (INCOMPAT_CSUM_SEED).
0x4000 Large directory >2GB or 3-level htree (INCOMPAT_LARGEDIR). Prior to this feature, directories could not be larger than 4GiB and could not have an htree more than 2 levels deep. If this feature is enabled, directories can be larger than 4GiB and have a maximum htree depth of 3.
0x8000 Data in inode (INCOMPAT_INLINE_DATA).
0x10000 Encrypted inodes are present on the filesystem. (INCOMPAT_ENCRYPT).

The superblock read-only compatible features field is a combination of any of the following:

Value Description
0x1 Sparse superblocks. See the earlier discussion of this feature (RO_COMPAT_SPARSE_SUPER).
0x2 This filesystem has been used to store a file greater than 2GiB (RO_COMPAT_LARGE_FILE).
0x4 Not used in kernel or e2fsprogs (RO_COMPAT_BTREE_DIR).
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! (RO_COMPAT_HUGE_FILE)
0x10 Group descriptors have checksums. In addition to detecting corruption, this is useful for lazy formatting with uninitialized groups (RO_COMPAT_GDT_CSUM).
0x20 Indicates that the old ext3 32,000 subdirectory limit no longer applies (RO_COMPAT_DIR_NLINK). A directory's i_links_count will be set to 1 if it is incremented past 64,999.
0x40 Indicates that large inodes exist on this filesystem (RO_COMPAT_EXTRA_ISIZE).
0x80 This filesystem has a snapshot (RO_COMPAT_HAS_SNAPSHOT).
0x100 Quota (RO_COMPAT_QUOTA).
0x200 This filesystem supports “bigalloc”, which means that file extents are tracked in units of clusters (of blocks) instead of blocks (RO_COMPAT_BIGALLOC).
0x400 This filesystem supports metadata checksumming. (RO_COMPAT_METADATA_CSUM; implies RO_COMPAT_GDT_CSUM, though GDT_CSUM must not be set)
0x800 Filesystem supports replicas. This feature is neither in the kernel nor e2fsprogs. (RO_COMPAT_REPLICA)
0x1000 Read-only filesystem image; the kernel will not mount this image read-write and most tools will refuse to write to the image. (RO_COMPAT_READONLY)
0x2000 Filesystem tracks project quotas. (RO_COMPAT_PROJECT)
0x8000 Verity inodes may be present on the filesystem. (RO_COMPAT_VERITY)

The s_def_hash_version field is one of the following:

Value Description
0x0 Legacy.
0x1 Half MD4.
0x2 Tea.
0x3 Legacy, unsigned.
0x4 Half MD4, unsigned.
0x5 Tea, unsigned.

The s_default_mount_opts field is any combination of the following:

Value Description
0x0001 Print debugging info upon (re)mount. (EXT4_DEFM_DEBUG)
0x0002 New files take the gid of the containing directory (instead of the fsgid of the current process). (EXT4_DEFM_BSDGROUPS)
0x0004 Support userspace-provided extended attributes. (EXT4_DEFM_XATTR_USER)
0x0008 Support POSIX access control lists (ACLs). (EXT4_DEFM_ACL)
0x0010 Do not support 32-bit UIDs. (EXT4_DEFM_UID16)
0x0020 All data and metadata are commited to the journal. (EXT4_DEFM_JMODE_DATA)
0x0040 All data are flushed to the disk before metadata are committed to the journal. (EXT4_DEFM_JMODE_ORDERED)
0x0060 Data ordering is not preserved; data may be written after the metadata has been written. (EXT4_DEFM_JMODE_WBACK)
0x0100 Disable write flushes. (EXT4_DEFM_NOBARRIER)
0x0200 Track which blocks in a filesystem are metadata and therefore should not be used as data blocks. This option will be enabled by default on 3.18, hopefully. (EXT4_DEFM_BLOCK_VALIDITY)
0x0400 Enable DISCARD support, where the storage device is told about blocks becoming unused. (EXT4_DEFM_DISCARD)
0x0800 Disable delayed allocation. (EXT4_DEFM_NODELALLOC)

The s_flags field is any combination of the following:

Value Description
0x0001 Signed directory hash in use.
0x0002 Unsigned directory hash in use.
0x0004 To test development code.

