PGLog::rewind_divergent_log use case

PGLog::rewind_divergent_log use case

[Loic Dachary]

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[Note: ceph-devel cc'ed]

Hi Sam,

While trying to create the conditions for the following to happen in PGLog::rewind_divergent_log:

    if (p == log.log.begin()) {
      // yikes, the whole thing is divergent!
      divergent.swap(log.log);
      break;
    }

https://github.com/ceph/ceph/blob/master/src/osd/PGLog.cc#L355

I created this ( matching the drawing attached ), and the test case works. 

    {
      pg_log_entry_t e;

      info.log_tail = log.tail = eversion_t(1, 1);
      newhead = eversion_t(1, 3);
      e.version = divergent_version = eversion_t(1, 5);
      e.soid.hash = 0x9;
      divergent_object = e.soid;
      e.op = pg_log_entry_t::DELETE;
      e.prior_version = prior_version = eversion_t(0, 2);
      log.log.push_back(e);
      log.head = e.version;
    }

https://github.com/dachary/ceph/commit/d592d7f10968c56139cb470a4111ddfbe08a4030#L0R136

But it does not look like a situation that is actually going to happen. I feel something is missing 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::rewind_divergent_log use case

[Samuel Just]

I think that case may be possible, and that the assert may be wrong.
We have probably never seen a problem there due to
osd_min_pg_log_entries.  It's also possible that there is a subtle bug
between there and calc_acting().
-Sam

On Sat, Jun 8, 2013 at 1:16 AM, Loic Dachary <loic@dachary.org> wrote:
> [Note: ceph-devel cc'ed]
>
> Hi Sam,
>
> While trying to create the conditions for the following to happen in PGLog::rewind_divergent_log:
>
>     if (p == log.log.begin()) {
>       // yikes, the whole thing is divergent!
>       divergent.swap(log.log);
>       break;
>     }
>
> https://github.com/ceph/ceph/blob/master/src/osd/PGLog.cc#L355
>
> I created this ( matching the drawing attached ), and the test case works.
>
>     {
>       pg_log_entry_t e;
>
>       info.log_tail = log.tail = eversion_t(1, 1);
>       newhead = eversion_t(1, 3);
>       e.version = divergent_version = eversion_t(1, 5);
>       e.soid.hash = 0x9;
>       divergent_object = e.soid;
>       e.op = pg_log_entry_t::DELETE;
>       e.prior_version = prior_version = eversion_t(0, 2);
>       log.log.push_back(e);
>       log.head = e.version;
>     }
>
> https://github.com/dachary/ceph/commit/d592d7f10968c56139cb470a4111ddfbe08a4030#L0R136
>
> But it does not look like a situation that is actually going to happen. I feel something is missing 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::rewind_divergent_log use case

[Loic Dachary]

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Hi Sam,

I took a look at calc_acting and I can't figure out enough to create a bug report. There are checks based on last_update and tail but it is difficult go infer how it relates to the rewind_divergent_log function. If you can connect the dots for me I'd be happy to write down a proper bug report ;-)

Cheers

On 06/11/2013 01:02 AM, Samuel Just wrote:
> I think that case may be possible, and that the assert may be wrong.
> We have probably never seen a problem there due to
> osd_min_pg_log_entries.  It's also possible that there is a subtle bug
> between there and calc_acting().
> -Sam
> 
> On Sat, Jun 8, 2013 at 1:16 AM, Loic Dachary <loic@dachary.org> wrote:
>> [Note: ceph-devel cc'ed]
>>
>> Hi Sam,
>>
>> While trying to create the conditions for the following to happen in PGLog::rewind_divergent_log:
>>
>>     if (p == log.log.begin()) {
>>       // yikes, the whole thing is divergent!
>>       divergent.swap(log.log);
>>       break;
>>     }
>>
>> https://github.com/ceph/ceph/blob/master/src/osd/PGLog.cc#L355
>>
>> I created this ( matching the drawing attached ), and the test case works.
>>
>>     {
>>       pg_log_entry_t e;
>>
>>       info.log_tail = log.tail = eversion_t(1, 1);
>>       newhead = eversion_t(1, 3);
>>       e.version = divergent_version = eversion_t(1, 5);
>>       e.soid.hash = 0x9;
>>       divergent_object = e.soid;
>>       e.op = pg_log_entry_t::DELETE;
>>       e.prior_version = prior_version = eversion_t(0, 2);
>>       log.log.push_back(e);
>>       log.head = e.version;
>>     }
>>
>> https://github.com/dachary/ceph/commit/d592d7f10968c56139cb470a4111ddfbe08a4030#L0R136
>>
>> But it does not look like a situation that is actually going to happen. I feel something is missing 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.
> --
> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

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


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