On 2022-02-19 17:46, Martin K. Petersen wrote:
Douglas,
What should the sd driver do when it gets the error in the subject
line? Try again, and again, and again, and again ...?
sd 2:0:1:0: [sdb] Unaligned partial completion (resid=3584, sector_sz=4096)
sd 2:0:1:0: [sdb] tag#407 CDB: Read(16) 88 00 00 00 00 00 00 00 00 00 00 00 00 01 00
Not very productive, IMO. Perhaps, after say 3 retries getting the
_same_ resid, it might rescan that disk. There is a big hint in the
logged data shown above: trying to READ 1 block (sector_sz=4096) and
getting a resid of 3584. So it got back 512 bytes (again and again
...). The disk isn't mounted so perhaps it is being prepared. And
maybe that preparation involved a MODE SELECT which changed the LB
size in its block descriptor, prior to a FORMAT UNIT.
The kernel doesn't inspect passthrough commands to track whether an
application is doing MODE SELECT or FORMAT UNIT. The burden is generally
on the application to do the right thing.
No, of course not. But the kernel should inspect all UAs especially the one
that says: CAPACITY DATA HAS CHANGED !
I'm assuming we're trying to read the partition table. Did the device
somehow get closed between the MODE SELECT and the FORMAT UNIT?
Nope, look up "format corrupt" state in SBC, there is a asc/ascq code for
that, and it was _not_ reported in this case. The disk was fine after those
two commands, it was sd or the scsi mid-level that didn't observe the UAs,
hence the snafu. Sending a READ command after a CAPACITY DATA HAS CHANGE
UA is "undefined behaviour" as the say in the C/C++ spec.
Also more and more settings in SCSI *** are giving the option to return an
error (even MEDIUM ERROR) if the initiator is reading a block that has never
been written. So if the sd driver is looking for a partition table (LBA 0 ?)
then you have a chicken and egg problem that retrying will not solve.
Another issue with that error message: what does "unaligned" mean in
this context? Surely it is superfluous and "Partial completion" is
more accurate (unless the resid is negative).
The "unaligned" term comes from ZBC.
The sd driver should take its lead from SBC, not ZBC.
Doug Gilbert
*** for example, FORMAT UNIT (FFMT=2)
