Horton

Re: Standalone synchronous master

MauMau <maumau307@gmail.com>

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From: "MauMau" <maumau307@gmail.com>
To: "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: 2014-01-09T12:57:42Z
Lists: pgsql-hackers 
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).

The cost is reasonable with RAID-1.  The sync rep requires high cost to get 
both durability and availability --- three servers.

Am I expecting too much?


Regards
MauMau