Re: [HACKERS] Quorum commit for multiple synchronous replication. - Mailing list pgsql-hackers

On Wed, Apr 12, 2017 at 2:36 AM, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> On Thu, Apr 6, 2017 at 4:17 PM, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>> On Thu, Apr 6, 2017 at 10:51 AM, Noah Misch <noah@leadboat.com> wrote:
>>> On Thu, Apr 06, 2017 at 12:48:56AM +0900, Fujii Masao wrote:
>>>> On Wed, Apr 5, 2017 at 3:45 PM, Noah Misch <noah@leadboat.com> wrote:
>>>> > On Mon, Dec 19, 2016 at 09:49:58PM +0900, Fujii Masao wrote:
>>>> >> Regarding this feature, there are some loose ends. We should work on
>>>> >> and complete them until the release.
>>>> >>
>>>> >> (1)
>>>> >> Which synchronous replication method, priority or quorum, should be
>>>> >> chosen when neither FIRST nor ANY is specified in s_s_names? Right now,
>>>> >> a priority-based sync replication is chosen for keeping backward
>>>> >> compatibility. However some hackers argued to change this decision
>>>> >> so that a quorum commit is chosen because they think that most users
>>>> >> prefer to a quorum.
>>>> >>
>>>> >> (2)
>>>> >> There will be still many source comments and documentations that
>>>> >> we need to update, for example, in high-availability.sgml. We need to
>>>> >> check and update them throughly.
>>>> >>
>>>> >> (3)
>>>> >> The priority value is assigned to each standby listed in s_s_names
>>>> >> even in quorum commit though those priority values are not used at all.
>>>> >> Users can see those priority values in pg_stat_replication.
>>>> >> Isn't this confusing? If yes, it might be better to always assign 1 as
>>>> >> the priority, for example.
>>>> >
>>>> > [Action required within three days.  This is a generic notification.]
>>>> >
>>>> > The above-described topic is currently a PostgreSQL 10 open item.  Fujii,
>>>> > since you committed the patch believed to have created it, you own this open
>>>> > item.  If some other commit is more relevant or if this does not belong as a
>>>> > v10 open item, please let us know.  Otherwise, please observe the policy on
>>>> > open item ownership[1] and send a status update within three calendar days of
>>>> > this message.  Include a date for your subsequent status update.  Testers may
>>>> > discover new open items at any time, and I want to plan to get them all fixed
>>>> > well in advance of shipping v10.  Consequently, I will appreciate your efforts
>>>> > toward speedy resolution.  Thanks.
>>>> >
>>>> > [1] https://www.postgresql.org/message-id/20170404140717.GA2675809%40tornado.leadboat.com
>>>>
>>>> Thanks for the notice!
>>>>
>>>> Regarding the item (2), Sawada-san told me that he will work on it after
>>>> this CommitFest finishes. So we would receive the patch for the item from
>>>> him next week. If there will be no patch even after the end of next week
>>>> (i.e., April 14th), I will. Let's wait for Sawada-san's action at first.
>>>
>>> Sounds reasonable; I will look for your update on 14Apr or earlier.
>>>
>>>> The items (1) and (3) are not bugs. So I don't think that they need to be
>>>> resolved before the beta release. After the feature freeze, many users
>>>> will try and play with many new features including quorum-based syncrep.
>>>> Then if many of them complain about (1) and (3), we can change the code
>>>> at that timing. So we need more time that users can try the feature.
>>>
>>> I've moved (1) to a new section for things to revisit during beta.  If someone
>>> feels strongly that the current behavior is Wrong and must change, speak up as
>>> soon as you reach that conclusion.  Absent such arguments, the behavior won't
>>> change.
>>>
>>>> BTW, IMO (3) should be fixed so that pg_stat_replication reports NULL
>>>> as the priority if quorum-based sync rep is chosen. It's less confusing.
>>>
>>> Since you do want (3) to change, please own it like any other open item,
>>> including the mandatory status updates.
>>
>> I agree to report NULL as the priority. I'll send a patch for this as well.
>>
>> Regards,
>>
>
> Attached two draft patches. The one makes pg_stat_replication.sync
> priority report NULL if in quorum-based sync replication. To prevent
> extra change I don't change so far the code of setting standby
> priority. The another one improves the comment and documentation. If
> there is more thing what we need to mention in documentation please
> give me feedback.

Attached is the modified version of the doc improvement patch.
Barring any objection, I will commit this version.

+    In term of performance there is difference between two synchronous
+    replication method. Generally quorum-based synchronous replication
+    tends to be higher performance than priority-based synchronous
+    replication. Because in quorum-based synchronous replication, the
+    transaction can resume as soon as received the specified number of
+    acknowledgement from synchronous standby servers without distinction
+    of standby servers. On the other hand in priority-based synchronous
+    replication, the standby server that the primary server must wait for
+    is fixed until a synchronous standby fails. Therefore, if a server on
+    low-performance machine a has high priority and is chosen as a
+    synchronous standby server it can reduce performance for database
+    applications.

This description looks misleading. A quorum-based sync rep is basically
more efficient when there are multiple standbys in s_s_names and you want
to replicate the transactions to some of them synchronously. I think that
this assumption should be documented explicitly. So I modified this
description. Please see the modified version in the attached patch.

+ /*
+ * Update priority of this WalSender, but note that in
+ * quroum-based sync replication, the value of
+ * sync_standby_priority has no effect.
+ */

This is not true because even quorum-based sync rep uses the priority
value to check whether the standby is async or sync. So I just remove this.

+ * In quorum-based sync replication we select the quorum sync
+ * standby without theirs priority. The all running active standbys
+ * are considered as a candidate for quorum sync standbys

Same as above.

Also I removed some descriptions that I thought unnecessary to add.

Regards,

-- 
Fujii Masao

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