five minutes rules - the price of one access of disc I/O

This is very interesting topic, they use following formula to compute access interval time:

BreakEvenIntervalinSeconds = (PagesPerMBofRAM / AccessesPerSecondPerDisk) × (PricePerDiskDrive / PricePerMBofRAM).

It is derived using formulas for the cost of RAM to hold a page in the buffer pool and the cost of a (fractional) disk to perform I/O every time a page is needed, equating these two costs, and solving the equation for the interva开发者_如何学运维l between accesses.

so the cost of disc I/O per access is PricePerDiskDrive / AccessesPerSecondPerDisk, My question is why disc I/O cost per access is computed like this?

The underlying assumption is that the limit to the life of a disk is how many disk seeks there are, while RAM has a fixed cost for its size, and a fixed lifetime regardless of how often it is accessed. This is reasonable because seeking to disk causes physical wear and tear, and when the disk goes, you lose the whole disk. By contrast RAM has no physical moving parts, and so does not wear out with use.

With that assumption, the cost of keeping data on disk depends on the frequency of access and the cost of the disk. The cost of keeping data in RAM depends on how much RAM you're using. What they are trying to find is the break even point between where it is cheaper to keep data on disk or in RAM.

However the equation as given is incomplete. While that equation identifies relevant factors, there is an important constant of proportionality missing. How many accesses can the average hard drive sustain? How long does RAM last on average? Those enter into the costs for keeping data on hard drives and RAM, and without them you are comparing apples and oranges.

This is indicative of my impression of the whole paper. It says a lot at great length, about an important topic, but the analysis is sloppy. They are slopping and leave critical things out, and don't do enough to help people understand what they are thinking and when their analysis is appropriate what you are doing. For instance if you are trying to maintain a low latency system, you have to keep all of your data in RAM. Period. If you're processing large data sets and don't want to pay to keep it all in RAM, then you will be streaming data to/from disk. If you're keeping data in a redundant format, for instance RAID, you are doing more seeks per read than they admit.