On Sun, 1 Jul 2012 10:08:40 +0300 Alexander Lyakas <alex.bolshoy@xxxxxxxxx> wrote: > Hi everybody, > I am trying to understand what happens when raid5 is recovering a > disk, and a write comes to a stripe that has not been recovered yet. > Does md first reconstruct the missing chunk and then applies the > write, or first the write is applied as if the array is still degraded > (and not recovering), and only later the missing chunk is > reconstructed (when the md_do_sync() loop gets to this area)? > I am looking at the stripe handling logic (kernel 2.6.38), can anybody > pls point me at the path that handle_stripe5() takes in that case? > > Hi Alex, The stripe is still degraded, so md/raid5 treats it like a write to a degraded array. Exactly what happens depends one which block is being written. If the block being written would be stored on the recovering devices, then md will perform a reconstruct-write. It will read the other data blocks, calculate the parity, and write out the parity and the changed data. Similarly if the parity block is on the recovering device a reconstruct-write will be needed. If some other block is being written, md will do a read-modify-write to calculate the new parity and then write out the parity and data. In this case the block on the recovering device will not be written. I hope that clarifies the situation. NeilBrown
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