ZFS - thanks

Patrick M. Hausen hausen at punkt.de
Thu Jul 9 13:43:04 UTC 2009


On Thu, Jul 09, 2009 at 03:21:48PM +0200, Tonix (Antonio Nati) wrote:

> I see a lot of people advicing to use ZFS RAID instead of HW RAID.
> I'm going to use HP duplicated iSCSI subsystems, which have autonomous 
> RAID, so I'm confused about this advice.
> Following the ZFS RAID stream, should I keep each disk alone in iSCSI and 
> let the ZFS make the RAID job?
> Should not HW RAID to be (a lot) more efficient?

You cannot escape the poor write performance of RAID 5 and
comparable setups with or without hardware. No matter how
much you cache, one time a block must be written to disk.

And for RAID 1 or 1+0 we found the impact on modern CPUs

So we switched to GEOM for mirroring a long time ago for
one simple reason: hardware replacement.

With a "hardware RAID" you need the precise brand and model
of the controller (worst case) to read a disk with valuable
data on it in case of a complete machine failure and replacement.
What, if that's not avaliable any more?

With software you just need an arbitrary machine with the
matching HDD interface (P-ATA, S-ATA, SCSI, ...)

This is my first attempt at software RAID-other-than-1,
but I'm really pleased with the results so far.

The system is a Fujitsu (former Fujitsu Siemens) SX 40
JBOD with a SAS host interface and SAS or S-ATA disks.
You can daisy chain 3 of these boxes with twelve disks
each to one server.
The host adapter in my server  doesn't have any RAID functions.
LSI something, easily replaced.

Reasonably priced, nice, scalable solution for our needs.
It's a datastore, so it doesn't do anything but backup
and restore. I would not use ZFS for something that needs
to be "up" 24x7 just yet. We had a couple of crashes before
we changed some memory parameters.

Kind regards,
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