Re: suddenly slow writes on XFS Filesystem

Stefan Priebe
2012-05-07T20:05:18Z

Am 07.05.2012 18:36, schrieb Stan Hoeppner:
> This shows what I originally suspected. Notice how top heavy this
> histogram is. Over half of your free space sits on little islands of
> 8MB or less. 17% is in islands of 60KB or less. This is heavily
> fragmented free space. Contrast this with an XFS from the opposite end
> of the aging spectrum that is only 1/3rd full and has seen very few
> deletes as it has aged:
...
>
> Notice how it is very bottom heavy, and that 85% of the free space is in
> large islands of 16GB to 24GB.
This totally makes sense too me. Thanks for this explanation.

> Stefan, at this point in your filesystem's aging process, it may not
> matter how much space you keep freeing up, as your deletion of small
> files simply adds more heavily fragmented free space to the pool. It's
> the nature of your workload causing this.
This makes sense - do you have any idea or solution for this? Are
Filesystems, Block layers or something else which suits this problem /
situation?

> What I would suggest is doing an xfsdump to a filesystem on another LUN
> or machine, expand the size of this LUN by 50% or more (I gather this is
> an external RAID), format it appropriately, then xfsrestore. This will
> eliminate your current free space fragmentation, and the 50% size
> increase will delay the next occurrence of this problem. If you can't
> expand the LUN, simply do the xfsdump/format/xfsrestore, which will give
> you contiguous free space.
But this will only help for a few month or perhaps a year.

Greets,
Stefan