On 1/25/23 05:36, Bart Van Assche wrote:

On 1/24/23 11:27, Bart Van Assche wrote:

quoted

Implementing duration limit support using the I/O priority mechanism 
makes it impossible to configure the I/O priority for commands that have 
a duration limit. Shouldn't the duration limit be independent of the I/O 
priority? Am I perhaps missing something?

(replying to my own e-mail)

In SAM-6 I found the following: "The device server may use the duration 
expiration time to determine the order of processing commands with
the SIMPLE task attribute within the task set. A difference in duration 
expiration time between commands may override other scheduling 
considerations (e.g., different times to access different logical block 
addresses or vendor specific scheduling considerations). Processing of a 
collection of commands with different command duration limit settings 
should cause a command with an earlier duration expiration time to 
complete with status sooner than a command with a later duration 
expiration time."

Do I understand correctly that it is optional for a SCSI device to 
interpret the command duration as a priority and that this is not mandatory?

This describes the expected behavior from the drive in terms of command
execution ordering when CDL is used. The text is a little "soft" and sound
as if this behavior is optional because CDL is a combination of time
limits AND a policy for each time limit. The policy of a CDL indicates
what the drive behavior should be if a command misses its time limit. In
short, there are 2 policies:
1) best-effort: the time limit is a hint of sorts. If the drive fails to
execute the command before the limit expires, the command is not aborted
and execution continues.
2) fast-fail: If the drive fails to execute the command before the time
limit expires, the command must be completed with an error immediately.

And CDL also has a field, settable by the user, that describes an allowed
performance degradation to achieve CDL scheduling in time. That is, most
important for the best-effort case to indicate how "serious" the user is
about the CDL limit "hint".

So as you can see I think, the SAM-6 text is vague because of the many
possible variations in scheduling policies that need to be implemented by
a drive.

-- 
Damien Le Moal
Western Digital Research