ZFS - moving from a zraid1 to zraid2 pool with 1.5tb disks

Chris Forgeron cforgeron at acsi.ca
Wed Jan 5 15:55:13 UTC 2011

First off, raidz2 and raidz1 with copies=2 are not the same thing. 

raidz2 will give you two copies of parity instead of just one. It also guarantees that this parity is on different drives. You can sustain 2 drive failures without data loss. 

raidz1 with copies=2 will give you two copies of all your files, but there is no guarantee that they are on different drives, and you can still only sustain 1 drive failure.

You'll have better space efficiency with raidz2 if you're using 9 drives. 

If I were you, I'd use your 9 disks as one big raidz, or better yet, get 10 disks, and make a stripe of 2 5 disk raidz's for the best performance. 

Save your SSD drive for the L2ARC (cache) or ZIL, you'll get better speed that way instead of throwing it away on a boot drive. 


-----Original Message-----
From: owner-freebsd-stable at freebsd.org [mailto:owner-freebsd-stable at freebsd.org] On Behalf Of Damien Fleuriot
Sent: January-05-11 5:01 AM
To: Damien Fleuriot
Cc: freebsd-stable at freebsd.org
Subject: Re: ZFS - moving from a zraid1 to zraid2 pool with 1.5tb disks

Hi again List,

I'm not so sure about using raidz2 anymore, I'm concerned for the performance.

Basically I have 9x 1.5T sata drives.

raidz2 and 2x raidz1 will provide the same capacity.

Are there any cons against using 2x raidz1 instead of 1x raidz2 ?

I plan on using a SSD drive for the OS, 40-64gb, with 15 for the system itself and some spare.

Is it worth using the free space for cache ? ZIL ? both ?

@jean-yves : didn't you experience problems recently when using both ?

Fleuriot Damien

On 3 Jan 2011, at 16:08, Damien Fleuriot <ml at my.gd> wrote:

> On 1/3/11 2:17 PM, Ivan Voras wrote:
>> On 12/30/10 12:40, Damien Fleuriot wrote:
>>> I am concerned that in the event a drive fails, I won't be able to 
>>> repair the disks in time before another actually fails.
>> An old trick to avoid that is to buy drives from different series or 
>> manufacturers (the theory is that identical drives tend to fail at 
>> the same time), but this may not be applicable if you have 5 drives 
>> in a volume :) Still, you can try playing with RAIDZ levels and probabilities.
> That's sound advice, although one also hears that they should get 
> devices from the same vendor for maximum compatibility -.-
> Ah well, next time ;)
> A piece of advice I shall heed though is using 1% less capacity than 
> what the disks really provide, in case one day I have to swap a drive 
> and its replacement is a few kbytes smaller (thus preventing a rebuild).
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