On Sun, January 8, 2012 00:28, Bob Friesenhahn wrote: > > I think that I would also be interested in a system which uses the > so-called spare disks for more protective redundancy but then reduces > that protective redundancy in order to use that disk to replace a > failed disk or to automatically enlarge the pool. > > For example, a pool could start out with four-way mirroring when there > is little data in the pool. When the pool becomes more full, mirror > devices are automatically removed (from existing vdevs), and used to > add more vdevs. Eventually a limit would be hit so that no more > mirrors are allowed to be removed. > > Obviously this approach works with simple mirrors but not for raidz. > > Bob > -- > Bob Friesenhahn > bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ > GraphicsMagick Maintainer, http://www.GraphicsMagick.org/ > _______________________________________________ > zfs-discuss mailing list > zfs-discuss@opensolaris.org > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss >
I actually disagree about raidz. I have often thought that a "dynamic raidz" would be a great feature. For instance, you have a 4-way raidz. What you are saying is you want the array to survive the loss of a single drive. So, from an empty vdev, it starts by writing 2 copies of each block, effectively creating a pair of mirrors. These are quicker to write and quicker to resilver than parity, and you would likely get a read speed increase too. As the vdev starts to get full, it starts using a parity based redundancy, and converting "older" data to this as well. Performance drops a bit, but it happens slowly. In addition, any older blocks not yet converted are still quicker to read and resilver. This is only a theory, but it is certainly something which could be considered. It would probably take a lot of rewriting of the raidz code, though. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
