On Mon, Mar 05, 2012 at 12:32:45PM +0100, Jacek Luczak wrote:

2012/3/4 Jacek Luczak [off-list ref]:

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2012/3/3 Jacek Luczak [off-list ref]:

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2012/3/2 Chris Mason [off-list ref]:

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On Fri, Mar 02, 2012 at 03:16:12PM +0100, Jacek Luczak wrote:

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2012/3/2 Chris Mason [off-list ref]:

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On Fri, Mar 02, 2012 at 11:05:56AM +0100, Jacek Luczak wrote:

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I've took both on tests. The subject is acp and spd_readdir used with
tar, all on ext4:
1) acp: http://91.234.146.107/~difrost/seekwatcher/acp_ext4.png
2) spd_readdir: http://91.234.146.107/~difrost/seekwatcher/tar_ext4_readir.png
3) both: http://91.234.146.107/~difrost/seekwatcher/acp_vs_spd_ext4.png

The acp looks much better than spd_readdir but directory copy with
spd_readdir decreased to 52m 39sec (30 min less).

Do you have stats on how big these files are, and how fragmented they
are?  For acp and spd to give us this, I think something has gone wrong
at writeback time (creating individual fragmented files).

How big? Which files?

All the files you're reading ;)

filefrag will tell you how many extents each file has, any file with
more than one extent is interesting.  (The ext4 crowd may have better
suggestions on measuring fragmentation).

Since you mention this is a compile farm, I'm guessing there are a bunch
of .o files created by parallel builds.  There are a lot of chances for
delalloc and the kernel writeback code to do the wrong thing here.

[Most of files are B and K size]

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All files scanned: 1978149
Files fragmented: 313 (0.015%) where 11 have 3+ extents
Total size of fragmented files: 7GB (~13% of dir size)

Ok, so I don't have a lot of great new ideas.  My guess is that inode
order and disk order for the blocks aren't matching up.  You can confirm
this with:

acp -b some_dir

You can also try telling acp to make a bigger read ahead window:

acp -s 4096 -r 128 some_dir

You can tell acp to scan all the files in the directory tree first
(warning, this might use a good chunk of ram)

acp -w some_dir

and you can combine all of these together  None of the above
will actually help in your workload, but it'll help narrow down what is
actually seeky on disk.

-chris