On Wed, Sep 8, 2010 at 06:59, Edward Ned Harvey <sh...@nedharvey.com> wrote: >> On Tue, Sep 7, 2010 at 4:59 PM, Edward Ned Harvey <sh...@nedharvey.com> >> wrote: >> >> I think the value you can take from this is: >> Why does the BPG say that? What is the reasoning behind it? >> >> Anything that is a "rule of thumb" either has reasoning behind it (you >> should know the reasoning) or it doesn't (you should ignore the rule of >> thumb, dismiss it as myth.) > > Let's examine the myth that you should limit the number of drives in a vdev > because of resilver time. The myth goes something like this: You shouldn't > use more than ___ drives in a vdev raidz_ configuration, because all the > drives need to read during a resilver, so the more drives are present, the > longer the resilver time. > > The truth of the matter is: Only the size of used data is read. Because > this is ZFS, it's smarter than a hardware solution which would have to read > all disks in their entirety. In ZFS, if you have a 6-disk raidz1 with > capacity of 5 disks, and a total of 50G of data, then each disk has roughly > 10G of data in it. During resilver, 5 disks will each read 10G of data, and > 10G of data will be written to the new disk. If you have a 11-disk raidz1 > with capacity of 10 disks, then each disk has roughly 5G of data. 10 disks > will each read 5G of data, and 5G of data will be written to the new disk. > If anything, more disks means a faster resilver, because you're more easily > able to saturate the bus, and you have a smaller amount of data that needs > to be written to the replaced disk.
It is not a question of a vdev with 6 disk vs a vdev with 12 disks. It is about 1 vdev with 12 disk or 2 vdev with 6 disks. If you have 2 vdev you have to read half the data compared to 1 vdev to resilver a disk. Or look at it this way, you will put more data on a 12 disk vdev than on a 6 disk vdev. IO other than the resilver will also slow the resilver down more if you have large vdevs. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
