[zfs-discuss] ZFS on RAID-5 array

Gordan Bobic gordan.bobic at gmail.com
Wed Apr 11 05:39:35 EDT 2018

Short answer: Don't.

Long answer:
By putting ZFS on a single virtual disk exposed by hardware RAID you will
lose ZFS' ability to auto-heal data, thus forfeiting a substantial fraction
of it's value.
Because the underlying RAID geometry is entirely opaque, it is virtually
guaranteed that you will end up with a lot of misaligned access which will
potentially halve sustained read throughput.
Sustained write throughput can be as low as half of what ZFS can achieve
because of RMW required by parity RAID on writes.

In some _very_ specific uses, you may be able to get better small random
read performance if you have ZFS on top of hardware RAID5.
If you can reduce the exposed RAID block size to as low as 8KB, you could
create zfs with ashift=13 to ensure the alignment at the bottom end is
unaffected, and set recordsize to 8KB to ensure fs blocks won't straddle
disk boundaries, and operations you are doing are mostly <= 8KB so the 8KB
recordsize is in fact appropriate, then having ZFS on hardware RAID5 would
potentially yield better random read performance than RAIDZ1 of same shape.
Sustained write throughput can still be as bad as half due to RMW on parity

On Wed, Apr 11, 2018 at 1:56 AM, H via zfs-discuss <
zfs-discuss at list.zfsonlinux.org> wrote:

> I have a fibre-channel disk array configured as RAID-5 which I am
> considering installing zfs on, the host OS will be CentOS 7.
> I believe I have read that zfs should ideally be installed directly on the
> harddisk but would still like opinions as to the pros and cons.
> _______________________________________________
> zfs-discuss mailing list
> zfs-discuss at list.zfsonlinux.org
> http://list.zfsonlinux.org/cgi-bin/mailman/listinfo/zfs-discuss
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