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>, Hannu Krosing <hannu@2ndQuadrant.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: 2014-01-09T15:55:22Z
Lists: pgsql-hackers 
On 01/09/2014 04:15 PM, MauMau wrote:
> From: "Hannu Krosing" <hannu@2ndQuadrant.com>
>> On 01/09/2014 01:57 PM, MauMau wrote:
>>> 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.
>
> Really?  RAID-1 is a-sync?
Not exactly, as there is no "master" just controller writing to two
equal disks.

But having a "degraded" mode makes it
more like async - it continues even with single disk and syncs later if
and when the 2nd disk comes back.

Cheers

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