| From: | Ants Aasma <ants(dot)aasma(at)eesti(dot)ee> |
|---|---|
| To: | Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru> |
| Cc: | Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Dilip Kumar <dilipbalaut(at)gmail(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Greg Stark <stark(at)mit(dot)edu>, Andres Freund <andres(at)2ndquadrant(dot)com>, Rajeev rastogi <rajeev(dot)rastogi(at)huawei(dot)com>, Markus Wanner <markus(at)bluegap(dot)ch>, Ants Aasma <ants(at)cybertec(dot)at>, Bruce Momjian <bruce(at)momjian(dot)us>, obartunov <obartunov(at)postgrespro(dot)ru>, Teodor Sigaev <teodor(at)postgrespro(dot)ru> |
| Subject: | Re: Proposal for CSN based snapshots |
| Date: | 2016-08-10 17:26:55 |
| Message-ID: | CA+CSw_v_c=DJ3xG5eZkCHwng_1=LTg25DTj5WfGfuqYwg8C39A@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Wed, Aug 10, 2016 at 7:54 PM, Alexander Korotkov
<a(dot)korotkov(at)postgrespro(dot)ru> wrote:
> Oh, I found that I underestimated complexity of async commit... :)
>
> Do I understand right that now async commit right as follows?
> 1) Async transaction confirms commit before flushing WAL.
> 2) Other transactions sees effect of async transaction only when its WAL
> flushed.
> 3) In the session which just committed async transaction, effect of this
> transaction is visible immediately (before WAL flushed). Is it true?
Current code simplified:
XactLogCommitRecord()
if (synchronous_commit)
XLogFlush()
ProcArrayEndTransaction() // Become visible
The issue we are discussing is that with CSNs, the "become visible"
portion must occur in CSN order. If CSN == LSN, then async
transactions that have their commit record after a sync record must
wait for the sync record to flush and become visible. Simplest
solution is to not require CSN == LSN and just assign a CSN value
immediately before becoming visible.
Regards,
Ants Aasma
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