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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