> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
> boun...@opensolaris.org] On Behalf Of Jim Klimov
> Logically, yes - I agree this is what we expect to be done.
> However, at least with the normal ZFS reading pipeline, reads
> of redundant copies and parities only kick in if the first
> read variant of the block had errors (HW IO errors, checksum
I haven't read or written the code myself personally, so I'm not authoritative.
But I certainly know I've heard it said on this list before, that when you
read a mirror, it only reads one side (as you said) unless there's an error;
this allows a mirror to read 2x faster than a single disk (which I confirm by
benchmarking.) However, a scrub reads both sides, all redundant copies of the
data. I'm personally comfortably confident assuming this is true also for
reading the redundant copies of raidzN data.
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