The s_encrypt_algos list can contain any of the following:

Value Description
0 Invalid algorithm (ENCRYPTION_MODE_INVALID).
1 256-bit AES in XTS mode (ENCRYPTION_MODE_AES_256_XTS).
2 256-bit AES in GCM mode (ENCRYPTION_MODE_AES_256_GCM).
3 256-bit AES in CBC mode (ENCRYPTION_MODE_AES_256_CBC).

Total size of the superblock is 1024 bytes.

3.2. 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.

In ext2, ext3, and ext4 (when the 64bit feature is not enabled), the block group descriptor was only 32 bytes long and therefore ends at bg_checksum. On an ext4 filesystem with the 64bit feature enabled, the block group descriptor expands to at least the 64 bytes described below; the size is stored in the superblock.

If gdt_csum is set and metadata_csum is not set, the block group checksum is the crc16 of the FS UUID, the group number, and the group descriptor structure. If metadata_csum is set, then the block group checksum is the lower 16 bits of the checksum of the FS UUID, the group number, and the group descriptor structure. Both block and inode bitmap checksums are calculated against the FS UUID, the group number, and the entire bitmap.

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. See the bgflags table below.
0x14 __le32 bg_exclude_bitmap_lo Lower 32-bits of location of snapshot exclusion bitmap.
0x18 __le16 bg_block_bitmap_csum_lo Lower 16-bits of the block bitmap checksum.
0x1A __le16 bg_inode_bitmap_csum_lo Lower 16-bits of the inode bitmap checksum.
0x1C __le16 bg_itable_unused_lo Lower 16-bits of unused inode count. If set, we needn't scan past the (sb.s_inodes_per_group - gdt.bg_itable_unused)th entry in the inode table for this group.
0x1E __le16 bg_checksum Group descriptor checksum; crc16(sb_uuid+group_num+bg_desc) if the RO_COMPAT_GDT_CSUM feature is set, or crc32c(sb_uuid+group_num+bg_desc) & 0xFFFF if the RO_COMPAT_METADATA_CSUM feature is set. The bg_checksum field in bg_desc is skipped when calculating crc16 checksum, and set to zero if crc32c checksum is used.
      These fields only exist if the 64bit feature is enabled and s_desc_size > 32.
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 __le32 bg_exclude_bitmap_hi Upper 32-bits of location of snapshot exclusion bitmap.
0x38 __le16 bg_block_bitmap_csum_hi Upper 16-bits of the block bitmap checksum.
0x3A __le16 bg_inode_bitmap_csum_hi Upper 16-bits of the inode bitmap checksum.
0x3C __u32 bg_reserved Padding to 64 bytes.

Block group flags can be any combination of the following:

Value Description
0x1 inode table and bitmap are not initialized (EXT4_BG_INODE_UNINIT).
0x2 block bitmap is not initialized (EXT4_BG_BLOCK_UNINIT).
0x4 inode table is zeroed (EXT4_BG_INODE_ZEROED).

3.3. 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.

NOTE: If BLOCK_UNINIT is set for a given block group, various parts of the kernel and e2fsprogs code pretends that the block bitmap contains zeros (i.e. all blocks in the group are free). However, it is not necessarily the case that no blocks are in use -- if meta_bg is set, the bitmaps and group descriptor live inside the group. Unfortunately, ext2fs_test_block_bitmap2() will return '0' for those locations, which produces confusing debugfs output.

3.4. Inode Table

Inode tables are statically allocated at mkfs time. Each block group descriptor points to the start of the table, and the superblock records the number of inodes per group. See the section on inodes for more information.

