On Wed, July 14, 2010 14:58, Daniel Taylor wrote: > I'm about the build a opensolaris NAS system, currently we have two drives > and are planning on adding two more at a later date (2TB enterprise level > HDD are a bit expensive!).
Do you really need them? Now? Maybe 1TB drives are good now, and then add a pair of 2TB in a year? > Whats the best configuration for setting up these drives bearing in mind I > want to expand in the future? Mirror now (pool consisting of one two-way mirror vdev). Add second mirror vdev to the pool when you need to expand. > I was thinking of mirroring the drives and then converting to raidz some > how? No way to convert to raidz. (That is, no magic simple way; you can of course put in new drives for the raidz and copy the data across.) > It will only be a max of 4 drives, the second two of which will be bought > later. 5 drives would be a lot better. You could keep a hot spare -- and you could expand mirror vdevs safely (never dropping below your normal redundancy level), too. You can add new vdevs to a pool. This is very useful for a growing system (until you run out of drive slots). You can expand an existing vdev by replacing all the drives (one at a time). It's a lot cleaner and safer with mirror vdevs than with raidz[ 23] vdevs. In a raid vdev, you can replace drives individually and wait for them to resilver. When each drive is done, replace the next. When you have replaced all of the drives, the vdev will then make the new space available. HOWEVER, doing this takes away a level of redundancy -- you take away a live drive. For a RAIDZ, that means no redundancy during the resilver (which takes a while on a 2TB drive, if you haven't noticed). And the resilver is stressing the drives, so if there's any incipient failure, it's more likely to show up during the resilver. Scary! (RAIDZ2 is better in that you still have one layer of redundancy when you take one drive out; but in a 4-drive chassis forget it!). In a mirror vdev, you can be much cleverer, IF you can connect the new drive while the old drives are all still present. Attach the new bigger drive as a THIRD drive to the mirror vdev, and wait for the resilver. You now have a three-way mirror, and you never dropped below a two-way mirror at any time during the process. Detach one small drive and attach a new big drive, and wait again. And detach the last small drive, and you have now expanded your mirror vdev without ever dropping below your normal redundancy. (There are variants on this; the key point is that a mirror vdev can be an n-way mirror for any value of n your hardware can support.) If your backups are good and your uptime requirements aren't really strict, of course the risks can be tolerated better. -- 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
