On Mon, Dec 10, 2001 at 10:49:28AM -0800, Matthew Dillon wrote:
> :For RAID3 that is true. For the other ones...
> :
> :>     performance without it - for reading OR writing.  It doesn't matter 
> :>     so much for RAID{1,10},  but it matters a whole lot for something like
> :>     RAID-5 where the difference between a spindle-synced read or write
> :>     and a non-spindle-synched read or write can be upwards of 35%.
> :
> :If you have RAID5 with I/O sizes that result in full-stripe operations.
> :
> :-- 
> :|   / o / /_  _              email:  [EMAIL PROTECTED]
> :|/|/ / / /(  (_)  Bulte              Arnhem, The Netherlands 
>     Well, for reads a non-stripe-crossing op would still work reasonably
>     well.  But for writes less then full-stripe operations without
>     spindle sync are going to be terrible due to the read-before-write
>     requirement (to calculate parity).  The disk cache is useless in that
>     case.

Modern disks do prereads and writes are streamed by tagged command
queueing which invalidates this for linear access.
For non linear access the syncronisation is shadowed partly by different
seek times and different load on the spindles.
The chance that the data and parity spindle have the heads on the same
track is absolutely small for random access.
With 15000 upm drives the maximum rotational delay is 4ms and the
average is 2ms which gives you an maximum of only 1ms to gain under
ideal conditions - which we don't have as I stated above.

B.Walter              COSMO-Project         http://www.cosmo-project.de
[EMAIL PROTECTED]         Usergroup           [EMAIL PROTECTED]

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