Storage Area Network as a tlug meeting topic

[Lennart Sorensen]

On Thu, Apr 30, 2009 at 01:36:02PM -0400, Lennart Sorensen wrote:
> Well remember that to access data on a harddisk, the head has to move to
> the right track (typical 3.5" SATA 7200rpm drive tends to do an average
> seek in about 10ms or so, which is of course an eternity for a computer),
> and then has to wait for the sector to rotate past the head (which takes
> 1/7200 of a second of course maximum, and on average 1/3600 of a second.)
> 
> The expensive disks running 15000rpm of course half the rotational
> delay, and tend to be 2.5" as well, which means a shorter seek distance,
> and often have better head controllers that move and settle faster,
> and often get the average seek down to the 4 to 5ms range.  So for random
> I/O those should be at least twice as fast as your typical desktop drive.
> 
> Now some people have found that the desktop drive can have better transfer
> rates because of higher areal densities and longer tracks (because they
> are 3.5" not 2.5"), and to get a very fast drive you take a modern 1TB
> drive for example, and only use the first 10% or so of the disk, which
> is at the outside (normally) and hence has the highest density of bits
> per revolution, and by only using 10% of the disk, the head movement
> now only covers about 5% of the normal distance, and the average seek
> goes way down as a result.  Sure you waste 90% of the disk, but get get
> a very fast disk as a result.
> 
> After all if you can reduce the average seek from 10ms to 3ms, and get
> a higher transfer rate when you do reach the right part of the disk,
> and you can buy such a 1TB drive (giving you 100GB effective disk)
> for $130, that's not bad.  Compared to a 73GB 15000rpm disk which has
> a lower transfer rate and similar average seek (but a lower rotational
> delay, which doesn't matter nearly as much as the seek time), costing
> probably $200 or $300.  Seems wasteful, but its an interesting method.
> You effectively turn a 1TB 7200rpm 3.5" disk into a 100GB 1.5" drive
> (except with the transfer rate of the outer part of the 3.5" drive).
> 
> I wonder if there will ever be common drives with multiple heads to
> reduce the seek time overhead.

Oh and of course solid state drives (flash based) avoid all the seek
and rotational delay entirely and are hence awesome for random I/O,
even if they can't always match raw transfer rate with harddisks.
You get to pay for it though (for now).

-- 
Len Sorensen
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