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Rick Heiges

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All of us have heard that if you just add memory to your system, it will work faster.  This is generally accepted and it makes alot of sense, but what is the math behind it?

About 2 years ago, I heard the presenter of a webcast that I was viewing put these two elements in a perspective that I relay on to others when speaking about memory.  Basically, Disk Access is measured in milliseconds; RAM Access is measured in nanoseconds.  There are 1 million nanoseconds in 1 millisecond.  In other words the magnitude of access time is on the order of 1 million; that's pretty big.  In real human time, I can't distinguish between a nanosecond and a millisecond, but there is a HUGE difference. 

Just about everyone can look at their watch or a clock and know how long a second takes.  If you sit and stare at that timepiece until 1 million seconds tick by, you will stare at it for 11.57 days!

(1 million seconds / (60 seconds) / (60 minutes) / (24 hours)) = 11.57 Days

As you can see, there is a HUGE difference in the access speeds between IO and RAM.  We all knew this inherently, but sometimes an example can really help - especially when budgeting season is imminent.

Published Monday, December 3, 2007 3:31 PM by RickHeiges
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jchang said:

its a little more complicated than this

raw memory access, ie, to get a single word (up to 8 bytes) takes 50-60ns on a single processor Opteron, because it has an integrated memory controller,

it takes 100-120ns on Intel systems because memory access must go through a memory controller

now I am talking about round-trip time, ie, the processor issues a memory request, and memory returns it to the processor. people frequently talk about MT/s, which is the time between each transfer, which is different,

however, there is a major difference between main memory and storage,

by the a solid state storage system is implement, the time to access a block, the same as accessing a disk block, the access time is now in the range of 30-100 microseconds,

this is still much faster the 15K disk random access of 5 milli-sec

disk drives have perfectly good sequential transfer rates,

so solid state storage is really good for super high random IO over a reasonable size

December 20, 2007 10:30 PM

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