[zfs-discuss] ZFS on RAID-5 array

Gordan Bobic gordan.bobic at gmail.com
Sun Apr 15 09:17:51 EDT 2018


On Sun, Apr 15, 2018 at 2:01 PM, Edward Ned Harvey (zfsonlinux) via
zfs-discuss <zfs-discuss at list.zfsonlinux.org> wrote:

> > From: zfs-discuss <zfs-discuss-bounces at list.zfsonlinux.org> On Behalf
> > Of H via zfs-discuss
> >
> > So I should configure all 15 disks as one RAIDZ2 or three RAIDZ1 of
> > five disks each?
>
> Either way is fine. You have to decide what performance/space/reliability
> characteristics are most suited to your needs. You could also go with 7
> mirrors + spare. Or raidz3, although raidz3 seems excessive.
>
> 7 mirrors: read IOPS of 14 disks, write IOPS of 7 disks, read throughput
> of 14 disks, write throughput of 7 disks, capacity of 7 disks, guaranteed
> survival of at least 1 failure, probable survival of 3 concurrent failures,
> max survivable 7 concurrent failures
>
> 3x raidz1: read IOPS of 3 disks, write IOPS of 3 disks, read throughput of
> 12 disks, write throughput of 3 disks, capacity of 12 disks, guaranteed
> survival of at least 1 failure, probable survival of 2 concurrent failures,
> max survival 3 concurrent failures
>

Read IOPS would be 3x 5/4 =15/4 = 3.75 disks


>
> 1x raidz2: read IOPS of 1 disk, write IOPS of 1 disk, read throughput of
> 13 disks, write throughput of 13 disks, capacity of 13 disks, guaranteed
> survival of 2 failures, max survival 2 concurrent failures
>

Read IOPS would be 15/13 disks.

Reason being that reads don't touch the parity disk unless recovery due to
checksum failure is needed. Since the parity is rotated, that means that
disks you didn't request a read from have those IOPS available to serve
different requests.
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