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

Chris Forgeron cforgeron at acsi.ca
Wed Jan 5 22:34:58 UTC 2011


Yup, but the second set (stripe of 2 raidz1's) can achieve slightly better performance, particularly on a system that has a lot of load. There's a number of blog articles that discuss that in more detail than I care to get into here. 
Of course, that's a bit of a moot point, as you're not going to heavily load a 9 drive system, like a 48 drive system, but.. 

In that example, the first (raidz2) would be a bit more safe as it could take 2 drives failing. The latter (2 raidz1's) would die if those two failing drives are within 1 raidz1 vdev. 

It all comes down to that final decision on how much risk do you want to take with your data, what your budget is, and what your performance requirements are. 

I'm starting to settle into a stripe of 6 vdevs that are each a 5 disk raidz1, with two hot-spares kicking about, and a collection of small SSD's adding up to either 500G or 1TB of SSD L2ARC. A bit more risk, but I'm also planning on having an entirely redundant (yet slower) SAN device that will get a daily ZFS send, so my worst nightmare is yesterday's data - Which I can stand. 

Oh - I am also a fan of buying drives at different time periods or from different suppliers.. I have seen entire 4 and 8 drive arrays fail within a month of the first drives going. Always really fun when you were too slack to handle the first drive failure, the second one put you in a tight spot the next week, and then the third one dies while you're madly trying to do data recovery.. :-)

Really, in a big enough array, I like to swap out older drives for newer ones every now and then and repurpose the old - Just to keep the dreaded complete failure at bay. Things you learn to do with cheap SATA drives..


-----Original Message-----
From: owner-freebsd-stable at freebsd.org [mailto:owner-freebsd-stable at freebsd.org] On Behalf Of Damien Fleuriot
Sent: Wednesday, January 05, 2011 5:55 PM
To: Chris Forgeron
Cc: freebsd-stable at freebsd.org
Subject: Re: ZFS - moving from a zraid1 to zraid2 pool with 1.5tb disks

Well actually...

raidz2:
- 7x 1.5 tb = 10.5tb
- 2 parity drives

raidz1:
- 3x 1.5 tb = 4.5 tb
- 4x 1.5 tb = 6 tb , total 10.5tb
- 2 parity drives in split thus different raidz1 arrays

So really, in both cases 2 different parity drives and same storage...

---
Fleuriot Damien

On 5 Jan 2011, at 16:55, Chris Forgeron <cforgeron at acsi.ca> wrote:

> 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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