On 09/25/2010 10:26 AM, Eric Sandeen wrote:
Andreas Dilger wrote:
On 2010-09-24, at 18:05, Taras Glek wrote:
I noticed that several random IO-heavy Firefox files got fragmented
easily. Our cache suffers most. The cache works by creating a flat
file and storing fixed-size entries in it. I though if I
fallocate() the file first, then all of the writes within the
allocated area would not cause additional fragmentation.
This doesn't seem to completely cure fragmentation with ext4 in
2.6.33. If I allocate a 4mb file, it gets more and more fragmented
over time. fallocate() does reduce fragmentation, but not as much
as I expected.
Have you checked filefrag immediately after fallocating the file? Is
it OK?
It may be that the issue is that an fallocate()'d file is using
"unwritten extents" and converting these extents to "normal" extents
may cause apparent fragmentation. However, depending on which
version of e2fsprogs/filefrag you are using, it may well be that
these extents only appear to be fragmented due to the different
extent types.
Agreed, please include filefrag (-v) output right after it's fallocated,
and also when you see this fragmentation, and then we'll have a better idea
about what you're seeing. And, the newer the filefrag the better. :)
Thanks for clarification. Turns out ext4 is performing as expected,
nevermind my previous message.
I was confused by discrepancy in number of extents reported by filefrag
1.41.10 with/without -v flag.
filefrag _CACHE_003_
_CACHE_003_: 17 extents found
filefrag -v _CACHE_003_
Filesystem type is: ef53
File size of _CACHE_003_ is 4194304 (1024 blocks, blocksize 4096)
ext logical physical expected length flags
0 0 232448 128
1 128 232576 1 unwritten
2 129 232577 95
3 224 232672 1 unwritten
4 225 232673 31
5 256 232704 1 unwritten
6 257 232705 63
7 320 232768 1 unwritten
8 321 232769 95
9 416 232864 1 unwritten
10 417 232865 255
11 672 233120 1 unwritten
12 673 233121 191
13 864 233312 1 unwritten
14 865 233313 127
15 992 233440 3
16 995 233443 29 unwritten,eof
_CACHE_003_: 1 extent found
Thanks,
Taras
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