Viji V Nair wrote:
> A 15k rpm SAS drive will give you a throughput of 12MB and 120 IOPS.
> Now you can calculate the number of disks, specifically spindles, for
> getting your desired throughput and IOPs
I think you mean 120MB/s for that first part. Regardless, presuming you
can provision a database just based on IOPS rarely works. It's nearly
impossible to estimate what you really need anyway for a database app,
given that much of real-world behavior depends on the cached in memory
vs. uncached footprint of the data you're working with. By the time you
put a number of disks into an array, throw a controller card cache on
top of it, then add the OS and PostgreSQL caches on top of those, you
are so far disconnected from the underlying drive IOPS that speaking in
those terms doesn't get you very far. I struggle with this every time I
talk with a SAN vendor. Their fixation on IOPS without considering
things like how sequential scans mixed into random I/O will get handled
is really disconnected from how databases work in practice. For
example, I constantly end up needing to detune IOPS in favor of
readahead to make "SELECT x,y,z FROM t" run at an acceptable speed on
big tables.
--
Greg Smith 2ndQuadrant Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com www.2ndQuadrant.com