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February 13, 2009

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the storage anarchist

Chad -

One note about that cache hit rate table - it uses 6ms as the example response time of a 15K rpm drive. This response time is BEST CASE for a 15Krpm drive, and is usually attained only if data is stored on a fraction of the drive (to minimize head movement). It is not uncommon to see heavily loaded 15K drives delivering 10 or even 20ms response times - under these same loads an EFD will still deliver virtually constant 1ms response.

Said another way, 2-6x is the WORST CASE improvement you can expect from EFDs.

Rob

> One note about that cache hit rate table - it uses 6ms as the example response time of a 15K rpm drive

Those must be old crusty drives.

> This response time is BEST CASE for a 15Krpm drive

Maybe 7 years ago, sure.

Here's a 450 GB FC or SAS interface, average seek of 3.4 ms:

http://www.seagate.com/docs/pdf/datasheet/disc/ds_cheetah_15k_6.pdf

Rob

never mind. it was late. 3.4 + 2 ms = 5.4 ms for response, not 3.4 ms you have to account for latency. It is late, sorry.

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