To supplement Jared's response... Inline Peter Teoh wrote:
Unlike memory, where access time almost zero, accessing the disk is much slower. So sometimes I thought it will be better to spread out the data - thus enable simultaneous read by the different heads in the disk, just like those RAID design. Ie, fragmentation via distribution the blocks out in the disk can improve performance - can such things happened?
In terms of reading data in memory, access time may be almost zero, but the time required to read a block in memory is not thousands of times faster than reading it from disk. I don't recall the exact figure from a Cary Millsap presentation ("Why You Should Focus on LIOs"?)some years ago, but the difference was something like 42 times. If you add in time required to create a read consistent version, a single logical i/o could take much more time than a physical i/o.
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