Re: Quorum commit for multiple synchronous replication.

On Wed, Dec 7, 2016 at 4:05 PM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> wrote:
> On Wed, Dec 7, 2016 at 2:49 PM, Kyotaro HORIGUCHI
> <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>> Aside from measurement of the two sorting methods, I'd like to
>> point out that quorum commit basically doesn't need
>> sorting. Counting conforming santdbys while scanning the
>> walsender(receiver) LSN list comparing with the target LSN is
>> O(n). Small refactoring of SyncRerpGetOldestSyncRecPtr would
>> enough to do that.

What does the target LSN mean here?

> Indeed, I haven't thought about that, and that's a no-brainer. That
> would remove the need to allocate and sort each array, what is simply
> needed is to track the number of times a newest value has been found.
> So what this processing would do is updating the write/flush/apply
> values for the first k loops if the new value is *older* than the
> current one, where k is the quorum number, and between k+1 and N the
> value gets updated only if the value compared is newer. No need to
> take the mutex lock for a long time as well.

Sorry, I could not understand this algorithm. Could you elaborate
this? It takes only O(n) times?

> By the way, the patch now
> conflicts on HEAD, it needs a refresh.

Thanks, I'll post the latest patch.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center