[mythtv-users] MythTV 0.23 Available

Richard Shaw hobbes1069 at gmail.com
Tue May 11 18:26:32 UTC 2010

On Tue, May 11, 2010 at 12:53 PM, Brian J. Murrell
<brian at interlinx.bc.ca> wrote:
> 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.

I think there needs to be some better documentation. I read through
the LVM howto at TLDP which left much to be desired as well as read a
few articles which mostly had to do with making backups from live
systems, which is all I could quickly find, hence some of my
assumptions. One of which apparently incorrectly stated that snapshots
are not persistent[1].


>> 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.

Sorry, I reworded what I was going to say and apparently didn't
proofread it well enough, but apparently one of the articles I was
reading was incorrect at the point is now moot.

>> 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
> other.

I think we're combining two different questions causing the confusion.
I understand the snapshot only tracks changes (and now I understand
those changes can be to the original file system or the snapshot).

>> 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.

Here I was really asking a separate question. How does this work in
the specific case of upgrading Myth? Do you upgrade the original
system or the snapshot? If you upgrade the snapshot and things go
wrong, I see how that would be helpful. You can drop the snapshot and
go back to where you were. If the upgrade goes right, how do you merge
the difference with the original file system? Or do you?


[1] http://ds9a.nl/lvm-howto/HOWTO//cvs/lvm-howto/output/lvm-howto-7.html

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