Richard Elling wrote:
In the disk, at the disk block level, there is fairly substantial ECC.
Yet, we still see data loss. There are many mechanisms at work here. One
that we have studied to some detail is superparamagnetic decay -- the
medium wishes to decay to a lower-enegy state, losing information in the process.
One way to "prevent" this is to rewrite the data -- basically resetting the
decay clock. The study we did on this says that rewriting your data once
per year is reasonable. Note that ZFS is COW, and scrubbing is currently a
read operation which will only write when data needs to be reconstructed.
I look at this as: rewrite-style scrubbing is preventative, read and verify
style scrubbing is prescriptive. Either is better than neither.
In short, use spares and scrub.
I see another purpose for rewrite-style scrubbing - it would be an
enabler for disk eviction. First you mark the disk you want to
evict as read-only, than start a rewrite scrub. When done, your disk
is free of data and can be taken out.
--
Henk Langeveld <[EMAIL PROTECTED]>
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