On 27/01/2009 05:16, Neil Brown wrote: [...]
Probably the only practical data structure for this would be a bitmap similar to the current write-intent bitmap. Is it really worth supporting this in raid5? Are the sorts of devices that will benefit from 'discard' requests likely to be used inside an md/raid5 array I wonder....
Assuming I've understood correctly, this usage map sounds to me like a useful thing to have for all RAIDs. When building the array in the first place, the initial sync is just writing a usage map saying it's all empty. Filesystem writes and discards update it appropriately. Then when we get failing sectors reported via e.g. SMART or a scrub operation we know whether they're on used or unused areas so whether it's worth attempting recovery.
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