ffs_copyonwrite panics

Hi,

I've been using -CURRENT last update in February for quite a long time
and few weeks ago decided to finally update it. The update was quite
unfortunate as system became very unstable: it just hangs few times a
day and panics sometimes.

Some things can be reproduced, some cannot. Reproducible ones:

1. background fsck always makes system hang
2. system crashes on operations with nullfs mounts (disabled that for
now)

The most annoying one is ffs_copyonwrite panic which I cannot reproduce.
The thing is that if I will run 'startx' on it with some X apps it will
panic just in few minutes. When I leave the box with nearly no stress
(just use it as internet gateway for my laptop) it behaves a little
better but will eventually crash in few hours anyway.

The even more annoying thing is that when I cannot save the dump,
because when the system boots and runs 'savecore' it leads to
fss_copyonwrite panic as well. The panic happens when about 90% complete
(as seem via ctrl-t).

Any ideas how to debug and get rid of this issue?

System arch is amd64. I don't know what other details could be useful.

Roman Bogorodskiy