mksnap_ffs regression?

[Andrea Venturoli]

Hello.

On several servers I manage, I take backups to an external HD and use 
mksnap_ffs to create snapshots.
I've never had troubles with this up to 11.3.

Lately I started doing this on a 12.1 server and noticed mksnap_ffs will 
hang the box for several minutes (services stuck, no login allowed, 
already established ssh sessions partially work; shutdown not feasible 
unless reset button is pressed).
N.B. This is an external drive, only mounted when needed and only 
accessed to make backups, so if *it* got stuck, it should not affect the 
whole system.

I decided to check this and took the external HD to my desktop (11.3): 
it worked perfectly.
I then upgraded my desktop to 12.1p2 and it started doing as above:

_ mksnap_ffs will work for several minutes under high I/O (the HD is 
6TB); meanwhile, the system is responsive;

_ then mksnap_ffs will drastically reduce its I/O (at least as measured 
with top), but will keep working for some other minutes: during this 
phase, I cannot open any new program; ThunderBird gets stuck, while 
FireFox still works (but cannot open any new window); audacity keeps 
playing the current truck, but will get stuck on moving to the next; 
already open terminal windows might partially work;

_ then mksnap_ffs will exit and everything will get back to normal.



This is of course unacceptable on a production system.
Is there any known problem with mksnap_ffs on 12.1?
Any tweak/patch I should try?

  bye & Thanks
	av.