internal vs. external snaphots

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

Since I have been experiencing some mysterious file system corruption on a fs 
that was fsck-clean before being mounted, I would like to periodically fsck 
that fs while mounted.
The problem is that fs is 6T large.

If I use an internal snapshot (fsck_ffs -X), the server stalls for about an 
hour, then panics (wapbl_deregister_allocation: out of resources) and, upon 
reboot, needs longer for replaying the journal than an fsck would take.

If I use an external snapshot (fsck_ffs -x /somewhere/else), the snapshot only 
takes a few seconds, but fsck_ffs complains about unreferenced files, missing 
blocks in the free list and bad summary information. I have tested both -x and 
-X on a smaller fs, and -X runs fine while -x complains.

So why do internal and external snapshots behave so differently? Are they just 
two completely diffrent aproaches to the same problem or what's the reason for 
using what seems to be a totally different strategy when the backup file is 
inside the fs itself?

Is there any documentation on the subject?