Re: Should queue_max_hw_sectors() always be <= max_dev_sectors?

["Martin K. Petersen" <martin.petersen@oracle.com>]

>>>>> "Andy" == Andy Grover <agrover@redhat.com> writes:

Hey Andy,

Andy> ...because limits.max_hw_sectors internally seems to be just the
Andy> controller limit, but what users of queue_max_hw_sectors
Andy> (including sysfs) really want is the lesser of
Andy> limits.max_hw_sectors and limits.max_dev_sectors.

max_dev_sectors is a limit reported by the target device for
READ/WRITE/VERIFY requests. Whereas max_hw_sectors describes the DMA
constraints of the controller.

It is a requirement for SG_IO, firmware updates, tape drives, etc. that
the controller limits are reported correctly since they do I/Os that are
much bigger than typical filesystem requests. When we tried to conflate
the two things broke in interesting ways.

I can't remember why the max_dev_sectors sysfs export patch didn't go
in. But instead (or in addition) we could entertain having a
max_rw_sectors_kb file that reflects the biggest READ/WRITE/VERIFY
request the device can consume if that's the information you're after?

-- 
Martin K. Petersen	Oracle Linux Engineering