Re: Overwrite faster than fallocate

Theodore Ts'o
2022-06-23T19:47:55Z

On Thu, Jun 23, 2022 at 02:28:47PM -0400, Santosh S wrote:
>
> What kind of write will stop an uninitialized extent from splitting?
> For example, I want to create a file, fallocate 512MB, and zero-fill
> it. But I want the file system to only create 4 extents so they all
> reside in the inode itself, and each extent represents the entire
> 128MB (so no splitting).

If you write into an unitialized extent, it *has* to be split, since
we have to record what has been initialized, and what has not. So for
example:

root@kvm-xfstests:/vdc# fallocate -l 1M test-file
root@kvm-xfstests:/vdc# filefrag -vs test-file
Filesystem type is: ef53
File size of test-file is 1048576 (256 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 255: 68864.. 69119: 256: last,unwritten,eof
test-file: 1 extent found
root@kvm-xfstests:/vdc# dd if=/dev/zero of=test-file bs=1k conv=notrunc bs=4k count=1 seek=10
1+0 records in
1+0 records out
4096 bytes (4.1 kB, 4.0 KiB) copied, 0.000252186 s, 16.2 MB/s
root@kvm-xfstests:/vdc# filefrag -vs test-file
Filesystem type is: ef53
File size of test-file is 1048576 (256 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 9: 68864.. 68873: 10: unwritten
1: 10.. 10: 68874.. 68874: 1:
2: 11.. 255: 68875.. 69119: 245: last,unwritten,eof
test-file: 1 extent found

However, if you write to an adjacent block, the extent will get split
--- and then we will merge it to the initialized block. So for
example, if we write to block 9:

root@kvm-xfstests:/vdc# dd if=/dev/zero of=test-file bs=1k conv=notrunc bs=4k count=1 seek=9
1+0 records in
1+0 records out
4096 bytes (4.1 kB, 4.0 KiB) copied, 0.000205357 s, 19.9 MB/s
root@kvm-xfstests:/vdc# filefrag -vs test-file
Filesystem type is: ef53
File size of test-file is 1048576 (256 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 8: 68864.. 68872: 9: unwritten
1: 9.. 10: 68873.. 68874: 2:
2: 11.. 255: 68875.. 69119: 245: last,unwritten,eof
test-file: 1 extent found

So if you eventually write all of the blocks, because of the split and
the merging behavior, eventually the extent tree will be put into an efficient state:

root@kvm-xfstests:/vdc# dd if=/dev/zero of=test-file bs=1k conv=notrunc bs=4k count=9 seek=0
...
root@kvm-xfstests:/vdc# filefrag -vs test-file
Filesystem type is: ef53
File size of test-file is 1048576 (256 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 10: 68864.. 68874: 11:
1: 11.. 255: 68875.. 69119: 245: last,unwritten,eof
test-file: 1 extent found
root@kvm-xfstests:/vdc# dd if=/dev/zero of=test-file bs=1k conv=notrunc bs=4k count=240 seek=11
...
root@kvm-xfstests:/vdc# filefrag -vs test-file
Filesystem type is: ef53
File size of test-file is 1048576 (256 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 250: 68864.. 69114: 251:
1: 251.. 255: 69115.. 69119: 5: last,unwritten,eof
test-file: 1 extent found
root@kvm-xfstests:/vdc# dd if=/dev/zero of=test-file bs=1k conv=notrunc bs=4k count=5 seek=251
...
root@kvm-xfstests:/vdc# filefrag -vs test-file
Filesystem type is: ef53
File size of test-file is 1048576 (256 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 255: 68864.. 69119: 256: last,eof
test-file: 1 extent found
root@kvm-xfstests:/vdc#

Bottom-line, there isn't just splitting, but there is also merging
going on. So it's not really something that you need to worry about.

Cheers,

- Ted