ext4: import group descriptors chapter from wiki page
Import the chapter about group descriptors from the on-disk format wiki page into the kernel documentation. Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Родитель
3db42a2139
Коммит
3c6ba09dd3
|
@ -7,3 +7,4 @@ The filesystem is sharded into a number of block groups, each of which
|
|||
have static metadata at fixed locations.
|
||||
|
||||
.. include:: super.rst
|
||||
.. include:: group_descr.rst
|
||||
|
|
|
@ -0,0 +1,170 @@
|
|||
.. 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: 1 1 1 77
|
||||
: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
|
||||
- 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.
|
||||
|
||||
.. _bgflags:
|
||||
|
||||
Block group flags can be any combination of the following:
|
||||
|
||||
.. list-table::
|
||||
:widths: 1 79
|
||||
:header-rows: 1
|
||||
|
||||
* - 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).
|
Загрузка…
Ссылка в новой задаче