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.

Cheers,

John.