[zfs-discuss] Slow read performance

Gordan Bobic gordan.bobic at gmail.com
Mon Apr 2 10:24:20 EDT 2018


Perhaps, but just because dd seems to benefit from bigger vdevs doesn't
mean that the real world workload won't be slower with bigger vdevs.

So what you are measuring is an indication of neither absolute nor relative
performance. It's not even apples vs. oranges, it's apples vs. supertankers
comparison.


On Mon, 2 Apr 2018, 15:03 Alex Vodeyko, <alex.vodeyko at gmail.com> wrote:

> Yes sure, "dd" is just a quick check.
> Because if I get with "dd" 1+ GB/s reads from 10 drives and < 0.5 GB/s
> from 30 (and even 60) drives - then something is fundamentally wrong.
>
> Thanks,
> Alex
>
>
> 2018-04-02 15:37 GMT+03:00 Gordan Bobic <gordan.bobic at gmail.com>:
> > You do realize that measuring sequential I/O performance with dd will
> > produce readings that are in no meaningful way correlatable to any
> > multi-user workload you are likely to throw at it, especially something
> as
> > metadata intensive (small random I/O) as Lustre, right?
> >
> > On Mon, 2 Apr 2018, 12:05 Alex Vodeyko via zfs-discuss,
> > <zfs-discuss at list.zfsonlinux.org> wrote:
> >>
> >> Yes, I'm now running zpool with two 12+3 raidz3 VDEVs. Read
> >> performance is definitely better than zpool with three 8+2 raidz2
> >> VDEVs (And still major read performance improvement comes from ashift
> >> = 9):
> >> - "dd" ashift = 9:  write = 2.3 GB/s, read = 1.8 GB/s
> >> - "dd" ashift = 12: write = 2.8 GB/s, read = 1.2 GB/s
> >> .
> >> I decided to have 2x 30-drive zpools (because it seems not too much
> >> performance difference between 30 and 60 drives in all my tested zpool
> >> configs). I will use Lustre filesystem, so decided to have 2x Lustre
> >> OSS servers with 30 drives each.
> >> Still choosing the correct zpool layout - from the benchmarks it seems
> >> raidz3 performance is only 100 MB/s worse.
> >> But it would be great to get the advice on the best zpool layout
> >> (raidz2 vs raidz3, 12+2, 13+2, 12+3) for 30 drives.
> >> I'm now for two 12+3 raidz3 VDEVs with ashift=9.
> >>
> >> Thanks,
> >> Alex
> >>
> >> 2018-04-01 23:20 GMT+03:00 Andreas Dilger <adilger at dilger.ca>:
> >> > On Mar 31, 2018, at 11:23 PM, Alex Vodeyko <alex.vodeyko at gmail.com>
> >> > wrote:
> >> >> To remind - all of the above came from zpool of six 8+2 raidz2 (60
> >> >> drives total)
> >> >>
> >> >> For comparison I've created one zpool with single 8+2 raidz2 (10
> >> >> drives) and rerun tests on it, so:
> >> >
> >> > Have you tried a different geometry, like 5x 10+2 RAID-Z2 VDEVs?  At
> one
> >> > time there was a bug in the block allocation code that made 8+2 not
> work as
> >> > well as 9+2 or 7+2.  That _should_ have been fixed in the 0.7.x
> version you
> >> > are running, but there might still be some problems.
> >> >
> >> > Cheers, Andreas
> >> >
> >> >
> >> >
> >> >
> >> >
> >> _______________________________________________
> >> zfs-discuss mailing list
> >> zfs-discuss at list.zfsonlinux.org
> >> http://list.zfsonlinux.org/cgi-bin/mailman/listinfo/zfs-discuss
>
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