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Re: Async Commit, v21 (now: v22)

From: Florian Weimer <fweimer(at)bfk(dot)de>
To: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
Cc: "Alvaro Herrera" <alvherre(at)commandprompt(dot)com>, "Peter Eisentraut" <peter_e(at)gmx(dot)net>, "Gregory Stark" <stark(at)enterprisedb(dot)com>, "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com>, "Bruce Momjian" <bruce(at)momjian(dot)us>, <pgsql-patches(at)postgresql(dot)org>
Subject: Re: Async Commit, v21 (now: v22)
Date: 2007-07-24 08:51:03
Message-ID: 82k5sqkww8.fsf@mid.bfk.de (view raw or flat)
Thread:
Lists: pgsql-patches
+    <para>
+     Asynchronous commit provides different behaviour to setting 
+     <varname>fsync</varname> = off, which is a server-wide
+     setting that will alter the behaviour of all transactions,
+     overriding the setting of <varname>synchronous_commit</varname>,
+     as well as risking much wider data loss.  With <varname>fsync</varname> 
+ 	= off the WAL written but not fsynced, so data is lost only in case
+ 	of a system crash. Both WAL and datafiles written within the last 
+ 	few seconds would be at risk, affecting all types of database 
+ 	transactions. The precise number depends upon whether your operating 
+ 	system is configured for automatic filesystem writeback and what the 
+ 	delay is set too; Linux currently defaults to 30 seconds.  With 
+ 	asynchronous commit the WAL is not written to disk at all at commit 
+ 	time, so data is lost if there is a database server crash, whether or
+ 	not the system crashes at the same time.
+    </para>

I think fsync=off also endagers metadata, while synchronous_commit=off
should be perfectly safe as far as the metadata is concerned.
Wouldn't this be worth mentioning as well?


-- 
Florian Weimer                <fweimer(at)bfk(dot)de>
BFK edv-consulting GmbH       http://www.bfk.de/
Kriegsstra├če 100              tel: +49-721-96201-1
D-76133 Karlsruhe             fax: +49-721-96201-99

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Subject: Re: Async Commit, v21 (now: v22)
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