Behavior of SCSI device for START STOP UNIT command

Gerry Houlder gerry.houlder at
Thu Nov 20 07:11:11 PST 2014

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There are a lot of configuration questions that affect the response in your
(1) Are Logical unit 0 and logical Unit 1 two separate disk drives, or are
they two virtual logical units that are on the same physical drive? This
determines whether the commands for each LUN are treated separately or
whether the commands are treated as if they are all for the same device.
(2) The standard terms for power down options are "idle" or "standby".
Which of these do you mean by sleep/power down? Recovery behavior is
usually different for idle versus standby.
(3) What are the setting in the CCF_IDLE and CCF_STANDBY fields of the
power Condition mode page? These settings determine the recovery behavior
|from idle and standby conditions, respectively, and may result in either
transitioning to active and processing an incoming command or responding
with CHECK CONDITION status to an incoming command.
(4) Disk drives can respond to some commands while in an idle or standby
condition. The exact list of such commands is vendor specific, but the idea
is that any command that doesn't require access to the disk might be
processed properly without the drive having to leave the idle or standby
condition. READ CAPACITY is in the gray area (might not require medium
access) where some vendors might treat this as a command that is serviced
while in a low power condition.
>From you list of choices, either response 1) or 2) is possible. Choice 3)
should never be possible. Also choice 4) all of the commands complete with
GOOD status is possible. Commands to a drive that is in a low power
condition may take longer to complete then if the drive were in the active
On Thu, Nov 20, 2014 at 4:06 AM, Munjal Mistry <munjal_mistry at>
>  Hello All,
> I have a query regarding the device behavior for changing the power
> condition during on-going read/write operation.
> *Scenario:*
> Suppose, the device is in active power mode and logical unit 0 is
> processing write command and logical unit 1 is processing read command. In
> between this process, host sends START STOP UNIT command which requests to
> change power mode to sleep/power down mode, followed by READ CAPACITY (10)
> command.
> In this case, which behavior of device is correct?
> (1) Device will wait for the READ and WRITE commands to completion and
> then move to pre-sleep/pre-power down mode and sends CHECK CONDITION status
> for READ CAPACITY (10) command.
> (2) Device will send response with GOOD status for READ CAPACITY (10)
> command and wait for completion of READ-WRITE command and then move to
> pre-sleep/pre-power down mode
> (3) Device stops processing READ-WRITE command and sends CHECK CONDITION
> status for READ-WRITE command and move to pre-sleep/pre-power down mode.
> Then device sends CHECK CONDITION status for READ CAPACITY (10) command.
> In-case any of the above mentioned behavior is not correct then it would
> be great if you could let me know the expected behavior of the device.
> Thanks in advance.
> --
> Warm Regards,
> Munjal Mistry

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