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

[Mike Fedyk <mfedyk@matchmail.com>]

Hi guys,

Reiser3 has worse fragmentation than ext3 in this test case.  Reiser4 
was better, but it had a bad worst case and still wasn't as good as xfs.

I'm especially interested in getting reiser3 to produce fewer fragments 
in this test.

Comments?

-------- Original Message --------
Subject: Re: [OT] block allocation algorithm [Was] Re: heavily 
fragmented	file system.. How to defrag it on-line??
Date: Mon, 15 Mar 2004 11:13:32 +0800
From: Isaac Claymore <clay@exavio.com.cn>
To: Andreas Dilger <adilger@clusterfs.com>
CC: ext3-users@redhat.com
References: <41089CB27BD8D24E8385C8003EDAF7AB06D296@karl.alexa.com> 
<20040304021707.GA13386@thunk.org> 
<20040309074643.GA1557@exavio.com.cn> 
<20040309080248.GB1144@schnapps.adilger.int>

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:


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.



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.


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.


> Cheers, Andreas
> --
> Andreas Dilger
> http://sourceforge.net/projects/ext2resize/
> http://www-mddsp.enel.ucalgary.ca/People/adilger/
> 
> 
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> Ext3-users mailing list
> Ext3-users@redhat.com
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-- 

Regards, Isaac
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