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Re: [dm-devel] DM_snapshot_cow filesystem (dmsetup create snapshot)

On 11/29/2011 12:22 AM, Frederick Grose wrote:
On Sun, Nov 27, 2011 at 8:08 PM, Douglas McClendon
<dmc lists filteredperception org
<mailto:dmc lists filteredperception org>> wrote:

    On 11/27/2011 04:31 PM, Frederick Grose wrote:

        On Sat, Oct 29, 2011 at 4:49 PM, Alasdair G Kergon
        <agk redhat com <mailto:agk redhat com>
        <mailto:agk redhat com <mailto:agk redhat com>>> wrote:

            markmc did some code that shows how to read the format a few
            ago here:

            Otherwise look at dm-snap-persistent.c in the kernel tree.


        Users can easily exhaust a LiveUSB snapshot overlay by writing
        too many changes to the OS filesystem, such as by performing
        a yum update.

        Would such an 'exhausted' overlay be amenable to some sort of
        data recovery or forensics?

        If so, how so; if not, why not?

    Can you not mount the exhausted dm snapshot?  If you mean data
    recovery or forensics beyond that, please elaborate.

    I know I've had issues 'read-only' mounting ext3(2?3?4?) filesystems
    before (on actually read-only block devices).  Maybe some flags that
    I've forgotten get past that, but worst case you can always fake a
    writable device with a 2nd overlay/snapshot going to a writable
    device, i.e. if you wanted to let fsck repair things.


A typical scenario goes like this:

A LiveCD is loaded onto a USB device with a relatively small
persistent overlay and no separate home.img filesystem.  The user
proceeds to use the system and save some information.  They may
execute a df command and see that there is lots of space
available on the rootfs.  Then they decide to execute a yum
update or yum install when there is insufficient space available.
(They do not know about dmsetup status.)  Their session will now
not shutdown normally. dmsetup status now shows

live-rw: 0 8388608 snapshot Invalid.

The device fails to complete a subsequent boot attempt.  The
startup log shows

You probably know this, but note there is a reset_overlay (or similar) boot flag which I think causes the overlay/snapshot to be reset, to get a booting system. So the rest of this is with the use-case in mind of trying to recover prior contents of the overlay/snapshot in a useful way.

mount: /dev/mapper/live-rw: can't read superblock

e2fsck /dev/dm-0 shows

Buffer I/O error on device dm-0, logical block 0 (and 1, 2, 3, 0)
e2fsck: Attempt to read block from the filesystem resulted in
short read while trying to open /dev/dm-0
Could this be a zero-length partition?

I recreated the above scenario on Fedora-16-Live-Desktop
installed with an overlay-size-mb of 100.  After updating to ~70%
of the overlay, I wrote with
dd if=/dev/zero of=/boot/load bs=1M count=10
3 times to exhaust the overlay.

With the defective device mounted on a good system, and loop
devices set up for ext3fs.img and the overlay file,

dmsetup create item --table "0 8388608 snapshot 7:1 7:2 P 8"
item: 0 8388608 snapshot Invalid

I don't know what that last line of output specifically means as far as subsequent expectations should go, though presumably others on this list can elaborate, or worst grep for the message in the code.

Maybe dmsetup is more cooperative with full overlays if they are created in read-only mode?(wild guess)

I.e. there is certainly the expectation that you won't be able to write to the device (or at a minimum to any chunk not already in the overlay(?)). So as I mentioned, the way I'd go about it would be to recreate it (I'm taking for granted above cmdline is correct), but in read-only mode, then either just dup that to a new larger device, or add a second read-write snapshot layer on top (pointing to a looped tmpfile somewhere). Then try to mount and/or fsck that.

But that presumes that the 'snapshot Invalid' isn't evidence that something is preventing even read-only access to the full snapshot. For that I defer to the people more familiar with the relevent code.

e2fsck /dev/dm-0 reports

e2fsck 1.41.14 (22-Dec-2010)
e2fsck: Attempt to read block from filesystem resulted in short
read while trying to open /dev/dm-0
Could this be a zero-length partition?

A hex editor view of the overlay file shows lots of content with
lots of zeros at the end of the file.

My questions are these:

Might there be a way to edit the overlay file to restore its
utility for the information written before the damaging write?

If so, how so; if not, why not?

Were it actually the case that full persistent snapshots become unusuable (short of 'editing'), that would seem like a bug needing to be fixed.


Suitable references would do as well.

Thanks!       --Fred

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