A few technical items on UFS2 and snapshots...

[Joe Schmoe]

(posted to -questions a few days back, but with no response)

Hi - a few questions about UFS2 and snapshots:

1. Is it dangerous to mount all 20 possible filesystem snapshots and _leave them mounted_ to use at any time ?  What about
automatically mounting all 20 snapshots at boot time ?

2. Related to the first question, it seems like I am getting space out of nowhere ... that is, if I fill up a drive, then
make a snapshot, then erase the drive and fill it again, then make another snapshot ... and do this 20 times, AND THEN mount
all 20 snapshots, it seems like I now have 20x as much disk space as before (granted, most of it is read-only) ... it seems
like I am getting something for nothing.  What am I missing here ?  What tradeoffs do I begin to make as I mount up more and
more snapshots and get more and more browsable space ?

3. When I mount a snapshot, as described in the man page, but then later mount -uw the snapshot ( to make that a writeable
mount) and, say, touch a file or create a file in the mounted snapshot ... what exactly am I doing ?  Have I corrupted the
snapshot ?  Is it still usable as a snapshot ?  Where does this space end up being used at if I write a file in a
write-enabled, mounted snapshot ?

4. This is not related to snapshots, but is a UFS2 question ... I see that if I am doing filesystem activity, and before I
can sync the disks, my machine crashes ... the machine sort of goes back in time when it reboots - the files or directories I
had created no longer exist when it reboots.   This is expected, I suppose, and makes sense.  However, it seems like I have
also seen the following behavior:

write file A
write file B
crash
file A exists, but B does not
write file B
crash
BOTH file A and B _no longer exist_

Is this possible ?  Have I really seen that behavior, or am I remembering it wrong ?  I swear that I have seen something like
this happen ... if this is possible, can someone explain how ?  It seems like it shouldn't be possible...

Thanks!

		
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