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Re: Maximum Possible Insert Performance?

From: William Yu <wyu(at)talisys(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Maximum Possible Insert Performance?
Date: 2003-11-26 16:44:59
Message-ID: bq2la9$2jh1$1@news.hub.org (view raw or flat)
Thread:
Lists: pgsql-performance
Josh Berkus wrote:
> William,
> 
> 
>>When my current job batch is done, I'll save a copy of the dir and give
>>the WAL on ramdrive a test. And perhaps even buy a Sandisk at the local
>>store and run that through the hooper.
> 
> 
> We'll be interested in the results.   The Sandisk won't be much of a 
> performance test; last I checked, their access speed was about 1/2 that of a 
> fast SCSI drive.   But it could be a feasability test for the more expensive 
> RAMdrive approach.

Some initial numbers. I simulated a CPU increase by underclocking the 
processors. Most of the time, performance does not scale linearly with 
clock speed but since I also underclocked the FSB and memory bandwidth 
with the CPU, it's nearly an exact match.

1.15GHz    6.14
1.53GHz    6.97 +33% CPU = +13.5% performance

I then simulated adding a heapload of extra memory by running my job a 
second time. Unfortunately, to keep my 25GB DB mostly cached in memory, 
the word heapload is too accurate.

Run 1      6.97
Run 2      7.99 +14%

I popped in an extra IDE hard drive to store the WAL files and that 
boosted the numbers by a little. From looking at iostat, the ratio 
looked like 300K/s WAL for 1MB/s data.

WAL+Data on same disk    6.97
WAL+Data separated       7.26 +4%

I then tried to put the WAL directory onto a ramdisk. I turned off 
swapping, created a tmpfs mount point and copied the pg_xlog directory 
over. Everything looked fine as far as I could tell but Postgres just 
panic'd with a "file permissions" error. Anybody have thoughts to why 
tmpfs would not work?


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