> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- > boun...@opensolaris.org] On Behalf Of Orvar Korvar > > Ok, so mirrors resilver faster. > > But, it is not uncommon that another disk shows problem during resilver (for > instance r/w errors), this scenario would mean your entire raid is gone, right?
Imagine, you have 8 disks configured as 4x 2-way mirrors. Capacity of 4 disks. Imagine for comparison, you have 6 disks configured as raidz2. Capacity of 4 disks. Imagine, in the event of a disk failure, the mirrored configuration resilvers 4x faster, which is a good estimate because each mirrored vdev has 1/4 as many objects on it. Yes it's possible for 2 disks failure to destroy the mirrored configuration, if they happen to both be partners of each other. But the probability of a 2nd disk failure being the partner of the first failed disk is only 1/7, and it only results in pool failure if it occurs within the resilver window, which is 4x less probable. You can work out the probabilities, but suffice it to say, the probability of pool failure using the mirrored configuration is not dramatically different from the probability of pool failure with the raidz configuration. If you want to know the precise probabilities, you have to fill in all the variables... Number of drives in each configuration, resilver times, MTTDL for each drive... etc. Sometimes the mirrors are more reliable, sometimes the raid is more reliable. Performance of the mirrors is always equal or better than performance of the raidz. Cost of the mirrors is always equal or higher than the cost of the raidz. > If you are using mirrors, and one disk crashes and you start resilver. Then the > other disk shows r/w errors because of the increased load - then you are > screwed? Because large disks take long time to resilver, possibly weeks? If one disk fails in a mirror, then one disk has increased load. If one disk fails in a raidz, then N disks have increased load. So no, I don't think this is a solid argument against mirrors. ;-) Incidentally, large disks only take weeks to resilver in a large raid configuration. That never happens in a mirrored configuration. ;-) > In that case, it would be preferable to use mirrors with 3 disks in each vdev. > Trimorrs. Each vdev should be one raidz3. If I'm not mistaken, a 3-way mirror is not implemented behind the scenes in the same way as a 3-disk raidz3. You should use a 3-way mirror instead of a 3-disk raidz3. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
