When disk transaction frequency is at a high level in busy servers, the wasted
cost of sorting does become an issue; also, it is not extremely rare for the
OS to sort the data into the worst case for the controller's ordering
algorithm, as the OS knows nothing (especially with SCSI-SCSI RAID
controllers) about the true disk geometry. Being able to disable the sorting
adds little to no complexity and and affords a small, but significant,
increase in speed. Having more control over the sorting is even better. In
benchmarks on Solaris with raw disk devices and a benchmarking program that
simulated different ordering algorithms, throughput was highest to the disks
(behind a cached controller with 64MB) when first-come first-served was used
to service the randomly generated requests. I can rewrite and re-run the
benchmark if anyone desires.
-- Robert Minichino Chief Engineer Denarius Enterprises, Inc.
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