3.5. Multiple Mount Protection

Multiple mount protection (MMP) is a feature that protects the filesystem against multiple hosts trying to use the filesystem simultaneously. When a filesystem is opened (for mounting, or fsck, etc.), the MMP code running on the node (call it node A) checks a sequence number. If the sequence number is EXT4_MMP_SEQ_CLEAN, the open continues. If the sequence number is EXT4_MMP_SEQ_FSCK, then fsck is (hopefully) running, and open fails immediately. Otherwise, the open code will wait for twice the specified MMP check interval and check the sequence number again. If the sequence number has changed, then the filesystem is active on another machine and the open fails. If the MMP code passes all of those checks, a new MMP sequence number is generated and written to the MMP block, and the mount proceeds.

While the filesystem is live, the kernel sets up a timer to re-check the MMP block at the specified MMP check interval. To perform the re-check, the MMP sequence number is re-read; if it does not match the in-memory MMP sequence number, then another node (node B) has mounted the filesystem, and node A remounts the filesystem read-only. If the sequence numbers match, the sequence number is incremented both in memory and on disk, and the re-check is complete.

The hostname and device filename are written into the MMP block whenever an open operation succeeds. The MMP code does not use these values; they are provided purely for informational purposes.

The checksum is calculated against the FS UUID and the MMP structure. The MMP structure (struct mmp_struct) is as follows:

Offset Type Name Description
0x0 __le32 mmp_magic Magic number for MMP, 0x004D4D50 (“MMP”).
0x4 __le32 mmp_seq Sequence number, updated periodically.
0x8 __le64 mmp_time Time that the MMP block was last updated.
0x10 char[64] mmp_nodename Hostname of the node that opened the filesystem.
0x50 char[32] mmp_bdevname Block device name of the filesystem.
0x70 __le16 mmp_check_interval The MMP re-check interval, in seconds.
0x72 __le16 mmp_pad1 Zero.
0x74 __le32[226] mmp_pad2 Zero.
0x3FC __le32 mmp_checksum Checksum of the MMP block.

3.6. 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.

All fields in jbd2 are written to disk in big-endian order. This is the opposite of ext4.

NOTE: Both ext4 and ocfs2 use jbd2.

The maximum size of a journal embedded in an ext4 filesystem is 2^32 blocks. jbd2 itself does not seem to care.

3.6.1. Layout

Generally speaking, the journal has this format:

Superblock descriptor_block (data_blocks or 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.

3.6.2. External Journal

Optionally, an ext4 filesystem can be created with an external journal device (as opposed to an internal journal, which uses a reserved inode). In this case, on the filesystem device, s_journal_inum should be zero and s_journal_uuid should be set. On the journal device there will be an ext4 super block in the usual place, with a matching UUID. The journal superblock will be in the next full block after the superblock.

1024 bytes of padding ext4 Superblock Journal Superblock descriptor_block (data_blocks or revocation_block) [more data or revocations] commmit_block [more transactions...]
      One transaction  

3.6.3. 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. See the jbd2_blocktype table below.
0x8 __be32 h_sequence The transaction ID that goes with this block.

The journal block type can be any one of:

Value Description
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.

3.6.4. 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
      Static information describing the journal.
0x0 journal_header_t (12 bytes) s_header Common header identifying this as a superblock.
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. Contrary to the comments, this field being zero does not imply that the journal is clean!
0x20 __be32 s_errno Error value, as set by jbd2_journal_abort().
      The remaining fields are only valid in a v2 superblock.
0x24 __be32 s_feature_compat; Compatible feature set. See the table jbd2_compat below.
0x28 __be32 s_feature_incompat Incompatible feature set. See the table jbd2_incompat below.
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 __u8 s_checksum_type Checksum algorithm used for the journal. See jbd2_checksum_type for more info.
0x51 __u8[3] s_padding2  
0x54 __u32 s_padding[42]  
0xFC __be32 s_checksum Checksum of the entire superblock, with this field set to zero.
0x100 __u8 s_users[16*48] ids of all file systems sharing the log. e2fsprogs/Linux don't allow shared external journals, but I imagine Lustre (or ocfs2?), which use the jbd2 code, might.

