[linux-lvm] Snapshots and disk re-use

Stuart D. Gathman stuart at bmsi.com
Thu Feb 24 14:50:13 UTC 2011

On Thu, 24 Feb 2011, Jonathan Tripathy wrote:

> > > However the origin will be written to as it will be in use...
> > There were 3 cases of what you might be asking.  One of the 3 cases was:
> > If you are taking snapshots for backup, then it was suggested to zero the
> > *-cow (which will have any blocks written to the origin since the snapshot
> > was taken) before deleting the snapshot.  However, I wasn't sure if this
> > was safe to while the origin is mounted, since writes to the origin consult
> > the *-cow to see whether origin blocks need to be copied before begin
> > overwritten.

> But I want to keep this data. As the data written to the origin while a
> respective snapshot is in use would be data during normal operation of a
> customer VPS.
> All I want to do is make sure that my use of snapshot for backup purposes (to
> rsync the snapshot to a remote server) doesn't leave loose data lying anywhere
> on physical disk.

Yes. To be more pedantic, the COW has copies of the original contents
of blocks written to the origin since the snapshot.  That is why you
need to clear it to achieve your stated purpose.  The origin blocks
are written normally to the *-real volume (you can see these in
the /dev/mapper directory).

I am only concerned that since the COW has both the saved origin blocks, and a
table of which blocks from the origin have been saved, that clearing
the table might cause problems for an active origin.  It should be safe
with the origin unmounted (but having to shutdown the VM defeats the
purpose of a snapshot!)  I am hoping an expert will weigh in.

	      Stuart D. Gathman <stuart at bmsi.com>
    Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
"Confutatis maledictis, flammis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.

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