From: | Andres Freund <andres(at)2ndquadrant(dot)com> |
---|---|
To: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> |
Cc: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Rajeev rastogi <rajeev(dot)rastogi(at)huawei(dot)com>, Ants Aasma <ants(at)cybertec(dot)at>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Markus Wanner <markus(at)bluegap(dot)ch> |
Subject: | Re: Proposal for CSN based snapshots |
Date: | 2014-05-30 15:27:21 |
Message-ID: | 20140530152721.GC30516@awork2.anarazel.de |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-hackers |
Hi,
On 2014-05-30 17:59:23 +0300, Heikki Linnakangas wrote:
> So, here's a first version of the patch. Still very much WIP.
Cool.
> One thorny issue came up in discussions with other hackers on this in PGCon:
>
> When a transaction is committed asynchronously, it becomes visible to other
> backends before the commit WAL record is flushed. With CSN-based snapshots,
> the order that transactions become visible is always based on the LSNs of
> the WAL records. This is a problem when there is a mix of synchronous and
> asynchronous commits:
>
> If transaction A commits synchronously with commit LSN 1, and transaction B
> commits asynchronously with commit LSN 2, B cannot become visible before A.
> And we cannot acknowledge B as committed to the client until it's visible to
> other transactions. That means that B will have to wait for A's commit
> record to be flushed to disk, before it can return, even though it was an
> asynchronous commit.
> I personally think that's annoying, but we can live with it. The most common
> usage of synchronous_commit=off is to run a lot of transactions in that
> mode, setting it in postgresql.conf. And it wouldn't completely defeat the
> purpose of mixing synchronous and asynchronous commits either: an
> asynchronous commit still only needs to wait for any already-logged
> synchronous commits to be flushed to disk, not the commit record of the
> asynchronous transaction itself.
I have a hard time believing that users won't hate us for such a
regression. It's pretty common to mix both sorts of transactions and
this will - by my guesstimate - dramatically reduce throughput for the
async backends.
> * Logical decoding is broken. I hacked on it enough that it looks roughly
> sane and it compiles, but didn't spend more time to debug.
I think we can live with it not working for the first few
iterations. I'll look into it once the patch has stabilized a bit.
> * I expanded pg_clog to 64-bits per XID, but people suggested keeping
> pg_clog as is, with two bits per commit, and adding a new SLRU for the
> commit LSNs beside it. Probably will need to do something like that to avoid
> bloating the clog.
It also influences how on-disk compatibility is dealt with. So: How are
you planning to deal with on-disk compatibility?
> * Add some kind of backend-private caching of clog, to make it faster to
> access. The visibility checks are now hitting the clog a lot more heavily
> than before, as you need to check the clog even if the hint bits are set, if
> the XID falls between xmin and xmax of the snapshot.
That'll hurt a lot in concurrent scenarios :/. Have you measured how
'wide' xmax-xmin usually is? I wonder if we could just copy a range of
values from the clog when we start scanning....
Greetings,
Andres Freund
--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
From | Date | Subject | |
---|---|---|---|
Next Message | Marc Mamin | 2014-05-30 15:31:48 | "pivot aggregation" with a patched intarray |
Previous Message | Heikki Linnakangas | 2014-05-30 15:14:43 | Re: Proposal for CSN based snapshots |