> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- > boun...@opensolaris.org] On Behalf Of Paul Piscuc > > looks promising. One element that we cannot determine is the optimum > number of disks in a raid-z pool. In the ZFS best practice guide, 7,9 and 11
There are several important things to consider: -1- Performance in usage. -2- Cost to buy disks & slots to hold disks. -3- Resilver / scrub time. You're already on the right track to answer #1 and #2. So I want to talk a little bit about #3 For typical usage on spindle hard disks, ZFS has a problem with resilver and scrub time. It will only resilver or scrub the used areas of disk, which seems like it would be faster than doing the whole disk, but since it ends up being a whole bunch of small sectors scattered about the disk, and typically most of the disk is used, and the order of resilver/scrub is not in disk order, it means you end up needing to do random seeks all over the disk, to read/write nearly the whole disk. The end result is a resilver time that can be 1-2 orders of magnitude larger than you expected. Like a week or three, if you have a bad configuration (lots of disks in a vdev) ... or 12-24 hours in the best case (mirrors and nothing else). The problem is linearly related to the number of used chunks in the degraded vdev, which is itself, usually approximated as a fraction of the total pool. So you minimize the problem if you use mirrors, and you maximize the problem if you make your pool from one huge raidzN vdev. On my disks, for a sun server where this was an issue for me ... If I needed to resilver the entire disk sequentially, including unused space, it would have required 2 hrs. I use ZFS mirrors, and it actually took 12 hrs. If I had made the pool one big raidzN, it would have needed 20 days. Until this problem is fixed, I recommend using mirrors only, and staying away from raidzN, unless you're going to build your whole pool out of SSD's. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
