zpool scrub speed

Jean-Michel Bruenn jean.bruenn at ip-minds.de
Sat May 7 18:59:29 EDT 2011

Hello (again),

if devsk got the same issue as i, this is easily reproducable. Launch
X11, start your ZFS stuff, mount it, and now run bonnie++ onto it with
some highers values.

i.e. bonnie++ -d /mountedzfsdir -n 512

As soon as it's at "reading with getc" the box starts to have high wait
shown in top while swap is not in use, ram is not in use, cpu shows 99%
idle. Used Hardware: quadcore 3,1 ghz, 4gb ram, 8gb swap, 4x500gb sata
with 32mb cache each.

Tried with raid5, raid10, zfs - everytime same result.

Tried with different hardware: dualcore 2ghz, 3gb ram, same amount of
discs, same result.

As soon as it's at writing (deleting directories and such things) i
can't move the mouse properly everything is hanging, and at some points
i have to wait like a minute that i can start to continue doing
anything (and again.. no swap in use, no ram in use, it's all free, not
even cached). 

So... this should be easily reproducable. Switching from PReempt to
non-preempt (which is now the default) helps a bit. switching from cfq
to deadline helps. according to a kernel dev this is a known bug and
the proper solution is using cgroups (as i've explained in the other
mail). This is only reproducable on HIGH I/O - Thats why bonnie or
"scrub" is good for reproducing it.

However, this was the case for "my" problems - switching to a box
without x11 solved it, zfs is totally stable, nothing is hanging. Would
be cool if you can verify, that you got the "same" issue (because then,
its not a zfs-issue - it's a kernel bug)


On Sat, 7 May 2011 15:23:33 -0700 (PDT) devsk
<devsku at gmail.com> wrote:

> Its not reproducible so far (as is typical with intermittent issues)
> and I have run scrub before multiple times.
> I reduced the ARC to 1.5GB but not to 128M. Previously, very low arc
> WAS one of the reasons for lock ups. So, I have never tried that
> again. And Brian had put in fixes for actually respecting the ARC
> limit in rc4, so I kept it at 2GB because I have 12GB in this box.
> -devsk
> On May 7, 3:15 pm, "Fajar A. Nugraha" <l... at fajar.net> wrote:
> > On Sun, May 8, 2011 at 5:09 AM, devsk <dev... at gmail.com> wrote:
> > > So, it looks like scrub speed is also
> > > determined by how much memory is fragmented i.e. on a box which is
> > > freshly booted, things are generally faster and as the time goes on
> > > and zfs allocates and arc_reclaim eventually reclaims memory
> > > repeatedly, things begin to slow down.
> >
> > Is the lockup reproducable? Can your repeat the process, but this time
> > setting zfs_arc_max and zfs_arc_min to the same value (something like
> > 128M or whatever sane value depends on your memory size) thru module
> > parameter?
> >
> > --
> > Fajar

Jean-Michel Bruenn <jean.bruenn at ip-minds.de>

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