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Jeff Garzik wrote:
Going in the wrong direction. Talk to the people who write OS
application software, because its they who generate the data access
patterns.
Lets be real - no one knows the actual physical data access patterns.
Quiz: In a typical multitasking OS with ten applications running each
SEQUENTIALLY reading or writing files on the same disk drive - what is
the data access pattern actually seen by the drive?
"read ahead" is a hint.
Read ahead is a hint and some OS's and some I/O subsystems (s/w and h/w)
pass this hint down to the lowest levels of the storage subsystem on a
COMMAND BY COMMAND baises - after all it requires only ONE BIT of data
in the s/w I/O command structures or in a device's R/W command parameters.
It allows the OS to hint to the device that
reading ahead is likely to be an overall win (or, if disabled, an
overall loss). If a data access pattern is largely random, involving
lots of seeks, one would be smart to disable read-ahead, BUT NOT ALL
READ CACHING.
The ATA SET FEATURES commands for controlling read ahead and write cache
were never intended to be used in this manner and would probably be very
ineffective is used in this manner.
Users are far more likely to want to disable read-ahead, than to disable
read caching.
Users? Explain please... Application program? File system? I/O driver?
Hale
--
++ Hale Landis ++ www.ata-atapi.com ++
