On Wed, Sep 28, 2011 at 8:21 AM, Edward Ned Harvey <opensolarisisdeadlongliveopensola...@nedharvey.com> wrote: > When a vdev resilvers, it will read each slab of data, in essentially time > order, which is approximately random disk order, in order to reconstruct the > data that must be written on the resilvering device. This creates two > problems, (a) Since each disk must fetch a piece of each slab, the random > access time of the vdev as a whole is approximately the random access time > of the slowest individual device. So the more devices in the vdev, the > worse the IOPS for the vdev... And (b) the more data slabs in the vdev, the > more iterations of random IO operations must be completed. > > In other words, during resilvers, you're IOPS limited. If your pool is made > of all SSD's, then problem (a) is basically nonexistent, since the random > access time of all the devices are equal and essentially zero. Problem (b) > isn't necessarily a problem... It's like, if somebody is giving you $1,000 > for free every month and then they suddenly drop down to only $500, you > complain about what you've lost. ;-) (See below.)
If you regularly spend all of the given $1,000, then you're going to complain hard when it suddenly drops to $500. > So again: Not a problem if you're making your pool out of SSD's. Big problem if your system is already using most of the available IOPS during normal operation. -- Fajar _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
