MAXPHYS bump for FreeBSD 13

[Alexander Motin]

On 14.11.2020 06:22, Hans Petter Selasky wrote:
> On 11/14/20 5:14 AM, Alexander Motin wrote:
>>> We currently have a MAXPHYS of 128k. This is the maximum size of I/Os
>>> that we normally use (though there are exceptions).
>>>
>>> I'd like to propose that we bump MAXPHYS to 1MB, as well as bumping
>>> DFLTPHYS to 1MB.
>>
>> I am all for the MAXPHYS change, as Warner told it was my proposition on
>> a chat.  ZFS uses blocks and aggregates I/O up to 1MB already and can
>> more potentially, and having I/O size lower then this just overflows
>> disk queues, increases processing overheads, complicates scheduling and
>> in some cases causes starvation.
>>
>> I'd just like to note that DFLTPHYS should probably not be changed that
>> straight (if at all), since it is used as a fallback for legacy code.
>> If it is used for anything else -- that should be reviewed and probably
>> migrated to some other constant(s)>
> Beware that many USB 2.0 devices will break if you try to transfer more
> than 64K. Buggy SCSI implementations!

Yes, thanks, I remember.  The code reports MAXPHYS only for
USB_SPEED_SUPER devices, relying on DFLTPHYS fallback in CAM otherwise.
 I think slower ones could just be hardcoded to 64KB to be certain.

-- 
Alexander Motin