> From: Phil Harman <phil.har...@gmail.com>
> Date: Tue, 10 Aug 2010 09:24:52 +0100
> To: Ian Collins <i...@ianshome.com>
> Cc: Terry Hull <t...@nrg-inc.com>, "zfs-discuss@opensolaris.org"
> <zfs-discuss@opensolaris.org>
> Subject: Re: [zfs-discuss] RAID Z stripes
> 
> On 10 Aug 2010, at 08:49, Ian Collins <i...@ianshome.com> wrote:
> 
>> On 08/10/10 06:21 PM, Terry Hull wrote:
>>> I am wanting to build a server with 16 - 1TB drives with 2 ­ 8 dri
>>> ve RAID Z2 arrays striped together. However, I would like the capa
>>> bility of adding additional stripes of 2TB drives in the future. W
>>> ill this be a problem? I thought I read it is best to keep the str
>>> ipes the same width and was planning to do that, but I was wonderi
>>> ng about using drives of different sizes. These drives would all b
>>> e in a single pool.
>>> 
>> It would work, but you run the risk of the smaller drives becoming
>> full and all new writes doing to the bigger vdev. So while usable,
>> performance would suffer.
> 
> Almost by definition, the 1TB drives are likely to be getting full
> when the new drives are added (presumably because of running out of
> space).
> 
> Performance can only be said to suffer relative to a new pool built
> entirely with drives of the same size. Even if he added 8x 2TB drives
> in a RAIDZ3 config it is hard to predict what the performance gap will
> be (on the one hand: RAIDZ3 vs RAIDZ2, on the other: an empty group vs
> an almost full, presumably fragmented, group).
> 
>> One option would be to add 2TB drives as 5 drive raidz3 vdevs. That
>> way your vdevs would be approximately the same size and you would
>> have the optimum redundancy for the 2TB drives.
> 
> I think you meant 6, but I don't see a good reason for matching the
> group sizes. I'm for RAIDZ3, but I don't see much logic in mixing
> groups of 6+2 x 1TB and 3+3 x 2TB in the same pool (in one group I
> appear to care most about maximising space, in the other I'm
> maximising availability)
> 
> The other issue is that of hot spares. In a pool of mixed size drives
> you either waste array slots (by having spares of different sizes) or
> plan to have unavailable space when small drives are replaced by large
> ones.


So do I understand correctly that really the "Right" thing to do is to build
a pool not only with a consistent strip width, but also to build it with
drives on only one size?   It also sounds like from a practical point of
view that building the pool full-sized is the best policy so that the data
can be spread relatively uniformly across all the drives from the very
beginning.  In my case, I think what I will do is to start with the 16
drives in a single pool and when I need more space, I'll create a new pool
and manually move the some of the existing data to the new pool to spread
the IO load.   

The other issue here seems to be RAIDZ2 vs RAIDZ3.  I assume there is not a
significant performance difference between the two for most loads, but
rather I choose between them based on how badly I want the array to stay
intact.  

-
Terry



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