| .. SPDX-License-Identifier: GPL-2.0 |
| |
| 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``: |
| |
| .. list-table:: |
| :widths: 8 8 24 40 |
| :header-rows: 1 |
| |
| * - 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 <quota>`__ file. |
| * - 0x244 |
| - \_\_le32 |
| - s\_grp\_quota\_inum |
| - Inode number of group `quota <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\_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. |
| |
| .. _super_state: |
| |
| The superblock state is some combination of the following: |
| |
| .. list-table:: |
| :widths: 8 72 |
| :header-rows: 1 |
| |
| * - Value |
| - Description |
| * - 0x0001 |
| - Cleanly umounted |
| * - 0x0002 |
| - Errors detected |
| * - 0x0004 |
| - Orphans being recovered |
| |
| .. _super_errors: |
| |
| The superblock error policy is one of the following: |
| |
| .. list-table:: |
| :widths: 8 72 |
| :header-rows: 1 |
| |
| * - Value |
| - Description |
| * - 1 |
| - Continue |
| * - 2 |
| - Remount read-only |
| * - 3 |
| - Panic |
| |
| .. _super_creator: |
| |
| The filesystem creator is one of the following: |
| |
| .. list-table:: |
| :widths: 8 72 |
| :header-rows: 1 |
| |
| * - Value |
| - Description |
| * - 0 |
| - Linux |
| * - 1 |
| - Hurd |
| * - 2 |
| - Masix |
| * - 3 |
| - FreeBSD |
| * - 4 |
| - Lites |
| |
| .. _super_revision: |
| |
| The superblock revision is one of the following: |
| |
| .. list-table:: |
| :widths: 8 72 |
| :header-rows: 1 |
| |
| * - 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. |
| |
| .. _super_compat: |
| |
| The superblock compatible features field is a combination of any of the |
| following: |
| |
| .. list-table:: |
| :widths: 16 64 |
| :header-rows: 1 |
| |
| * - 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). |
| * - 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). |
| |
| .. _super_incompat: |
| |
| The superblock incompatible features field is a combination of any of the |
| following: |
| |
| .. list-table:: |
| :widths: 16 64 |
| :header-rows: 1 |
| |
| * - 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). |
| |
| .. _super_rocompat: |
| |
| The superblock read-only compatible features field is a combination of any of |
| the following: |
| |
| .. list-table:: |
| :widths: 16 64 |
| :header-rows: 1 |
| |
| * - 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 <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). |
| |
| .. _super_def_hash: |
| |
| The ``s_def_hash_version`` field is one of the following: |
| |
| .. list-table:: |
| :widths: 8 72 |
| :header-rows: 1 |
| |
| * - Value |
| - Description |
| * - 0x0 |
| - Legacy. |
| * - 0x1 |
| - Half MD4. |
| * - 0x2 |
| - Tea. |
| * - 0x3 |
| - Legacy, unsigned. |
| * - 0x4 |
| - Half MD4, unsigned. |
| * - 0x5 |
| - Tea, unsigned. |
| |
| .. _super_mountopts: |
| |
| The ``s_default_mount_opts`` field is any combination of the following: |
| |
| .. list-table:: |
| :widths: 8 72 |
| :header-rows: 1 |
| |
| * - 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) |
| |
| .. _super_flags: |
| |
| The ``s_flags`` field is any combination of the following: |
| |
| .. list-table:: |
| :widths: 8 72 |
| :header-rows: 1 |
| |
| * - Value |
| - Description |
| * - 0x0001 |
| - Signed directory hash in use. |
| * - 0x0002 |
| - Unsigned directory hash in use. |
| * - 0x0004 |
| - To test development code. |
| |
| .. _super_encrypt: |
| |
| The ``s_encrypt_algos`` list can contain any of the following: |
| |
| .. list-table:: |
| :widths: 8 72 |
| :header-rows: 1 |
| |
| * - 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. |
