Areca vs. ZFS performance testing.

Jeremy Chadwick koitsu at
Wed Nov 12 23:43:03 PST 2008

On Wed, Nov 12, 2008 at 10:57:58PM +0000, Dieter wrote:
> >> For the array(s)
> >> 9 x ST31000340AS  1tb disks
> >> 1 x ST31000333AS  1tb disk (trying to swap this for a ST31000340AS)
> > There seems to be little difference between enabling and disabling the
> > disk cache on the Areca.  This leads me to two conclusions:
> > 	1. Disabling the write cache does nothing on Seagate drives.
> > 	2. IO to the drives is so slow that a write cache is irrelevant.
> I have a couple of the ST31000340AS 1TB disks as well as older lower capacity
> Seagates, and turning the write cache on/off makes a MASSIVE (roughly 10:1)
> difference in write speed.
> Jeremy reports "about 13%" with Seagate ST3120026AS:
> Perhaps there is something about the Areca or the testing?  Is the write cache
> really getting turned on/off?

The Areca controller he has can do caching of its own (it has 256MBytes
of cache).  Meaning, if you disable write cache on the disks (but not
the Areca controller itself), all of the caching being done is purely
controller-based.  The actual disk writes between the controller and the
disk will, of course, be "slow" -- but between the OS and the
controller, things should appear fast.

Let me outline the 4 test scenarios (I thought I did this in my original
mail to Danny, but I believe I also said "don't get caught up in
excessive granularity because it'll just confuse people now" -- case in

- Areca cache disabled, disk write cache enabled
- Areca cache disabled, disk write cache disabled
- Areca cache enabled, disk write cache enabled
- Areca cache enabled, disk write cache disabled [**]

As I understand it, Danny performed the tests with the [**]

| Jeremy Chadwick                                jdc at |
| Parodius Networking              |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |

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