The difference between ZFS scrubs and resilvers
At one level, asking what the difference is between scrubs and resilvers sounds silly; resilvering is replacing disks, while scrubbing is checking disks. But if you look at the ZFS code in the kernel things become much less clear, because both scrubs and resilvers use a common set of code to do all their work and it's actually not at all easy to tell what happens differently between them. Since I have actually dug through this section of ZFS code just the other day, I want to write down what the differences are while I remember them.
Both scrubs and resilvers traverse all of the metadata in the pool (in a non-sequential order), and both wind up reading all data. However, scrubs do this more thoroughly for mirrored vdevs; scrubs read all sides of a mirror, while resilvers only read one copy of the data (well, one intact copy). On raidz vdevs there is no difference here, as both scrubs and resilvers read both the data blocks and the parity blocks. This implies that a scrub on mirrored vdevs does more IO and (importantly) more checking than a resilver does. After a resilver of mirrored vdevs, you know that you have at least one intact copy of every piece of the pool, while after an error-free scrub of mirrored vdevs, you know that all ZFS metadata and data on all disks is fully intact.
For resilvers but not scrubs (at least normally), ZFS will sort of attempt to write everything back to the disks again, as I covered in the sidebar of last entry. As far as I can tell from the code, ZFS always skips even trying to write things back to disks that are either known to have good data (for example they were read from) or that are believed to be good because their DTL says that the data is clean on them (I believe that this case only really applies for mirrored vdevs). For scrubs, the only time ZFS submits writes to the disks is if it detects actual errors.
(Although I don't entirely understand how you get into this situation, it appears that a scrub of a pool with a 'replacing' disk behaves a lot like a resilver as far as that disk is concerned. As you would expect and want, the scrub doesn't try to read from the new disk and, like resilvers, it tries to write everything back to the disk.)
Since we only have mirrored vdevs on our fileservers, what really matters to us here is the difference in what gets read between scrubs and resilvers. On the one hand, resilvers put less read load on the disks, which is good for reducing their impact. On the other hand, resilvering isn't quite as thorough a check of the pool's total health as a scrub is.
PS: I'm not sure if either a scrub or a resilver reads the
ZIL. Based on some code in zil.c,
I suspect that it's checked only during scrubs, which would make
sense. Alternately, zil.c is reusing
for non-scrub IO for some of its side effects, but that would
be a bit weird.