zfs as layer distributor

Dimitar Vasilev dimitar.vassilev at gmail.com
Mon Oct 6 05:25:48 UTC 2008

2008/10/6 Andrew Snow <andrew at modulus.org>

> Dimitar Vasilev wrote:
>> Hi all,
>> Does someone use zfs as layer distributor on the top of hardware raid  -
>> (RAID10,RAID6,etc)?
> I've found ZFS works faster when given more than one disk device.  The
> reason being, it is smart about writing journal logs and metadata copies to
> different devices, resulting in higher performance by using idle disks.  It
> also provides more "channels" for write clustering so higher throughput on
> write-heavy loads.
> Secondly if you use ZFS to provide RAID1 or RAID5, due to checksumming it
> can be smarter about which data it chooses in the event of a checksum
> failure.  Hardware RAID can only do this with RAID6.
> Finally, when ZFS issues "flush cache" command to the disk for metadata and
> journal logs, there is less data to flush when you give it multiple smaller
> devices.  If you have a single monolithic RAID device with a large (eg.
> 256mb) cache, it can ruin performance while the RAID card flushes its entire
> cache.  (This can be disabled with a sysctl).
> - Andrew

Thanks Andrew,
I have an Areca 1120 with RAID-6 and on the top of it a zfs as a layer
So far I can tell the following:
1)works nice and fast
2)can be pain in the rear if your controller spits one of the disks due to
power surge/etc.
3) zfs snapshots caused some crashes and bad descriptors on 7.0-stable as of
3 months behind- but it's somewhat expected.
I'm thinking of raidz2 and setting the disks as pass-through.
Would love if someone to hear if someone has tested hardware raid6 and zfs
over it.
Best regards,

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