Alex.
On 6/15/06, Mark Lewis <mark.lewis@xxxxxxxx> wrote:
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
---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
