30 TB RAID6 + XFS slow write performance

[Michael Monnerie]

On Donnerstag, 21. Juli 2011 Dave Chinner wrote:
> No, they'll get sunit aligned but default, which would be on 64k
> boundaries.

OK, so only when <quote Dave> "swalloc mount option set and the 
allocation is for more than a swidth of space it will align to swidth 
rather than sunit" </quote Dave>.

So even when I specify swalloc but a file is generated with only 4KB, it 
will very probably be sunit aligned on disk.
 
> > That way, all stripes of a 1GB partition would be full when 
> > there are roughly 1170 files (1170*896KiB ~ 1GB). What would happen
> > when  I create other files - is XFS "full" then, or would it start
> > using sub- stripes? If sub-stripes, would they start at su
> > (=64KiB) distances, or at single block (e.g. 4KiB) distances?
> 
> It starts packing files tightly into remaining free space when no
> free aligned extents are availble for allocation in the AG.

That means for above example, that 16384 x 2KiB files could be created, 
and each be sunit aligned on disk. Then all sunit start blocks are full, 
so additional files will be sub-sunit "packed", is it this?

That would mean fragmentation is likely to occur from that moment, if 
there are files that grow. And files >64KiB are immediately fragmented 
then. At this time, there are only 16384 * 2KiB = 32MiB used, which is 
3,125% of the disk. I can't believe my numbers, are they true?
OK, this is a worst case scenario, and as you've said before, any 
filesystem can be considered full at 85% fill grade. But it's incredible 
how quickly you could fuck up a filesystem when using su/sw and writing 
small files.

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

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