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 ^ 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_wtime_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[2]	s_pad	Zero padding.
0x27C	__le32	s_reserved[96]	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. Not implemented (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)

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+desc) if the RO_COMPAT_GDT_CSUM feature is set, or crc32c(sb_uuid+group_desc) & 0xFFFF if the RO_COMPAT_METADATA_CSUM feature is set.
			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.