| Message ID | 20230124190308.127318-1-niklas.cassel@wdc.com |
|---|---|
| Headers | show |
| Series | Add Command Duration Limits support | expand |
On 1/24/23 11:59, Keith Busch wrote: > On Tue, Jan 24, 2023 at 11:29:10AM -0800, 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? > > BLK_STS_TIMEOUT should be used if the target device doesn't provide any > response to the command. The DURATION_LIMIT status is used when the device > completes a command with that status. Hi Keith, From SPC-6: "The MAX ACTIVE TIME field specifies an upper limit on the time that elapses from the time at which the device server initiates actions to access, transfer, or act upon the specified data until the time the device server returns status for the command." My interpretation of the above text is that the SCSI command duration limit specifies a hard limit, the same type of limit reported by the status code BLK_STS_TIMEOUT. It is not clear to me from the patch description why a new status code is needed for reporting that the command duration limit has been exceeded. Thanks, Bart.
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.
On 1/25/23 05:32, Bart Van Assche wrote: > On 1/24/23 11:59, Keith Busch wrote: >> On Tue, Jan 24, 2023 at 11:29:10AM -0800, 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? >> >> BLK_STS_TIMEOUT should be used if the target device doesn't provide any >> response to the command. The DURATION_LIMIT status is used when the device >> completes a command with that status. > > Hi Keith, > > From SPC-6: "The MAX ACTIVE TIME field specifies an upper limit on the > time that elapses from the time at which the device server initiates > actions to access, transfer, or act upon the specified data until the > time the device server returns status for the command." > > My interpretation of the above text is that the SCSI command duration > limit specifies a hard limit, the same type of limit reported by the > status code BLK_STS_TIMEOUT. It is not clear to me from the patch > description why a new status code is needed for reporting that the > command duration limit has been exceeded. As explained, this allows differentiating the "drive gave a response" (BLK_STS_DURATION_LIMIT) from the "drive is not responding" case with BLK_STS_TIMEOUT. We took care of mapping BLK_STS_DURATION_LIMIT to ETIME (timer expired) for user space too, to not overload ETIMEDOUT used with BLK_STS_TIMEOUT. We can certainly improve the commit message to describe all of this in more details. > > Thanks, > > Bart.