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Re: cheaper snapshots

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Hannu Krosing <hannu(at)2ndquadrant(dot)com>
Cc: Ants Aasma <ants(dot)aasma(at)eesti(dot)ee>, Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, pgsql-hackers(at)postgresql(dot)org, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: cheaper snapshots
Date: 2011-07-29 14:23:27
Message-ID: CA+TgmoaM=_9mm9-PFsNU6NTgfbF=CEeNZcaXFXZLxOXwtg256w@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Fri, Jul 29, 2011 at 10:20 AM, Hannu Krosing <hannu(at)2ndquadrant(dot)com> wrote:
>> An additional point to think about: if we were willing to insist on
>> streaming replication, we could send the commit sequence numbers via a
>> side channel rather than writing them to WAL, which would be a lot
>> cheaper.
>
> Why do you think that side channel is cheaper than main WAL ?

You don't have to flush it to disk, and you can use some other lock
that isn't as highly contended as WALInsertLock to synchronize it.

>> That might even be a reasonable thing to do, because if
>> you're doing log shipping, this is all going to be super-not-real-time
>> anyway.
>
> But perhaps you still may want to preserve visibility order to be able
> to do PITR to exact transaction "commit", no ?

Maybe.  In practice, I suspect most people won't be willing to pay the
price a feature like this would exact.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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