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

From: "fkater(at)googlemail(dot)com" <fkater(at)googlemail(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Inserting 8MB bytea: just 25% of disk perf used?
Date: 2010-01-14 14:29:03
Message-ID: 20100114142903.GB2973@comppasch2 (view raw or flat)
Thread:
Lists: pgsql-performance
Hello together,

I need to increase the write performance when inserting
bytea of 8MB. I am using 8.2.4 on windows with libpq.

The test setting is simple:

I write 100x times a byte array (bytea) of 8 MB random data
into a table having a binary column (and oids and 3 other
int columns, oids are indexed). I realized that writing 8 MB
of 0-bytes is optimized away. With random data, the disk
space now is filled with 800MB each run as expected. I use a
transaction around the insert command.

This takes about 50s, so, 800MB/50s = 16MB/s.

However the harddisk (sata) could write 43 MB/s in the worst
case! Why is write performance limited to 16 MB/s?


Some more hints what I do:

I use PQexecParams() and the INSERT ... $001 notation to NOT
create a real escapted string from the data additionally but
use a pointer to the 8MB data buffer.

I altered the binary column to STORAGE EXTERNAL.

Some experiments with postgresql.conf (fsync off,
shared_buffers=1000MB, checkpoint_segments=256) did not
change the 50s- much (somtimes 60s sometimes a little less).

4 Core CPU 3 Ghz, WinXP, 1 TB SATA disk.


Do you have any further idea why 16MB/s seems to be the
limit here?

Thank You
 Felix



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Next:From: Ivan VorasDate: 2010-01-14 14:49:16
Subject: Re: Inserting 8MB bytea: just 25% of disk perf used?
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