Horton

Re: Standalone synchronous master

Hannu Krosing <hannu@2ndquadrant.com>

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From: Hannu Krosing <hannu@2ndQuadrant.com>
To: MauMau <maumau307@gmail.com>, Andres Freund <andres@2ndquadrant.com>, "Joshua D. Drake" <jd@commandprompt.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Kevin Grittner <kgrittn@ymail.com>, Bruce Momjian <bruce@momjian.us>, Heikki Linnakangas <hlinnakangas@vmware.com>, Simon Riggs <simon@2ndQuadrant.com>, Rajeev rastogi <rajeev.rastogi@huawei.com>, pgsql-hackers@postgresql.org
Date: 2014-01-09T14:33:06Z
Lists: pgsql-hackers 
On 01/09/2014 01:57 PM, MauMau wrote:
> From: "Andres Freund" <andres@2ndquadrant.com>
>> On 2014-01-08 14:42:37 -0800, Joshua D. Drake wrote:
>>> If we have the following:
>>>
>>> db0->db1:down
>>>
>>> Using the model (as I understand it) that is being discussed we have
>>> increased our failure rate because the moment db1:down we also lose
>>> db0. The
>>> node db0 may be up but if it isn't going to process transactions it is
>>> useless. I can tell you that I have exactly 0 customers that would
>>> want that
>>> model because a single node failure would cause a double node failure.
>>
>> That's why you should configure a second standby as another (candidate)
>> synchronous replica, also listed in synchronous_standby_names.
>
> Let me ask a (probably) stupid question.  How is the sync rep
> different from RAID-1?
>
> When I first saw sync rep, I expected that it would provide the same
> guarantees as RAID-1 in terms of durability (data is always mirrored
> on two servers) and availability (if one server goes down, another
> server continues full service).
What you describe is most like A-sync rep.

Sync rep makes sure that data is always replicated before confirming to
writer.


Cheers

-- 
Hannu Krosing
PostgreSQL Consultant
Performance, Scalability and High Availability
2ndQuadrant Nordic OÜ