Re: zpool frag

On Sunday, September 21, 2014 06:12:09 PM Steven Hartland wrote:
> ----- Original Message -----
> 
> > From: "Peter Wemm" <peter_at_wemm.org>
> > 
> > On Sunday, September 21, 2014 11:06:10 AM Allan Jude wrote:
> > > On 2014-09-21 04:57, Beeblebrox wrote:
> > > > FRAG means fragmentation, right? Zpool fragmentation? That's news to
> > > > me.
> > > > If
> > > > this is real how do I fix it?
> > > > 
> > > > NAME      SIZE  ALLOC   FREE   FRAG  EXPANDSZ    CAP  DEDUP  HEALTH
> > > > ALTROOT pool1      75.5G  53.7G  21.8G    60%         -    71%  1.00x
> > > > ONLINE  - pool2      48.8G  26.2G  22.6G    68%         -    53% 
> > > > 1.00x
> > > > ONLINE  - pool3       204G   177G  27.0G    53%         -    86% 
> > > > 1.11x
> > > > ONLINE  -
> > > 
> > > It is not something you 'fix', it is just a metric to help you
> > > understand the performance of your pool. The higher the fragmentation,
> > > the longer it might take to allocate new space, and obviously you will
> > > have more random seek time while reading from the pool.
> > > 
> > > As Steven mentions, there is no defragmentation tool for ZFS. You can
> > > zfs send/recv or backup/restore the pool if you have a strong enough
> > > reason to want to get the fragmentation number down.
> > > 
> > > It is a fairly natural side effect of a copy-on-write file system.
> > > 
> > > Note: the % is not the % fragmented, IIRC, it is the percentage of the
> > > free blocks that are less that a specific size. I forget what that size
> > > is.
> > 
> > I fear that the information presented in its current form is going to
> > generate lots of fear and confusion.
> > 
> > The other thing to consider is that this gets much, much worse as the pool
> > fills up.  Even UFS has issues with fragmentation when it fills, but ZFS
> > is far more sensative to it.  In the freebsd.org cluster we have a health
> > check alert at 80% full, but even that's probably on the high side.
> 
> This "should" be less of an issue if you have the spacemap_histogram feature
> enabled on the pool, which IIRC if your seeing FRAG details should be the
> case.

Hopefully so.  The catch though is when its been run without it until recently 
it can be a bit of a surprise.


-- 
Peter Wemm - peter_at_wemm.org; peter_at_FreeBSD.org; peter_at_yahoo-inc.com; KI6FJV
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