On 10 Aug 2010, at 10:22, Ian Collins <i...@ianshome.com> wrote: > On 08/10/10 09:12 PM, Andrew Gabriel wrote: >> Phil Harman wrote: >>> 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 drive RAID >>>>> Z2 arrays striped together. However, I would like the capability of >>>>> adding additional stripes of 2TB drives in the future. Will this be a >>>>> problem? I thought I read it is best to keep the stripes the same width >>>>> and was planning to do that, but I was wondering about using drives of >>>>> different sizes. These drives would all be 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) >> >> Another option - use the new 2TB drives to swap out the existing 1TB drives. >> If you can find another use for the swapped out drives, this works well, and >> avoids ending up with sprawling lower capacity drives as your pool grows in >> size. This is what I do at home. The freed-up drives get used in other >> systems and for off-site backups. Over the last 4 years, I've upgraded from >> 1/4TB, to 1/2TB, and now on 1TB drives. >> > I have been doing the same. > > The reason I mentioned performance (and I did mean 6 drives!) is in order to > get some space on a budget I replaced one mirror in a stripe with bigger > drives. The others soon became nearly full and most of the IO went to the > bigger pair, so I lost nearly all the benefit of the stripe. I have also > grown stripes and seen similar issues and I had to remove and replace large > chunks of data to even things out. > > I really think mixing vdev sizes is a bad idea.
I'd agree if this was a new pool, but this question was about expanding an existing pool (which is nearly full and where the performance is presumably acceptable). Adding another vdev, whatever its size, is a simple zero downtime option for growing the pool (adding another pool would fragment the name space). With a similar number of spindles in a similar RAID configuration, performance is unlikely to get worse, indeed (as already noted) it is likely to get better until the new vdev fills up. Many systems only need to be good enough, not optimum. The best is often the enemy of the good. Anyone using RAIDZn is cost conscious to some degree (or why not just go for a hige stripe of 4-way mirrored SSDs and be done with it?) _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
