SCSI Power Management

Jim McGrath jmcgrath at QNTM.COM
Wed Jun 5 20:43:36 PDT 1996


* From the SCSI Reflector, posted by:
* "Jim McGrath" <jmcgrath at qntm.com>
*
                      Subject:                              Time:  7:32 PM
  OFFICE MEMO         SCSI Power Management                 Date:  6/5/96


The current draft SCSI specification calls for devices to return a CHECK
CONDITION to the host if a command is issued to the device when the
device is unable to execute the command due to being in a low power
mode.  Thus if the host issues a READ that requires disk access, and the
device
is in STANDBY mode (motor turned off), it is suppose to fail the command.

This seems to be unnecessaryily harsh and not workable.  If the device
was put into STANDBY due the the STANDBY counter expiring, then the
host did not necessary know that the device was in a low power mode
to being with.  This will probably confuse the host and cause all sorts of
problems.  In ATA, the device will under these conditions resume the
ACTIVE state (i.e. spin up) and execute the command.  That appears to be
much better.  Moreover, desktop systems will want AT and SCSI to behave
the same no matter what.

Does this maek sense?  ( have not worked on the power management stuff
for some time).  Comments?

Jim






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