Hi all,
I'm doing some test using a small BTRFS volume on CentOS 6.5 x86_64 (I
know that CentOS 6 use an old kernel and btrfs version and I plan to
replicate the same test on Fedora 20).
From my understanding, disabling CoW and fallocate a file should give a
non-fragmented file. The followind commands show that:
[root@blackhole test]# fallocate test.img -l 1G
[root@blackhole test]# sync
[root@blackhole test]# filefrag -v test.img
Filesystem type is: 9123683e
File size of test.img is 1073741824 (262144 blocks, blocksize 4096)
ext logical physical expected length flags
0 0 269312 262144 eof
test.img: 1 extent found
As you can see, I have a single, continuous block stream.
However, write some 4k blocks into the file leads to fragmentation:
[root@blackhole test]# for id in `seq 1 32`; do dd if=/dev/zero
of=test.img bs=4k count=1 seek=$id conv=notrunc,nocreat
oflag=direct,sync; done
...
[root@blackhole test]# filefrag -v test.img
Filesystem type is: 9123683e
File size of test.img is 1073741824 (262144 blocks, blocksize 4096)
ext logical physical expected length flags
0 0 269312 1
1 1 269313 1
2 2 531456 269314 31
3 33 269345 531487 262111 eof
test.img: 3 extents found
If I don't use fallocate to reserve space, using a simple dd to write 0s
to the target file, the fragmentation do not occour:
[root@blackhole test]# dd if=/dev/zero of=test.img bs=2M count=512
[root@blackhole test]# sync
[root@blackhole test]# for id in `seq 1 32`; do dd if=/dev/zero
of=test.img bs=4k count=1 seek=$id conv=notrunc,nocreat
oflag=direct,sync; done
...
[root@blackhole test]# filefrag -v test.img
Filesystem type is: 9123683e
File size of test.img is 1073741824 (262144 blocks, blocksize 4096)
ext logical physical expected length flags
0 0 269312 262144 eof
test.img: 1 extent found
So, my question is: why writing to a fallocated file produce
fragmentation, even with CoW disabled?
Regards.
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: [email protected] - [email protected]
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