Re: General questions: self-tests / ATA attributes / SCSI sense / smart return status

Michael Woon wrote:
> Hi Smartmontools devs,
>
>
> I'm writing for a bit of clarity about self-tests and what records 
> they produce, and the smart health call.
>
>
> As I understand from the documentation, calling "smartctl -H":
>
> -returns the result of the SMART RETURN STATUS command -/or- /checks 
> if any ATA attributes exceed thresholds in ATA drives

Yes.


> -checks for any error codes in the SCSI sense buffer.
>

It checks ASC/ASCQ in SCSI IE log page (if supported) or in result of 
REQUEST SENSE command.

Most remaining answers are for ATA/SATA only. SCSI/SAS differs 
considerably. Some SCSI expert on this list might want to answer.


> As I understand from the documentation, fore- and back-ground checks 
> update the self-test error log and certain ATA attributes, when they run.

There is no ATA "self-test error log". On completion of a self-test, a 
new entry is usually added to the ATA self-test log(s). The ATA error 
log(s) are typically not updated on read errors found during a self-test.


> My main questions are:
>
> -what does SMART RETURN STATUS evaluate?

Anything the author of the drive firmware decided to evaluate :-)

Recent versions of ATA ACS standards say:
"The SMART RETURN STATUS command causes the device to communicate the 
reliability status of the device to the host."
If command returns failure(0x2c,0xf4): "The device has detected a 
threshold exceeded condition."

Note that ATA SMART Attributes are not part of the standard. The SMART 
READ THRESHOLDS command was declared obsolete in ATA-4 (1998).


>
> alternatively stated:
> -does the command -only- look at ATA attributes stored in the table 
> and error codes in the SCSI sense buffer? or is the content of the 
> self-test error log also a factor?
>

SMART RETURN STATUS does not return failure if any Read/Write error 
occured. It usually will return failure if the number of spare blocks 
for reallocation is below some threshold.


> bottom line:
> -If I want to be sure of the health of a disk, can I trust the smart 
> health status (to include the result of the self-tests) or do I have 
> to look at -both- the health status and the self-test error log?

If you want to proactively replace drives, I would recommend to watch 
the number of reallocated sectors (e.g. use smartd with '-R 9! -r 9!' 
directive). A failing SMART STATUS may occur (too?) late.


>
> or do I have the wrong angle on this:
> -simply watch for a '0' exit code for an "all okay"?
>

It depends, see above.


> minor questions about the exit codes:
> -is it possible to have a set bit 3 (device failing) without a set bit 
> 4 (attributes over threshold), and vice versa?

Yes: if SMART RETURN STATUS returned failure but there is no attribute 
<= threshold in the SMART DATA block, the SMART READ THRESHOLD command 
did not work, etc...


> -at what point does a SCSI drive set the 6th bit in the error code? I 
> have drives (SAS) that have some errors in their smartctl output, but 
> don't set this bit when smartctl is run on them.

For some historic reason, bit 6 was never implemented for SCSI.


> -does bit 7 really only work for SATA drives? (SCSI drives have a self 
> test log too)

Yes, it works "better" for ATA because newer long tests without error 
clear the bit.

Thanks,
Christian


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