Documentation/filesystems/ext4/group_descr.rst

Source file repositories/reference/linux-study-clean/Documentation/filesystems/ext4/group_descr.rst

File Facts

System
Linux kernel
Corpus path
Documentation/filesystems/ext4/group_descr.rst
Extension
.rst
Size
5442 bytes
Lines
174
Domain
Support Tooling And Documentation
Bucket
Documentation
Inferred role
Support Tooling And Documentation: documentation
Status
atlas-only

Why This File Exists

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

Dependency Surface

Detected Declarations

Annotated Snippet

.. SPDX-License-Identifier: GPL-2.0

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``.

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - 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

Annotation

Implementation Notes