Due to an error during an update to the system kernel on a database server backing a web application, we ran for a month (mid-December to mid-January) with the WAL files in the pg_xlog subdirectory on the same 40-spindle array as the data -- only the OS was on a separate mirrored pair of drives. When we found the problem, we moved the pg_xlog directory back to its own mirrored pair of drives and symlinked to it. It's a pretty dramatic difference. Graph attached, this is response time for a URL request (like what a user from the Internet would issue) which runs 15 queries and formats the results. Response time is 85 ms without putting WAL on its own RAID; 50 ms with it on its own RAID. This is a real-world, production web site with 1.3 TB data, millions of hits per day, and it's an active replication target 24/7. Frankly, I was quite surprised by this, since some of the benchmarks people have published on the effects of using a separate RAID for the WAL files have only shown a one or two percent difference when using a hardware RAID controller with BBU cache configured for write-back. No assistance needed; just posting a performance data point. -Kevin
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