> How is an internal snapshot defined?
I'm not completely sure. Ask hannken@.
> The term seems to only exist in dump(8).
And fsck_ffs(8).
> Is it a snapshot for which the backing store is on the same disk
> the snapshot is done?
I suppose. hannken@ should know.
My experience is that I regularily take external snapshots (using fssconfig(8))
while doing TSM backups, and this works fine even for multi-TB filesystems.
However, taking an internal snapshot (fsck -X) on that filesystem starved the
machine, eventually paniced it (some WAPBL resource exhaustion) and the log
replay on reboot took something like half an hour. On much smaller filesystems,
fsck -X works fine.