Thanks, Neil, for the clear explanation. Alex. On Mon, Jul 2, 2012 at 12:44 AM, NeilBrown <neilb@xxxxxxx> wrote: > On Sun, 1 Jul 2012 16:36:51 +0300 Alexander Lyakas <alex.bolshoy@xxxxxxxxx> > wrote: > >> Thanks, Neil! >> That clarifies. >> >> Does this also mean, that when md_do_sync() gets to such >> already-reconstructed stripe, it might reconstruct it once again, >> unless the stripe stays in the stripe cache? > > Yes, it will reconstruct it, and that might be "again" if the reconstructed > block has already been written. If the stripe is still in the cache, I think > it will still write that block out again, but won't need to reconstruct it. > > NeilBrown > > >> >> Thanks for helping, >> Alex. >> >> >> On Sun, Jul 1, 2012 at 11:00 AM, NeilBrown <neilb@xxxxxxx> wrote: >> > 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 > -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
