On 8 May 2017, Phil Turmel verbalised: > On 05/08/2017 10:50 AM, Nix wrote: > >> I wonder... scrubbing is not very useful with md, particularly with RAID >> 6, because it does no writes unless something mismatches, > > This is wrong. The purpose of scrubbing is to expose any sectors that > have degraded (as Wol describes) to the point of generating a read > error. A "check" scrub only writes back to the sectors that report a > URE, giving the drive firmware a chance to fix or relocate the sector. > > A check scrub will NOT write on mismatch, just increment the mismatch > counter. This is the recommended regular scrubbing operation. You want > to know when mismatches occur. And... then what do you do? On RAID-6, it appears the answer is "live with a high probability of inevitable corruption". That's not very good. (AIUI, if a check scrub finds a URE, it'll rewrite it, and when in the common case the drive spares it out and the write succeeds, this will not be reported as a mismatch: is this right?) >> If there was a way to get md to *rewrite* everything during scrub, >> rather than just checking, this might help (in addition to letting the >> drive refresh the magnetization of absolutely everything). > > This is actually counterproductive. Rewriting everything may refresh > the magnetism on weakening sectors, but will also prevent the drive from > *finding* weakening sectors that really do need relocation. If a sector weakens purely because of neighbouring writes or temperature or a vibrating housing or something (i.e. not because of actual damage), so that a rewrite will strengthen it and relocation was never necessary, surely you've just saved a pointless bit of sector sparing? (I don't know: I'm not sure what the relative frequency of these things is. Read and write errors in general are so rare that it's quite possible I'm worrying about nothing at all. I do know I forgot to scrub my old hardware RAID array for about three years and nothing bad happened...) -- NULL && (void) -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
