MAXPHYS bump for FreeBSD 13

[Hans Petter Selasky]

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!

--HPS