Re: [PATCH v3 02/18] block: introduce BLK_STS_DURATION_LIMIT

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 1/25/23 04:29, Bart Van Assche wrote:
> On 1/24/23 11:02, Niklas Cassel wrote:
>> Introduce the new block IO status BLK_STS_DURATION_LIMIT for LLDDs to
>> report command that failed due to a command duration limit being
>> exceeded. This new status is mapped to the ETIME error code to allow
>> users to differentiate "soft" duration limit failures from other more
>> serious hardware related errors.
> 
> What makes exceeding the duration limit different from an I/O timeout 
> (BLK_STS_TIMEOUT)? Why is it important to tell the difference between an 
> I/O timeout and exceeding the command duration limit?

If the device fail to execute a command in time, it will either
1) Fail the command with an error and sense data set (policy 0xf for the
time limit)
2) Return a success status for the command with sense data set telling the
host "data not available". This (weird) case is in essence equivalent to
(1) but was defined to avoid the penalty of a queue abort with SATA drives
(NCQ command errors always result in all on-going commands being aborted).

In both cases, the drive is still responsive and operational.
BLK_STS_TIMEOUT is used if a command timed-out, indicating that the drive
is *not* responding. BLK_STS_TIMEOUT thus generally mean "something is
wrong" (not always, but most of the time.

So we cetainly do not want to overload BLK_STS_TIMEOUT to indicate failed
CDL IOs as that would not allow the user to distinguished from more
serious hardware issues.


-- 
Damien Le Moal
Western Digital Research