Re: rationale for a PGLog::merge_old_entry case

[Loic Dachary <loic@dachary.org>]

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Hi Sam,

Thanks for the explanation. I misread the third case, my description was incorrect. I amended https://github.com/ceph/ceph/pull/340 accordingly.

Cheers

On 06/03/2013 10:28 PM, Samuel Just wrote:
> In all three cases, we know the authoritative log does not contain an
> entry for oe.soid, therefore:
> 
> If oe.prior_version > log.tail, we must already have processed an
> earlier entry for that object resulting in the object being correctly
> marked missing (or not) (specifically, the entry for
> oe.prior_version).
> 
> If log.tail >= oe.prior_version > eversion_t(), the missing entry
> should have need set at oe.prior_version (revise_need).
> oe.prior_version cannot be divergent because all divergent entries
> must fall within the log (otherwise, we would have backfilled).
> 
> If oe.prior_version == eversion_t(), the object no longer exists, and
> the object should be removed from the missing set.
> 
> Hope that helps.
> -Sam
> 
> On Sun, Jun 2, 2013 at 4:09 AM, Loic Dachary <loic@dachary.org> wrote:
>> Hi Sam,
>>
>> TL;DR:
>>
>> When there no new entry, what is the rationale for merge_old_entry to remove the object from missing only if the tail is eversion_t() and the object prior_version is also eversion_t() ?
>> https://github.com/dachary/ceph/blob/f58299db098d5f18c817b516fa6ffaa76245e57d/src/osd/PGLog.cc#L330
>>
>> Long version:
>>
>> The conditions are created with:
>>
>>     info.log_tail = eversion_t();
>>     oe.soid.hash = 1;
>>     oe.op = pg_log_entry_t::DELETE;
>>     oe.prior_version = eversion_t();
>>
>>     missing.add(oe.soid, eversion_t(1,1), eversion_t());
>>
>> as shown in
>> https://github.com/dachary/ceph/blob/f58299db098d5f18c817b516fa6ffaa76245e57d/src/test/osd/TestPGLog.cc#L467
>> I double checked with gdb and when called with
>>
>> EXPECT_FALSE(merge_old_entry(t, oe, info, remove_snap, dirty_log));
>> https://github.com/dachary/ceph/blob/f58299db098d5f18c817b516fa6ffaa76245e57d/src/test/osd/TestPGLog.cc#L481
>>
>> it reaches
>>
>> missing.rm(oe.soid, missing.missing[oe.soid].need);
>> https://github.com/dachary/ceph/blob/f58299db098d5f18c817b516fa6ffaa76245e57d/src/osd/PGLog.cc#L330
>>
>> and the expected side effects are observed:
>>
>>     EXPECT_FALSE(dirty_log);
>>     EXPECT_TRUE(remove_snap.empty());
>>     EXPECT_TRUE(t.empty());
>>     EXPECT_FALSE(missing.have_missing());
>>     EXPECT_TRUE(log.empty());
>>     EXPECT_EQ(0U, ondisklog.length());
>> https://github.com/dachary/ceph/blob/f58299db098d5f18c817b516fa6ffaa76245e57d/src/test/osd/TestPGLog.cc#L483
>>
>> Cheers
>>
>> --
>> Loïc Dachary, Artisan Logiciel Libre
>> All that is necessary for the triumph of evil is that good people do nothing.
>>

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


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