Re: Question about raid5 disk recovery logic

[NeilBrown <neilb@suse.de>]

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On Sun, 1 Jul 2012 10:08:40 +0300 Alexander Lyakas <alex.bolshoy@gmail.com>
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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