On 01/09/2014 02:01 AM, Jim Nasby wrote:
> On 1/8/14, 6:05 PM, Tom Lane wrote:
>> Josh Berkus<j...@agliodbs.com>  writes:
>>> >On 01/08/2014 03:27 PM, Tom Lane wrote:
>>>> >>What we lack, and should work on, is a way for sync mode to have
>>>> M larger
>>>> >>than one.  AFAICS, right now we'll report commit as soon as
>>>> there's one
>>>> >>up-to-date replica, and some high-reliability cases are going to
>>>> want
>>>> >>more.
>>> >"Sync N times" is really just a guarantee against data loss as long as
>>> >you lose N-1 servers or fewer.  And it becomes an even
>>> >lower-availability solution if you don't have at least N+1 replicas.
>>> >For that reason, I'd like to see some realistic actual user demand
>>> >before we take the idea seriously.
>> Sure.  I wasn't volunteering to implement it, just saying that what
>> we've got now is not designed to guarantee data survival across failure
>> of more than one server.  Changing things around the margins isn't
>> going to improve such scenarios very much.
>> It struck me after re-reading your example scenario that the most
>> likely way to figure out what you had left would be to see if some
>> additional system (think Nagios monitor, or monitors) had records
>> of when the various database servers went down.  This might be
>> what you were getting at when you said "logging", but the key point
>> is it has to be logging done on an external server that could survive
>> failure of the database server.  postmaster.log ain't gonna do it.
> Yeah, and I think that the logging command that was suggested allows
> for that *if configured correctly*.
*But* for relying on this, we would also need to make logging
which would probably not go down well with many people, as it makes things
even more fragile from availability viewpoint (and slower as well).


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

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