Re: Overwrite faster than fallocate

Santosh S
2022-06-23T21:55:43Z

On Thu, Jun 23, 2022 at 3:43 PM Theodore Ts'o <tytso@mit.edu> wrote:
>
> 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

Nice! Thank you.