Optimum dev_openings value

[Glen Barber]

Hi,

I have three machines running 8.0-RELEASE, each with two mirrored Seagate
SCSI disks using ZFS.  In making sure these machines are performing as
best as possible, I'm trying to find what the best practices are with
regards to dev_openings values, and what determines the default
dev_openings value.

Two machines have identical ahc(4) Adaptec AIC7899 Ultra160 SCSI adapters,
but display different defaults, which I have not had luck finding an
explanation:

	h04# camcontrol tags da0 -v
	(pass0:ahc1:0:0:0): dev_openings  33  
	(pass0:ahc1:0:0:0): dev_active    0   
	(pass0:ahc1:0:0:0): devq_openings 33
	(pass0:ahc1:0:0:0): devq_queued   0   
	(pass0:ahc1:0:0:0): held          0   
	(pass0:ahc1:0:0:0): mintags       2   
	(pass0:ahc1:0:0:0): maxtags       255
	          
	h21# camcontrol tags da0 -v
	(pass0:ahc0:0:0:0): dev_openings  64  
	(pass0:ahc0:0:0:0): dev_active    0   
	(pass0:ahc0:0:0:0): devq_openings 64
	(pass0:ahc0:0:0:0): devq_queued   0   
	(pass0:ahc0:0:0:0): held          0   
	(pass0:ahc0:0:0:0): mintags       2   
	(pass0:ahc0:0:0:0): maxtags       255
	
	h04# camcontrol devlist
	<SEAGATE ST318406LC 0109>          at scbus1 target 0 lun 0 (pass0,da0)
	<SEAGATE ST318406LC 010A>          at scbus1 target 1 lun 0 (pass1,da1)
	     
	h21# camcontrol devlist 
	<SEAGATE ST3146855LC 0003>         at scbus0 target 0 lun 0 (pass0,da0)
	<SEAGATE ST3146855LC 0003>         at scbus0 target 1 lun 0 (pass1,da1)

Is there an obvious explanation for the difference here?

The third machine uses sym(4), which I expect would cause a different
default dev_openings (currently defaulted to 44).

Is there a general rule to adjusting this value?

-- 
Glen Barber