| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
| Cc: | Simon Riggs <simon(at)2ndquadrant(dot)com>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Scaling XLog insertion (was Re: Moving more work outside WALInsertLock) |
| Date: | 2012-03-07 15:04:19 |
| Message-ID: | 7130.1331132659@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> Just to keep things in perspective -- For a commit record to reach one
> megabyte, it would have to be a transaction that drops over 43k tables.
> Or have 64k smgr inval messages (for example, a TRUNCATE might send half
> a dozen of these messages). Or have 262k subtransactions. Or
> combinations thereof.
> Now admittedly, a page is only 8 kB, so for a commit record to be "many
> pages long" (that is, >=3) it would require about 1500 smgr inval
> messages, or, say, about 250 TRUNCATEs (of permanent tables with at
> least one toastable field and at least one index).
What about the locks (if running hot-standby)?
> So they are undoubtely rare. Not sure if as rare as Higgs bosons.
Even if they're rare, having a major performance hiccup when one happens
is not a side-effect I want to see from a patch whose only reason to
exist is better performance.
regards, tom lane
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