Re: Quorum commit for multiple synchronous replication.

On Mon, Nov 28, 2016 at 8:03 PM, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> On Sat, Nov 26, 2016 at 10:27 PM, Michael Paquier
> <michael.paquier@gmail.com> wrote:
>> On Tue, Nov 15, 2016 at 7:08 PM, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>>> Attached latest version patch incorporated review comments. After more
>>> thought, I agree and changed the value of standby priority in quorum
>>> method so that it's not set 1 forcibly. The all standby priorities are
>>> 1 If s_s_names = 'ANY(*)'.
>>> Please review this patch.
>>
>> Sorry for my late reply. Here is my final lookup.
>
> Thank you for reviewing!
>
>>  <synopsis>
>> -<replaceable class="parameter">num_sync</replaceable> ( <replaceable
>> class="parameter">standby_name</replaceable> [, ...] )
>> +[ANY] <replaceable class="parameter">num_sync</replaceable> (
>> <replaceable class="parameter">standby_name</replaceable> [, ...] )
>> +FIRST <replaceable class="parameter">num_sync</replaceable> (
>> <replaceable class="parameter">standby_name</replaceable> [, ...] )
>>  <replaceable class="parameter">standby_name</replaceable> [, ...
>> This can just be replaced with [ ANY | FIRST ]. There is no need for
>> braces as the keyword is not mandatory.
>>
>> +        is the name of a standby server.
>> +        <literal>FIRST</> and <literal>ANY</> specify the method used by
>> +        the master to control the standby servres.
>>          </para>
>> s/servres/servers/.
>>
>>             if (priority == 0)
>>                 values[7] = CStringGetTextDatum("async");
>>             else if (list_member_int(sync_standbys, i))
>> -               values[7] = CStringGetTextDatum("sync");
>> +               values[7] = SyncRepConfig->sync_method == SYNC_REP_PRIORITY ?
>> +                   CStringGetTextDatum("sync") : CStringGetTextDatum("quorum");
>>             else
>>                 values[7] = CStringGetTextDatum("potential");
>> This can be simplified a bit as "quorum" is the state value for all
>> standbys with a non-zero priority when the method is set to
>> SYNC_REP_QUORUM:
>>             if (priority == 0)
>>                 values[7] = CStringGetTextDatum("async");
>> +           else if (SyncRepConfig->sync_method == SYNC_REP_QUORUM)
>> +               values[7] = CStringGetTextDatum("quorum");
>>             else if (list_member_int(sync_standbys, i))
>>                 values[7] = CStringGetTextDatum("sync");
>>             else
>>
>> SyncRepConfig data is made external to syncrep.c with this patch as
>> walsender.c needs to look at the sync method in place, no complain
>> about that after considering if there could be a more elegant way to
>> do things without this change.
>
> Agreed.
>
>> While reviewing the patch, I have found a couple of incorrectly shaped
>> sentences, both in the docs and some comments. Attached is a new
>> version with this word-smithing. The patch is now switched as ready
>> for committer.
>
> Thanks. I found a typo in v7 patch, so attached latest v8 patch.

+        qsort(write_array, len, sizeof(XLogRecPtr), cmp_lsn);
+        qsort(flush_array, len, sizeof(XLogRecPtr), cmp_lsn);
+        qsort(apply_array, len, sizeof(XLogRecPtr), cmp_lsn);

In quorum commit, we need to calculate the N-th largest LSN from
M quorum synchronous standbys' LSN. N would be usually 1 - 3 and
M would be 1 - 10, I guess. You used the algorithm using qsort for
that calculation. But I'm not sure if that's enough effective algorithm
or not.

If M (i.e., number of quorum sync standbys) is enough large,
your choice would be good. But usually M seems not so large.

Thought?

Regards,

-- 
Fujii Masao