[mythtv-users] MythTV 0.23 Available

Brian J. Murrell brian at interlinx.bc.ca
Tue May 11 17:53:30 UTC 2010

On Tue, 2010-05-11 at 12:26 -0500, Richard Shaw wrote: 
> I'm trying to understand how this would work. If we were talking about
> something like Virtualbox I would understand. The snapshot seems to
> track changes without modifying the original disk image so if you run
> into a problem you delete the snapshot and you're back to your
> pre-upgrade condition.

LVM snapshots would (IMHO) really be better named a "fork".  Once you
take a snapshot of an LVM device (which is nothing more than a block
device on which your filesystem lives) you now have two block devices
which are readable and writable and at the moment of the snapshot,
identical.  You can mount the filesystem on the snapshotted device and
read and write to it without causing any changes to the
device/filesystem that the snapshot was taken from.

Creating a snapshot is really no different than that "ghost" technique
previously mentioned except that it's instant (i.e. you don't have to
wait for the ghost/copy operation to complete) and more space efficient
as the snapshot only uses as much space as is needed to track the
differences between the "origin" (the device that was snapshotted) and
the snapshot.

> As far as I can tell, LVM snapshots work differently. The underlying
> file system continues to be modified but it copies the to be modified
> chucks to the snapshot first to present an unmodified file system.

Hrm.  I'm afraid there were too many typos in there for me to grasp what
you are trying to say.

> Other than letting you make a live backup (such as dd'ing the who
> volume) I don't see any real advantage.

As I said, a snapshot is the space and time efficient equivalent of
dding from one block device to another.

> You still need enough space
> somewhere to make a complete copy.

No.  You only need as much space as is required to store the "changes"
you make to one of the devices.  If you took a snapshot and did nothing
with either the snapshot or the origin, you would use no extra space at
all but have two block devices that were (virtual) "copies" of each

> If you run into an issue, how
> exactly does this help you?

Since a snapshot is a block device as fully functional as the origin it
was snapshotted from, you can mount the snapshot exactly as if it were
the origin.  You can even reboot from it if you need to.

Anything you could do by dding from one block device (i.e. filesystem)
to another block device (i.e. being able to boot from either) you can do
with LVM snapshots.


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