I've just found the root of the problem with my NetBSD-2.0F freezing on 
heavy I/O. It turns out it's not softdep fault, but the ATA RAID. I 
should have investigated it further before jumping to conclusions. I was 
using it on Promise FastTrak 100-Lite ATA RAID controller, /usr was 
residing on /dev/ld0e.  It's either the hardware, or NetBSD drivers for 
it are a bit flaky, I've switched to NetBSD software RAID on a regular 
Promise Ultra100 TX2 ATA controller and everything seems to be fine now.