From: Darrick J. Wong <darrick.wong@xxxxxxxxxx> Import the chapter about bitmaps from the on-disk format wiki page into the kernel documentation. Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx> --- Documentation/filesystems/ext4/ondisk/bitmaps.rst | 28 +++++++++++++++++++++ Documentation/filesystems/ext4/ondisk/globals.rst | 1 + 2 files changed, 29 insertions(+) create mode 100644 Documentation/filesystems/ext4/ondisk/bitmaps.rst diff --git a/Documentation/filesystems/ext4/ondisk/bitmaps.rst b/Documentation/filesystems/ext4/ondisk/bitmaps.rst new file mode 100644 index 000000000000..c7546dbc197a --- /dev/null +++ b/Documentation/filesystems/ext4/ondisk/bitmaps.rst @@ -0,0 +1,28 @@ +.. SPDX-License-Identifier: GPL-2.0 + +Block and inode Bitmaps +----------------------- + +The data block bitmap tracks the usage of data blocks within the block +group. + +The inode bitmap records which entries in the inode table are in use. + +As with most bitmaps, one bit represents the usage status of one data +block or inode table entry. This implies a block group size of 8 \* +number\_of\_bytes\_in\_a\_logical\_block. + +NOTE: If ``BLOCK_UNINIT`` is set for a given block group, various parts +of the kernel and e2fsprogs code pretends that the block bitmap contains +zeros (i.e. all blocks in the group are free). However, it is not +necessarily the case that no blocks are in use -- if ``meta_bg`` is set, +the bitmaps and group descriptor live inside the group. Unfortunately, +ext2fs\_test\_block\_bitmap2() will return '0' for those locations, +which produces confusing debugfs output. + +Inode Table +----------- +Inode tables are statically allocated at mkfs time. Each block group +descriptor points to the start of the table, and the superblock records +the number of inodes per group. See the section on inodes for more +information. diff --git a/Documentation/filesystems/ext4/ondisk/globals.rst b/Documentation/filesystems/ext4/ondisk/globals.rst index c0dea6297e7f..5891ee137749 100644 --- a/Documentation/filesystems/ext4/ondisk/globals.rst +++ b/Documentation/filesystems/ext4/ondisk/globals.rst @@ -8,3 +8,4 @@ have static metadata at fixed locations. .. include:: super.rst .. include:: group_descr.rst +.. include:: bitmaps.rst
