Re: Why does one get mismatches?

[Luca Berra <bluca@comedia.it>]

On Tue, Mar 02, 2010 at 02:04:47AM -0800, Michael Evans wrote:
>On Mon, Mar 1, 2010 at 11:36 PM, Luca Berra <bluca@comedia.it> wrote:
>> On Tue, Mar 02, 2010 at 04:01:00PM +1100, Neil Brown wrote:
>>>> 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?
>>>
>>> These is no inconsistency here that a filesystem would not equally expect
>>> from a single device.
>>> After the crash-while-writing B', it should expect to see either B or B',
>>> and it does, depending on which device is primary.
>>>
>>> Nothing to see here.
>>
>> I will try to explain better,
>> the problem is not related to the confusion between B or B'
>>
>> the problem is that on one disk we have B' _without_ C.
>>
>You're demanding full atomic commits; this is precisely what journals
>and /barriers/ are for.
>
>Are you are bypassing them in a quest for performance and paying for
>it on crashes?
>Or is this a hardware bug?
>Or is it some glitch in the block device layering leading to barrier
>requests not being honored?
I just asked for confirmation that with /barriers/ the scenario above
would not happen.

L.

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