[RFC, PATCH 0/2] xfs: dynamic speculative preallocation for delalloc

[Michael Monnerie]

On Donnerstag, 14. Oktober 2010 Ivan.Novick at emc.com wrote:
> I have seen ~50% performance improvement in read rate when changing
> from small extents to large extents with XFS. Essentially going from
> not using allocsize to setting 1gb allocsize.  Also GB/s class
> filesystem.

In case of a database, I'd say 1GB allocsize seems fine. But when you're 
at /var/log/, it could be a penalty, as when you have 50 open log files 
which get permanently appended, you have 50GB preallocated which almost 
surely won't be needed before the log is rotated anyway.

The question is: what is a good value for preallocation?

I'd guess for a database 1GB seems reasonable, for /var/log one full 
stripe (nr-data-disks * stripe size), for webserver/ftp data too, as 
well as a fileserver (storing office documents) and mailserver (storing 
each mail as a separate file or into one flat file).
Or, when you have an app that writes files of specific size, use the 
"normal maximum" file size. We have a special server storing files of 
normally 50M-1G size, there I set allocsize=1024k, so that files will be 
in one junk on disk. When sometimes a file is bigger, then it will be 
fragmented but at reasonable sizes so it won't matter too much.

Is that reasonable?

Are there guides to help with that? I'd like to write a FAQ entry if 
there is (can ever be?) a consensus of what would be a good value.

-- 
mit freundlichen Grüssen,
Michael Monnerie, Ing. BSc

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