A few technical items on UFS2 and snapshots...

From: Joe Schmoe (non_secure_at_yahoo.com)
Date: 06/23/04

  • Next message: Danny Braniss: "waiting on sbwait" 
    Date: Tue, 22 Jun 2004 17:28:50 -0700 (PDT)
To: freebsd-hackers@freebsd.org

    (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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