Browse Prior Art Database

Real-Time Performance for Small Computer System Interface Disk Arrays

IP.com Disclosure Number: IPCOM000117312D
Original Publication Date: 1996-Feb-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 2 page(s) / 67K

Publishing Venue

IBM

Related People

Fellela, TA: AUTHOR [+3]

Abstract

In realtime systems, it is important that the transfer times to disk array (RAID) subsystem remain nearly constant. The idea disclosed in this paper is turn off individual disk idle time functions and coordinate these functions from the host computer system or array controller. This insures the greatest amount of overlap in the execution of these functions and minimizes the impact on data transfer times.

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Real-Time Performance for Small Computer System Interface Disk Arrays

      In realtime systems, it is important that the transfer
times to disk array (RAID) subsystem remain nearly constant.  The
idea disclosed in this paper is turn off individual disk idle time
functions and coordinate these functions from the host computer
system or array controller.  This insures the greatest amount of
overlap in the execution of these functions and minimizes the impact
on data transfer times.

      In one application, we had a requirement to transmit data to
and from Small Computer System Interface (SCSI) disk arrays at a
sustained rate of 34 Megabytes per second.  This rate had to be
sustained for periods up to 3 hours or even longer.  Since a single
SCSI-2 bus can only acheive a maximum rate of 20 Megabytes per
second, two buses were used and the information was stripped across
the two buses on a block by block basis.  The target disks were RAID
devices composed of 9 disk drives.  Since a typical disk drive at
that time had transfer rates of only 4 to 5 Megabytes per second, the
RAID devices stripped the data blocks internally.  This gave the RAID
devices maximum performance rates in the 16 to 19 Megabyte per second
range.

      However, each individual drive performs idle time on a periodic
basis.  For example, Thermal Compensation (or Thermal Recalibration)
is done every 24 seconds for the first hour from being powered on,
every 2  minutes thereafter, and anytime the disk's internal
controller detects  a problem.  The Thermal Recalibration (TCAL)
function only takes 210 milliseconds, but if the 18 disks do not
perform TCAL at precisely the  same time, up to 3.78 seconds every 24
seconds can be lost.  This is because a transfe...