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Re: Streamy FS writes found detrimental.

From: Daniel Berlin <dan_at_dberlin.org>
Date: 2002-02-26 21:11:21 CET

On Tue, 2002-02-26 at 15:02, cmpilato@collab.net wrote:
> Daniel Berlin <dan@dberlin.org> writes:
> > Up the log buffer size, and the cache size, or yer gonna get screwed.
> > Royally most likely.
> Okay, I keep seeing comments like this, and I'm confused as to why
> they'll help. So I'll have fewer logfile of larger sizes, right?

The log buffer is what it buffers to *before* writing out the log file.
If you are doing a ton of logging operations, increasing the log buffer
will make it hit the disk a lot less.

The db cache is used for both reads and writes, much like an OS's buffer
cache does.
Because of how our db operations are performed, if the cache hit rate is
below <90%, your performance will drop through the floor.

The DB cache defaults to 256k. This used to give us just passable hit
rates (89% on the strings file). If we are doing more writes now, it's
going to force a lot more pages from the cache, and our cache hit rate
is probably much lower.

So the cache size needs to be increased.

> total sum of log operations written is going to be the same
> regardless, right?
Yes, but it's *when* it writes them that matters.
If it has to flush because the log buffer is full, everything waits.
If it's doing it while committing a transaction, it's writing to the
disk anyway, so it's nowhere near as costly.

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Received on Sat Oct 21 14:37:10 2006

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