Re: [Qemu-devel] [PATCH v2 1/1] quorum: Change vote rules for 64 bits hash

[Max Reitz]

On 19.02.2016 12:24, Alberto Garcia wrote:
> On Fri 19 Feb 2016 09:26:53 AM CET, Wen Congyang <address@hidden> wrote:
> 
>>>> If quorum has two children(A, B). A do flush sucessfully, but B
>>>> flush failed.  We MUST choice A as winner rather than just pick
>>>> anyone of them. Otherwise the filesystem of guest will become
>>>> read-only with following errors:
>>>>
>>>> end_request: I/O error, dev vda, sector 11159960
>>>> Aborting journal on device vda3-8
>>>> EXT4-fs error (device vda3): ext4_journal_start_sb:327: Detected abort 
>>>> journal
>>>> EXT4-fs (vda3): Remounting filesystem read-only
>>>
>>> Hi Xie,
>>>
>>> Let's see if I'm getting this right:
>>>
>>> - When Quorum flushes to disk, there's a vote among the return values of
>>>   the flush operations of its members, and the one that wins is the one
>>>   that Quorum returns.
>>>
>>> - If there's a tie then Quorum choses the first result from the list of
>>>   winners.
>>>
>>> - With your patch you want to give priority to the vote with result == 0
>>>   if there's any, so Quorum would return 0 (and succeed).
>>>
>>> This seems to me like an ad-hoc fix for a particular use case. What
>>> if you have 3 members and two of them fail with the same error code?
>>> Would you still return 0 or the error code from the other two?
>>
>> For example:
>> children.0 returns 0
>> children.1 returns -EIO
>> children.2 returns -EPIPE
>>
>> In this case, quorum returns -EPIPE now(without this patch).
>>
>> For example:
>> children.0 returns -EPIPE
>> children.1 returns -EIO
>> children.2 returns 0
>> In this case, quorum returns 0 now.
> 
> My question is: what's the rationale for returning 0 in case a) but not
> in case b)?
> 
>   a)
>     children.0 returns -EPIPE
>     children.1 returns -EIO
>     children.2 returns 0
> 
>   b)
>     children.0 returns -EIO
>     children.1 returns -EIO
>     children.2 returns 0
> 
> In both cases you have one successful flush and two errors. You want to
> return always 0 in case a) and always -EIO in case b). But the only
> difference is that in case b) the errors happen to be the same, so why
> does that matter?
> 
> That said, I'm not very convinced of the current logics of the Quorum
> flush code either, so it's not even a problem with your patch... it
> seems to me that the code should follow the same logics as in the
> read/write case: if the number of correct flushes >= threshold then
> return 0, else select the most common error code.

I'm not convinced of the logic either, which is why I waited for you to
respond to this patch. :-)

Intuitively, I'd expect Quorum to return an error if flushing failed for
any of the children, because, well, flushing failed. I somehow feel like
flushing is different from a read or write operation and therefore
ignoring the threshold would be fine here. However, maybe my intuition
is just off.

Anyway, regardless of that, if we do take the threshold into account, we
should not use the exact error value for voting but just whether an
error occurred or not. If all but one children fail to flush (all for
different reasons), I find it totally wrong to return success. We should
then just return -EIO or something.

Max

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