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Diffstat (limited to 'Documentation/filesystems/ext4/blockgroup.rst')
| -rw-r--r-- | Documentation/filesystems/ext4/blockgroup.rst | 17 |
1 files changed, 10 insertions, 7 deletions
diff --git a/Documentation/filesystems/ext4/blockgroup.rst b/Documentation/filesystems/ext4/blockgroup.rst index 46d78f860623..7cbf0b2b778e 100644 --- a/Documentation/filesystems/ext4/blockgroup.rst +++ b/Documentation/filesystems/ext4/blockgroup.rst @@ -1,7 +1,10 @@ .. SPDX-License-Identifier: GPL-2.0 +Block Groups +------------ + Layout ------- +~~~~~~ The layout of a standard block group is approximately as follows (each of these fields is discussed in a separate section below): @@ -60,7 +63,7 @@ groups (flex_bg). Leftover space is used for file data blocks, indirect block maps, extent tree blocks, and extended attributes. Flexible Block Groups ---------------------- +~~~~~~~~~~~~~~~~~~~~~ Starting in ext4, there is a new feature called flexible block groups (flex_bg). In a flex_bg, several block groups are tied together as one @@ -78,7 +81,7 @@ if flex_bg is enabled. The number of block groups that make up a flex_bg is given by 2 ^ ``sb.s_log_groups_per_flex``. Meta Block Groups ------------------ +~~~~~~~~~~~~~~~~~ Without the option META_BG, for safety concerns, all block group descriptors copies are kept in the first block group. Given the default @@ -105,9 +108,9 @@ descriptors. Instead, the superblock and a single block group descriptor block is placed at the beginning of the first, second, and last block groups in a meta-block group. A meta-block group is a collection of block groups which can be described by a single block group descriptor -block. Since the size of the block group descriptor structure is 32 -bytes, a meta-block group contains 32 block groups for filesystems with -a 1KB block size, and 128 block groups for filesystems with a 4KB +block. Since the size of the block group descriptor structure is 64 +bytes, a meta-block group contains 16 block groups for filesystems with +a 1KB block size, and 64 block groups for filesystems with a 4KB blocksize. Filesystems can either be created using this new block group descriptor layout, or existing filesystems can be resized on-line, and the field s_first_meta_bg in the superblock will indicate the first @@ -117,7 +120,7 @@ Please see an important note about ``BLOCK_UNINIT`` in the section about block and inode bitmaps. Lazy Block Group Initialization -------------------------------- +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A new feature for ext4 are three block group descriptor flags that enable mkfs to skip initializing other parts of the block group |