How ZFS resilvering saved us
I've said nasty things about ZFS before and I'll undoubtedly say some in the future, but today, for various reasons, I want to take the positive side and talk about how ZFS has saved us. While there are a number of ways that ZFS routinely saves us in the small, there's been one big near miss that stands out.
Our fundamental environment is ZFS pools with vdevs of mirror pairs of disks. This setup costs space but, among other things, it's safe from multi-disk failures unless you lose both sides of a single mirror pair (at which point you've lost a vdev and thus the entire pool). One day we came very close to that: one side of a mirror pair died more or less completely and then, as we were resilvering on to a spare disk, the other side of the mirror started developing read errors. This was especially bad because read errors generally had the effect of locking up this particular fileserver (for reasons we don't understand). This was particularly bad because in Solaris 10 update 8, rebooting a locked up fileserver causes the pool resilver to lose all progress to date and start again from scratch.
ZFS resilver saved us here in two ways. The obvious way is that it didn't give up on the vdev when the second disk had some read errors. Many RAID systems would have shrugged their shoulders, declared the second disk bad too, and killed the RAID array (losing all data on it). ZFS was both able and willing to be selective, declaring only specific bits bad instead of ejecting the whole disk and destroying the pool.
(We were lucky in that no metadata was damaged, only file contents, and we had all of the damaged files in backups.)
The subtle way is how ZFS let us solve the problem of successfully resilvering the pool despite the fileserver's 'eventually lock up after enough read errors' behavior. Because ZFS told us what the corrupt files were when it found them and because ZFS only resilvers active data, we could watch the pool's status during the resilver, see what files were reported as having unrepairable problems, and then immediately delete them; this effectively fenced the bad spots on the disk off from the fileserver so that it wouldn't trip over them and explode (again). With a traditional RAID system and a whole-device resync it would have been basically impossible to fence the RAID resync away from the bad disk blocks. At a minimum this would have made the resync take much, much longer.
The whole experience was very nerve-wracking, because we knew we were only one glitch away from ZFS destroying a very large pool. But in the end ZFS got us through and we able to tell users that we had very strong assurances that no other data had been damaged by the disk problems.