On Thu, June 3, 2010 10:50, Garrett D'Amore wrote: > On Thu, 2010-06-03 at 10:35 -0500, David Dyer-Bennet wrote: >> On Thu, June 3, 2010 10:15, Garrett D'Amore wrote: >> > Using a stripe of mirrors (RAID0) you can get the benefits of multiple >> > spindle performance, easy expansion support (just add new mirrors to >> the >> > end of the raid0 stripe), and 100% data redundancy. If you can >> afford >> > to pay double for your storage (the cost of mirroring), this is IMO >> the >> > best solution. >> >> Referencing "RAID0" here in the context of ZFS is confusing, though. >> Are >> you suggesting using underlying RAID hardware to create virtual volumes >> to >> then present to ZFS, or what? > > RAID0 is basically the default configuration of a ZFS pool -- its a > concatenation of the underlying vdevs. In this case the vdevs should > themselves be two-drive mirrors. > > This of course has to be done in the ZFS layer, and ZFS doesn't call it > RAID0, any more than it calls a mirror RAID1, but effectively that's > what they are.
Kinda mostly, anyway. I thought we recently had this discussion, and people were pointing out things like the striping wasn't physically the same on each drive and such. >> > Note that this solution is not quite as resilient against hardware >> > failure as raidz2 or raidz3. While the RAID1+0 solution can tolerate >> > multiple drive failures, if both both drives in a mirror fail, you >> lose >> > data. >> >> In a RAIDZ solution, two or more drive failures lose your data. In a >> mirrored solution, losing the WRONG two drives will still lose your >> data, >> but you have some chance of surviving losing a random two drives. So I >> would describe the mirror solution as more resilient. >> >> So going to RAIDZ2 or even RAIDZ3 would be better, I agree. > >>From a data resiliency point, yes, raidz2 or raidz3 offers better > protection. At a significant performance cost. The place I care about performance is almost entirely sequential read/write -- loading programs, and loading and saving large image files. I don't know a lot of home users that actually need high IOPS. > Given enough drives, one could probably imagine using raidz3 underlying > vdevs, with RAID0 striping to spread I/O across multiple spindles. I'm > not sure how well this would perform, but I suspect it would perform > better than straight raidz2/raidz3, but at a significant expense (you'd > need a lot of drives). Might well work that way; it does sound about right. >> In an 8-bay chassis, there are other concerns, too. Do I keep space >> open >> for a hot spare? There's no real point in a hot spare if you have only >> one vdev; that is, 8-drive RAIDZ3 is clearly better than 7-drive RAIDZ2 >> plus a hot spare. And putting everything into one vdev means that for >> any >> upgrade I have to replace all 8 drives at once, a financial problem for >> a >> home server. > > This is one of the reasons I don't advocate using raidz (any version) > for home use, unless you can't afford the cost in space represented by > mirroring and a hot spare or two. (The other reason ... for my use at > least... is the performance cost. I want to use my array to host > compilation workspaces, and for that I would prefer to get the most > performance out of my solution. I suppose I could add some SSDs... but > I still think multiple spindles are a good option when you can do it.) > > In an 8 drive chassis, without any SSDs involved,I'd configure 6 of the > drives as a 3 vdev stripe consisting of mirrors of 2 drives, and I'd > leave the remaining two bays as hot spares. Btw, using the hot spares > in this way potentially means you can use those bays later to upgrade to > larger drives in the future, without offlining anything and without > taking too much of a performance penalty when you do so. And the three 2-way mirrors is exactly where I am right now. I don't have hot spares in place, but I have the bays reserved for that use. In the latest upgrade, I added 4 2.5" hot-swap bays (which got the system disks out of the 3.5" hot-swap bays). I have two free, and that's the form-factor SSDs come in these days, so if I thought it would help I could add an SSD there. Have to do quite a bit of research to see which uses would actually benefit me, and how much. It's not obvious that either l2arc or zil on SSD would help my program loading, image file loading, or image file saving cases that much. There may be more other stuff than I really think of though. -- David Dyer-Bennet, d...@dd-b.net; http://dd-b.net/ Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/data/ Photos: http://dd-b.net/photography/gallery/ Dragaera: http://dragaera.info _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
