possible usb3-connected hard drive spin down causing lag

Hi,

I have a usb3-connected harddrive. dmesg shows this:
[...]
da0: <ADATA HD710 0> Fixed Direct Access SPC-4 SCSI device
[...]

running current-r367806-arm64

I think it might be auto-spinning-down or auto-sleeping. It's
making initial interaction lag of 2-3 seconds. Is there a 
sysctl or something somewhere where I can tell it to never 
sleep? Or is that something I'd need to contact the 
manufacturer about? Or is there an alternative strategy 
like tmpfs. It's not a "green" drive but I guess it might
be "green" in that it's usb3 powered.

I have vfs.read_max=128 in /etc/sysctl.conf
zdb has ashift=12

In case it's relevant, the filesystem on the disk is zfs. Once
"woken up", inferaction is quick, as expected. 

thanks,
-- 
J.