best settings for usb2 and attached disks, and sdcards

Hi,
[cc'd to arm_at_ and fs_at_ where it's also relevant]

I have a number of rpi3 & rpi3 machines. Usually I want to attach a usb keydrive to them so that the sdcard isn't thrashed. They're all running -current. usr/src and usr/ports at least are mounted on the keydrive.

When initially updating eg the ports tree, svn will time out/crash because of the poor write performance of these devices in a rpi2/3 context. The fs on the usb keys is always ufs2. I have tried mounting these devices as -o async and also in fstab but this parameter seems not to 'take' in that mount doesn't report the async property set:

[...]
/dev/da0p2 on /ext (ufs, local, noatime, soft-updates)
[...]

was mounted with the command "mount -o async,noatime,rw /dev/da0s2 /ext"
but I can't tell if async is on or just ignored, no error message. And I still have to run svnlite cleanup /ext/ports until svnlite stops bailing out. When newfs was written, I passed -t  to it to enable trim, which seems to make a difference on this for deletes but not writes. Can anyone please suggest anything I can so to speed up disk i/o? And is async being applied or ignored?

thanks,
-- 
  John
  tech-lists_at_zyxst.net