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Re: [OT] block allocation algorithm [Was] Re: heavily fragmented file system.. How to defrag it on-line??



Isaac Claymore wrote:
On Tue, Mar 09, 2004 at 01:02:48AM -0700, Andreas Dilger wrote:

On Mar 09, 2004 15:46 +0800, Isaac Claymore wrote:

I've got a workload that several clients tend to write to separate files
under a same dir simultaneously, resulting in heavily fragmented files.
And, even worse, those files are rarely read simultaneously, thus read
performance degrades quite alot.

I'm wondering whether there's any feature that helps alleviating
fragmentation in such workloads. Does writing to different dirs(of a same
filesystem) help?

Very much yes. Files allocated from different directories will get blocks from different parts of the filesystem (if available), so they should be less fragmented. In 2.6 there is a heuristic that files opened by different processes allocate from different parts of a group, even within the same directory, but that only really helps if the files themselves aren't too large (i.e. under 8MB or so).



Thanks, I did some test on this last weekend, and here're the results in
case someone is interested:


Yes, very interesting. It would be nice if JFS was in this comparison.


Also, the next step is running each filesystem under your workload (Like test 5) and compare the fragmentation and performance over a longer period of time.

Read below for some more comments...


Test environment:


kernel: 2.6.3 with latest reiser4 patches applied.
OS: Debian testing/unstable
HW: Intel(R) Pentium(R) 4 CPU 1.80GHz, 256M RAM

For each FS configuration, my test went on as: dumping 3 files of 1G
each simultaneously, and measure the fragmentation with 'filefrag'.

Each test iteration was done on a freshly formatted filesystem.

Here goes the figures & my evaluations:

1. reiser3, 3 files under a same dir:

sandbox:/mnt/foo [1016]# dd if=/dev/zero of=f0 bs=16M count=64&;dd if=/dev/zero of=f1 bs=16M count=64&;dd if=/dev/zero of=f2 bs=16M count=64&;wait

sandbox:/mnt/foo [1018]# filefrag f0 f1 f2
f0: 470 extents found
f1: 461 extents found
f2: 470 extents found


My Evaluation: badly fragmented!



2. reiser3, 3 files under 3 different dirs:


sandbox:/mnt [1028]# dd if=/dev/zero of=dir0/foo bs=16M count=64&;dd if=/dev/zero of=dir1/foo bs=16M count=64&;dd if=/dev/zero of=dir2/foo bs=16M count=64&;wait

sandbox:/mnt [1029]# filefrag dir0/foo dir1/foo dir2/foo
dir0/foo: 448 extents found
dir1/foo: 462 extents found
dir2/foo: 443 extents found


My Evaluation: still bad, spreading the files under different dirs did no visible good.

How about with 10 dirs?





3. ext3, 3 files under a same dir:


sandbox:/mnt/foo [1041]# dd if=/dev/zero of=f0 bs=16M count=64&;dd if=/dev/zero of=f1 bs=16M count=64&;dd if=/dev/zero of=f2 bs=16M count=64&;wait

sandbox:/mnt/foo [1044]# filefrag f0 f1 f2
f0: 202 extents found, perfection would be 9 extents
f1: 207 extents found, perfection would be 9 extents
f2: 208 extents found, perfection would be 9 extents


My Evaluation: much better than reiser3, yet far from perfection.




4. ext3, 3 files under 3 different dirs:

sandbox:/mnt [1054]# dd if=/dev/zero of=dir0/foo bs=16M count=64&;dd
if=/dev/zero of=dir1/foo bs=16M count=64&;dd if=/dev/zero of=dir2/foo bs=16M count=64&;wait


sandbox:/mnt [1056]# sandbox:/mnt [1056]# filefrag dir0/foo dir1/foo dir2/foo
dir0/foo: 91 extents found, perfection would be 9 extents
dir1/foo: 9 extents found
dir2/foo: 95 extents found, perfection would be 9 extents


My Evaluation: spreading the files under different dirs DID help quite alot! but can we get even better result by spread the files more sparsely? (see next test)


5. still ext3, mkdir 10 dirs first, then dumping the files under the 1st, 5th, and 9th dirs:

sandbox:/mnt [1085]# dd if=/dev/zero of=dir0/foo bs=16M count=64&;dd if=/dev/zero of=dir4/foo bs=16M count=64&;dd if=/dev/zero of=dir9/foo bs=16M count=64&;wait

sandbox:/mnt [1086]# filefrag dir{0,4,9}/foo
dir0/foo: 11 extents found, perfection would be 9 extents
dir4/foo: 11 extents found, perfection would be 9 extents
dir9/foo: 10 extents found, perfection would be 9 extents


My Evaluation: almost perfect!




6. XFS, 3 files under a same dir:

sandbox:/mnt/foo [1112]# dd if=/dev/zero of=f0 bs=16M count=64&;dd if=/dev/zero of=f1 bs=16M count=64&;dd if=/dev/zero of=f2 bs=16M count=64&;wait

sandbox:/mnt/foo [1114]# filefrag f0 f1 f2
f0: 25 extents found
f1: 11 extents found
f2: 20 extents found


My Evaluation: this'd be the BEST result I got, when dumping into a same dir.





7. XFS, dumping into 3 dirs among ten, similar to test 5:


sandbox:/mnt [1127]# dd if=/dev/zero of=dir0/foo bs=16M count=64&;dd if=/dev/zero of=dir4/foo bs=16M count=64&;dd if=/dev/zero of=dir9/foo bs=16M count=64&;wait

sandbox:/mnt [1128]# filefrag dir0/foo dir4/foo dir9/foo
dir0/foo: 1 extent found
dir4/foo: 1 extent found
dir9/foo: 1 extent found


My Evaluation: impressed! cant be any better now.



How about with 3 dirs?



8. Reiser4, 1 dir


sandbox:/mnt/foo [1155]# dd if=/dev/zero of=f0 bs=16M count=64&;dd if=/dev/zero of=f1 bs=16M count=64&;dd if=/dev/zero of=f2 bs=16M count=64&;wait

sandbox:/mnt/foo [1156]# filefrag f0 f1 f2
f0: 45 extents found
f1: 6011 extents found
f2: 45 extents found


My Evaluation: far better than it's brother reiser3. the 6011 extents of f1 was weird, i'd have done more iterations to get an average, just blame lazy me ;)



9. Reiser4, 3 dirs among 10:

sandbox:/mnt [1165]# dd if=/dev/zero of=dir0/foo bs=16M count=64&;dd if=/dev/zero of=dir4/foo bs=16M count=64&;dd if=/dev/zero of=dir9/foo bs=16M count=64&;wait

sandbox:/mnt [1167]# filefrag dir{0,4,9}/foo
dir0/foo: 42 extents found
dir4/foo: 50 extents found
dir9/foo: 46 extents found


My Evaluation: nice figures, really. and unlike its elder brother, using more dirs DID help.

How about with 3 dirs?


Mike



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