PGLog::merge_log clarification

PGLog::merge_log clarification

[Loic Dachary]

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Hi Sam,

When calling merge_log on the following : http://pastealacon.com/32449 ( where the first column are the versions of the log entries and the second column are the versions of the olog entries ) I expect 5,2 and 6,1 from log to be added to divergent
https://github.com/ceph/ceph/blob/master/src/osd/PGLog.cc#L493
however, because the 1 from 6,1 compares lower than the 2 of 3,2 ( lower_bound ), the loop stops
https://github.com/ceph/ceph/blob/master/src/osd/PGLog.cc#L490
and divergent is empty. 

I am missing something and a hint would be much appreciated :-)

Cheers

-- 
Loïc Dachary, Artisan Logiciel Libre
All that is necessary for the triumph of evil is that good people do nothing.




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Re: PGLog::merge_log clarification

[Samuel Just]

Within a single log, the version must be always increasing.  The
reason for the epoch is that you might have two logs:

   a       b
(1, 1) (1, 1)
(1, 2) (1, 2)
(1, 3)
        (2, 3)
        (2, 4)

If a dies after persisting (1, 3) but before sending it to b, b might
become primary and go on accepting writes (2, 3) and (2, 4).  Later, a
comes back and we determine that (1, 3) is divergent using the epoch
numbers.
-Sam

On Tue, Jun 4, 2013 at 1:18 PM, Loic Dachary <loic@dachary.org> wrote:
> Hi Sam,
>
> When calling merge_log on the following : http://pastealacon.com/32449 ( where the first column are the versions of the log entries and the second column are the versions of the olog entries ) I expect 5,2 and 6,1 from log to be added to divergent
> https://github.com/ceph/ceph/blob/master/src/osd/PGLog.cc#L493
> however, because the 1 from 6,1 compares lower than the 2 of 3,2 ( lower_bound ), the loop stops
> https://github.com/ceph/ceph/blob/master/src/osd/PGLog.cc#L490
> and divergent is empty.
>
> I am missing something and a hint would be much appreciated :-)
>
> Cheers
>
> --
> Loïc Dachary, Artisan Logiciel Libre
> All that is necessary for the triumph of evil is that good people do nothing.
>
>
>
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