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Date:      Wed, 16 Dec 2009 15:19:35 -0800
From:      Matt Simerson <matt@corp.spry.com>
To:        freebsd-fs@freebsd.org
Subject:   Re: ZFS RaidZ2 with 24 drives?
Message-ID:  <E9C46E04-1A81-4EE5-909E-557EA08D16A9@corp.spry.com>
In-Reply-To: <deb820500912161320n43c552d7rf84264332574a701@mail.gmail.com>
References:  <568624531.20091215163420@pyro.de> <42952D86-6B4D-49A3-8E4F-7A1A53A954C2@spry.com> <957649379.20091216005253@pyro.de> <26F8D203-A923-47D3-9935-BE4BC6DA09B7@corp.spry.com> <deb820500912161320n43c552d7rf84264332574a701@mail.gmail.com>

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On Dec 16, 2009, at 1:20 PM, Thomas Burgess wrote:

> On Wed, Dec 16, 2009 at 3:43 PM, Matt Simerson <matt@corp.spry.com>  
> wrote:
>
> On Dec 15, 2009, at 3:52 PM, Solon Lutz wrote:
>
> Why do you use JBOD? You can configure a passthrough for all drives,
> explicitly degrading the Areca to a dumb sata controller...
>
> Why would I bother?  Both ways present each disk to FreeBSD.  Based  
> on my understanding (and an answer received from Areca support), the  
> only reason I'd bother manually configuring some disks for  
> passthrough is if I wanted to use some disks in a RAID array and  
> others as raw disks. Configuring JBOD mode configures ALL the disks  
> on the controller as passthrough devices.
>
> I think the main reason is that ZFS is better when it has raw drives.

I've heard that numerous times. Perhaps it is true in some cases. Such  
as when using a RAID controller from 1999. Or a $30 RAID adapter.

I've built several ZFS systems using on-board SATA/SAS controllers, a  
couple of 24-disk systems with the Marvell SATA controllers used in  
the Sun x4500, and three 24-disk systems using the Areca 1231ML. Using  
the Areca as a hardware RAID controller with RAID volumes has proven  
to perform better and be much more reliable than when using raw disks.

> Some of the features of ZFS don't work as well without having access  
> to the drives in this way, and other features don't work at all.

The last time I compared the performance of ZFS using dumb Marvell  
SATA controllers versus the Areca with RAID, them features you speak  
of weren't worth the bits used to say them.  On the two systems I  
described in this thread, the one using RAID significantly outperforms  
the one configured as JBOD. And in the case of the Areca, JBOD =  
passthrough = raw disks.

> In general, it's always best to let ZFS handle the raid stuff and  
> not use the hardware raid settings.

Because you said so?

I'd like to see some evidence to back that statement up. The only time  
I've seen better ZFS performance numbers than what I'm getting with  
FreeBSD 8 ZFS + Areca RAID6 is when I tested OpenSolaris with them  
Marvell SATA controllers. But that was in August of 2008, and ZFS on  
FreeBSD performs much better now. Some updated benchmarks would be  
welcome.

Matt




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