Following on to my blog on the Five Minute Rule and in-memory databases here I decided to quickly and informally recalculate the 4KB break-even point based on current technology (rather than use the 2007 numbers) The results are as follows:
- A 1TB SATA Disk with 4.2ms average latency and 126MB/s max transfer rate costs $100 here
- A 4GB DDR3 ECC memory card costs $33 here (I picked fairly expensive ECC memory… I could have gone with the $18 average price mentioned here)
- Apply the Gray/Putzolu formula: Break Even Interval = (Pages per MB of RAM/Accesses per Second per Disk) * ($ per Disk Drive/$ per MB of RAM)
And we find that today the break-even point for a 4KB block of data is 55 minutes…
Again… this means that for any 4KB block of data… or for any database table where there are 4KB blocks that are touched… within a 55 minute window it is more cost-effective to keep the data in-memory than to move it back and forth from disk. If the data is compressed the duration increases with the compression so that a table with 2X compression should reside in-memory if accessed on the average every 110 minutes.