On Tue, Jan 26, 2010 at 11:11 PM, Greg Smith <g...@2ndquadrant.com> wrote:
> 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. > > Yes, you are right. There are catches in the SAN controllers also. SAN vendors wont give that much information regarding their internal controller design. They will say they have 4 external 4G ports, you should also check how many internal ports they have and the how the controllers are operating, in Active-Active or Active- Standby mode. > -- > Greg Smith 2ndQuadrant Baltimore, MD > PostgreSQL Training, Services and Support > g...@2ndquadrant.com www.2ndQuadrant.com > >
