Estimating hot data size

[Chris Hoover <revoohc@xxxxxxxxx>]

All,

I'm trying to estimate the size of my hot data set, and wanted to get some validation that I'm doing this correctly.

Basically, I'm using the sum(heap_blks_read + idx_blks_read) from pg_statio_all_tables, and diffing the numbers over a period of time (1 hour at least).  Is this a fair estimate?  The reason for doing this is we are looking at new server hardware, and I want to try and get enough ram on the machine to keep the hot data in memory plus provide room for growth.

Thanks,

Chris

Example:

Time	Total Blocks
2011-02-16 11:25:34.621874-05	123,260,464,427.00

2011-02-16 12:25:46.486719-05

123,325,880,943.00

To get the hot data for this hour (in KB), I'm taking:

 (123,325,880,943.00 - 123,260,464,427.00)* 8 = 523,332,128KB

Correct?