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During scans (scrubs or resilvers), it sorts the blocks in each transaction
group by block offset; the result can be a significant improvement. (On my
test system just now, which I put some effort to introduce fragmentation into
the pool since I set it up yesterday, a scrub went from 1h2m to 33.5m with the
changes.) I've seen similar rations on production systems.
Approved by: Alexander Motin
Obtained from: ZFS On Linux
Relnotes: Yes (improved scrub performance, with tunables)
Differential Revision: https://reviews.freebsd.org/D15562
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