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