zfs as layer distributor

Andrew Snow andrew at modulus.org
Mon Oct 6 00:02:09 UTC 2008

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

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