The journal compat features are any combination of the following:

Value Description
0x1 Journal maintains checksums on the data blocks. (JBD2_FEATURE_COMPAT_CHECKSUM)

The journal incompat features are any combination of the following:

Value Description
0x1 Journal has block revocation records. (JBD2_FEATURE_INCOMPAT_REVOKE)
0x2 Journal can deal with 64-bit block numbers. (JBD2_FEATURE_INCOMPAT_64BIT)
0x4 Journal commits asynchronously. (JBD2_FEATURE_INCOMPAT_ASYNC_COMMIT)
0x8 This journal uses v2 of the checksum on-disk format. Each journal metadata block gets its own checksum, and the block tags in the descriptor table contain checksums for each of the data blocks in the journal. (JBD2_FEATURE_INCOMPAT_CSUM_V2)
0x10 This journal uses v3 of the checksum on-disk format. This is the same as v2, but the journal block tag size is fixed regardless of the size of block numbers. (JBD2_FEATURE_INCOMPAT_CSUM_V3)

Journal checksum type codes are one of the following. crc32 or crc32c are the most likely choices.

Value Description
1 CRC32
2 MD5
3 SHA1
4 CRC32C

3.6.5. 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 any of the following formats, depending on which journal feature and block tag flags are set.

If JBD2_FEATURE_INCOMPAT_CSUM_V3 is set, the journal block tag is defined as struct journal_block_tag3_s, which looks like the following. The size is 16 or 32 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. See the table jbd2_tag_flags for more info.
0x8 __be32 t_blocknr_high Upper 32-bits of the location of where the corresponding data block should end up on disk. This is zero if JBD2_FEATURE_INCOMPAT_64BIT is not enabled.
0xC __be32 t_checksum Checksum of the journal UUID, the sequence number, and the data block.
      This field appears to be open coded. It always comes at the end of the tag, after t_checksum. 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 the j_uuid field in struct journal_s, but only tune2fs touches that field.

The journal tag flags are any combination of the following:

Value Description
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.

If JBD2_FEATURE_INCOMPAT_CSUM_V3 is NOT set, the journal block tag is defined as struct journal_block_tag_s, which looks like the following. The size is 8, 12, 24, or 28 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 __be16 t_checksum Checksum of the journal UUID, the sequence number, and the data block. Note that only the lower 16 bits are stored.
0x6 __be16 t_flags Flags that go with the descriptor. See the table jbd2_tag_flags for more info.
      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 the j_uuid field in struct journal_s, but only tune2fs touches that field.

If JBD2_FEATURE_INCOMPAT_CSUM_V2 or JBD2_FEATURE_INCOMPAT_CSUM_V3 are set, the end of the block is a struct jbd2_journal_block_tail, which looks like this:

Offset Type Name Descriptor
0x0 __be32 t_checksum Checksum of the journal UUID + the descriptor block, with this field set to zero.

3.6.6. 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 tag.

3.6.7. Revocation Block

A revocation block is used to prevent replay of a block in an earlier transaction. This is used to mark blocks that were journalled at one time but are no longer journalled. Typically this happens if a metadata block is freed and re-allocated as a file data block; in this case, a journal replay after the file block was written to disk will cause corruption.

NOTE: This mechanism is NOT used to express “this journal block is superseded by this other journal block”, as the author (djwong) mistakenly thought. Any block being added to a transaction will cause the removal of all existing revocation records for that block.

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.

If JBD2_FEATURE_INCOMPAT_CSUM_V2 or JBD2_FEATURE_INCOMPAT_CSUM_V3 are set, the end of the revocation block is a struct jbd2_journal_revoke_tail, which has this format:

Offset Type Name Description
0x0 __be32 r_checksum Checksum of the journal UUID + revocation block

3.6.8. 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. See jbd2_checksum_type for more info.
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. If JBD2_FEATURE_INCOMPAT_CSUM_V2 or JBD2_FEATURE_INCOMPAT_CSUM_V3 are set, the first __be32 is the checksum of the journal UUID and the entire commit block, with this field zeroed. If JBD2_FEATURE_COMPAT_CHECKSUM is set, the first __be32 is the crc32 of all the blocks already written to the transaction.
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.