[dm-devel] [PATCH] dm-raid: add RAID discard support

[Brassow Jonathan]

On Sep 23, 2014, at 11:21 PM, NeilBrown wrote:

> 
> For RAID5 it is not acceptable to return "zeros or the original content".
> This is because of the possibility of RMW cycles to update the parity block.
> As "discard" the doesn't treat all blocks in a stripe in an identical way
> will cause the parity block to become incorrect, and it will stay incorrect
> through multiple RMW cycles until an RCW corrects it.
> A device failure while the parity block is incorrect will lead to data
> corruption.
> 
> The only real solution I can think of is to maintain an 'allocated' bitmap
> and clear bits when DISCARDing.  When a write happens to a location that is
> not 'allocated' the whole  bit worth gets resynced (or maybe zeroed-out).
> 
> Grumble.

I see, sure.

Are there problems with this 'allocated' bitmap idea when bitrot is considered?  Scrubbing (check/repair) assumes that the bitmap cannot be trusted and does a complete scan of the array - dealing with mismatches.  If bitrot happens in the 'allocated' bitmap...

 brassow