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fs-ext4-ondisk-layout (other features (Block and Inode Allocation Policy…
fs-ext4-ondisk-layout
other features
Special inodes
0 ~ 11
Block and Inode Allocation Policy
multi-block allocator
8k
delayed allocation
keep a file's data blocks in the same
block group as its inode
all the inodes in a directory are placed in the same block group as the
directory, when feasible
disk volume is cut up into 128MB block groups
Bigalloc
Inline Data
Lazy Block Group Initialization
reduce mkfs time
the inode bitmap and inode tables in a group are uninitialized
even the group descriptors can be
uninitialized
need to enable group descriptor checksum
Meta Block Groups
META_BG
partitioned into many metablock groups
a cluster of block groups whose group
descriptor structures can be stored in a single disk block
the group descriptor into the first group of each metablock group
non META_BG
all block group descriptors copies are kept
in the first BG
size of the block group descriptor
structure is 32 bytes
block group
standard block group
group0 padding(1024 bytes)
ext4 superblock (1 block)
group descriptors
reserved GDT blocks
Data Block Bitmap( 1 block)
inode Bitmap (1 block)
inode Table
data blocks
blocks
block size
often 4K
sb.s_blocks_per_group
often 32768 blocks
Flexible Block Groups
one logical block group
several block group tied together
the bitmap spaces and the inode table
space in the first block group
Checksums
metadata checksums
ext4 and jbd2 data
structures
the Super Block
sparse_super