On Thu, 2006-06-15 at 18:24 -0400, Tom Lane wrote: > I agree with Brian's suspicion that the SATA drive isn't properly > fsync'ing to disk, resulting in bogusly high throughput. However, > ISTM a well-configured SAN ought to be able to match even the bogus > throughput, because it should be able to rely on battery-backed > cache to hold written blocks across a power failure, and hence should > be able to report write-complete as soon as it's got the page in cache > rather than having to wait till it's really down on magnetic platter. > Which is what the SATA drive is doing ... only it can't keep the promise > it's making for lack of any battery backup on its on-board cache. It really depends on your SAN RAID controller. We have an HP SAN; I don't remember the model number exactly, but we ran some tests and with the battery-backed write cache enabled, we got some improvement in write performance but it wasn't NEARLY as fast as an SATA drive which lied about write completion. The write-and-fsync latency was only about 2-3 times better than with no write cache at all. So I wouldn't assume that just because you've got a write cache on your SAN, that you're getting the same speed as fsync=off, at least for some cheap controllers. -- Mark Lewis
