Re: Why does one get mismatches?

[Luca Berra <bluca@comedia.it>]

On Thu, Feb 25, 2010 at 08:39:36AM +1100, Neil Brown wrote:
>On Wed, 24 Feb 2010 11:12:09 -0500
>"Martin K. Petersen" <martin.petersen@oracle.com> wrote:
>
>> So realistically both disk blocks are wrong and there's a window until
>> the new, correct block is written.  That window will only cause problems
>> if there is a crash and we'll need to recover.  My main concern here is
>> how big the discrepancy between the disks can get, and whether we'll end
>> up corrupting the filesystem during recovery because we could
>> potentially be matching metadata from one disk with journal entries from
>> another.
>
>After a crash, md will only read from one of the devices (the first) until a
>resync has completed.  So there should be no room for more confusion than you
>would expect on a single device.

After thinking more about this i could come up with another concern
about write ordering.

example
app writes block A, B, C
md writes A on both disks
md writes B on disk1
app writes B again (B')
md writes B' on disk2
now md would write B' again on both disks, but the system crashes
(note, C is never written due to crash)

Disk 1 contains A and B in the correct order, it is missing C and B' but we
dont care, app should be able to recover from a crash

Disk 2 contains A and B', but they are wrongly ordered because C is
missing

If in the above case A and C are data blocks and B contains a journal
related to A and C, booting from disk 2 could result in inconsistent
data.

can the above really happen?
would using barriers remove the above concern?
am i missing something else?

L.

-- 
Luca Berra -- bluca@comedia.it
         Communication Media & Services S.r.l.
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