Re: splitting data into multiple tables

On Tue, Jan 26, 2010 at 11:11 PM, Greg Smith <greg(at)2ndquadrant(dot)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
> greg(at)2ndQuadrant(dot)com www.2ndQuadrant.com
>
>