Super Block - Kernel Docs

https://docs.kernel.org/next/filesystems/ext4/super.html

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

Sise

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

Sise 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

Sise 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

Sise 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

__u8

s_encryption_level

Versioning level for encryption.

0x177

__u8

s_reserved_pad

Padding to next 32bits.

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 field.

0x278

__u8

s_first_error_time_hi

Upper 8 bits of the s_first_error_time field.

0x279

__u8

s_last_error_time_hi

Upper 8 bits of the s_last_error_time field.

0x27A

__u8

s_first_error_errcode

0x27B

__u8

s_last_error_errcode

0x27C

__le16

s_encoding

Filename charset encoding.

0x27E

__le16

s_encoding_flags

Filename charset encoding flags.

0x280

__le32

s_orphan_file_inum

Orphan file inode number.

0x284

__le32

s_reserved[94]

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

Continue

Remount read-only

Panic

The filesystem creator is one of the following:

Value

Description

Linux

Hurd

Masix

FreeBSD

Lites

The superblock revision is one of the following:

Value

Description

Original format

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 uninitialised 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).

0x400

Fast commits supported. Although fast commits blocks are backward incompatible, fast commit blocks are not always present in the journal. If fast commit blocks are present in the journal, JBD2 incompat feature (JBD2_FEATURE_INCOMPAT_FAST_COMMIT) gets set (COMPAT_FAST_COMMIT).

0x1000

Orphan file allocated. This is the special file for more efficient tracking of unlinked but still open inodes. When there may be any entries in the file, we additionally set proper rocompat feature (RO_COMPAT_ORPHAN_PRESENT).

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 can be present. (INCOMPAT_ENCRYPT).

0x20000

Directories can be marked case-insensitive. (INCOMPAT_CASEFOLD).

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 uninitialised 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)

0x10000

Indicates orphan file may have valid orphan entries and thus we need to clean them up when mounting the filesystem (RO_COMPAT_ORPHAN_PRESENT).

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 committed 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

Invalid algorithm (ENCRYPTION_MODE_INVALID).

256-bit AES in XTS mode (ENCRYPTION_MODE_AES_256_XTS).

256-bit AES in GCM mode (ENCRYPTION_MODE_AES_256_GCM).

256-bit AES in CBC mode (ENCRYPTION_MODE_AES_256_CBC).

Total size of the superblock is 1024 bytes.