> 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
> mismatch).

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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  • [zfs-discuss] Scr... Jim Klimov
    • Re: [zfs-dis... Karl Wagner
      • Re: [zfs... Edward Ned Harvey (opensolarisisdeadlongliveopensolaris)
        • Re: ... Jim Klimov
          • ... Edward Ned Harvey (opensolarisisdeadlongliveopensolaris)
          • ... Timothy Coalson
            • ... Jim Klimov
              • ... Karl Wagner
                • ... Jim Klimov
                • ... Ray Arachelian
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                • ... Jim Klimov
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                • ... Edward Ned Harvey (opensolarisisdeadlongliveopensolaris)
                • ... Tomas Forsman

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