Re: [HACKERS] Sync vs. fsync during checkpoint

Tom Lane wrote:

> "Zeugswetter Andreas SB SD" <ZeugswetterA(at)spardat(dot)at> writes:
>> So Imho the target should be to have not much IO open for the checkpoint,
>> so the fsync is fast enough, even if serial.
>
> The best we can do is push out dirty pages with write() via the bgwriter
> and hope that the kernel will see fit to write them before checkpoint
> time arrives. I am not sure if that hope has basis in fact or if it's
> just wishful thinking. Most likely, if it does have basis in fact it's
> because there is a standard syncer daemon forcing a sync() every thirty
> seconds.

Looking at the response time charts I did for showing how vacuum delay
is doing, it seems at least on Linux there is hope that that is the
case. Those charts have just a regular 5 minute checkpoint with enough
checkpoint segments for that, and no other sync effort done at all.

The system has a hard time to handle a larger scaled test DB, so it is
definitely well saturated with IO. The charts are here:

http://developer.postgresql.org/~wieck/vacuum_cost/

>
> That means that instead of an I/O storm every checkpoint interval,
> we get a smaller I/O storm every 30 seconds. Not sure this is a big
> improvement. Jan already found out that issuing very frequent sync()s
> isn't a win.

In none of those charts I can see any checkpoint caused IO storm any
more. Charts I'm currently doing for 7.4.1 show extremely clear spikes
at checkpoints. If someone is interested in those as well I will put
them up.

Jan

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