Previous to this putback ZFS did not do any prefetching of data when resilering or scrubbing a pool. This made such operations more time consuming that the would need too be. Since resilvering a large pool can take days, anything that can speed up such operations can make quite a difference in time spent without sufficient replication of data. Fortunately faster resileving for zpool is on its way into OpenSolaris with the putback of "6678033 resilver code should prefetch". The gain of this will of course depend on your pool, but I'll try to find time for testing so that I can get back with some numbers in a later post.
Since scrub and resilvering shares the same code, this should improve scrubbing performance as well. Scrub prefetch was mentioned in the KCA 2009 keynote.
Talks I have given
2 weeks ago