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Re: Inserting 8MB bytea: just 25% of disk perf used?

From: "fkater(at)googlemail(dot)com" <fkater(at)googlemail(dot)com>
To: Hannu Krosing <hannu(at)2ndquadrant(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Inserting 8MB bytea: just 25% of disk perf used?
Date: 2010-01-18 16:04:20
Message-ID: 20100118160420.GF2913@comppasch2 (view raw or flat)
Thread:
Lists: pgsql-performance
Hannu Krosing:

> did you also test this with fsync=off ?

Yes. No significant difference.


> I suspect that what you are seeing is the effect of randomly writing to
> the index files. While sequential write performance can be up to
> 80MB/sec on modern drives, sequential writes are an order of magnitude
> slower. And at your data sizes you are very likely to hit a 
> CHECKPOINT, which needs to do some random writes.

Yes, from the server log I noticed that I hit checkpoints
too early and too often. I tried the astronomical value of
1000 for checkpoint_segments to not hit a single one for the
whole test run (copying 800 MB) -- even though that is no
good idea in practice of course.

It took even longer then. Probably because the server
created a lot of 16 MB log files (about 300 in my case)
which is presumly more costy (at least for the first run?)
than overwriting existing files. I am not too much into
that, though, since this is not a solution anyway on the
long run IMHO.

Thanks again.

 Felix
 


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