[mythtv-users] MythTV updates, back out plan, lvm snapshots

[Brent Bolin]

Replies noted in message

On Sun, Feb 26, 2012 at 9:13 AM, Brian J. Murrell <brian at interlinx.bc.ca> wrote:
> On 12-02-26 08:05 AM, Brent Bolin wrote:
>> Hello All,
>
> Hi,
>
>> I've know about lvm snapshots for a while but haven't used it.
>
> Welcome to 1990.  :-)
>
>> Assuming core OS is all in a single lv.
>
> Not a requirement.  I have my /usr and /var in separate LVs from / and
> just snapshot all of them.  i.e.:

I gave up years ago doing it the hard way.

>
> $ sudo lvs
>  LV                   VG          Attr   LSize   Origin      Snap%  Move Log Copy%  Convert
> ,,,  ubuntu_root          rootvol_tmp owi-ao 500.00m
>  ubuntu_root-20111213 rootvol_tmp swi-a- 500.00m ubuntu_root  49.09
>  ubuntu_root-20120209 rootvol_tmp swi-a- 500.00m ubuntu_root  43.17
>  ubuntu_root-20120214 rootvol_tmp swi-a- 500.00m ubuntu_root  36.15
>  ubuntu_root-20120220 rootvol_tmp swi-a- 500.00m ubuntu_root  15.43
>  ubuntu_usr           rootvol_tmp owi-ao   2.49g
>  ubuntu_usr-20111213  rootvol_tmp swi-a-   2.49g ubuntu_usr   34.00
>  ubuntu_usr-20120209  rootvol_tmp swi-a-   2.49g ubuntu_usr   29.71
>  ubuntu_usr-20120214  rootvol_tmp swi-a-   2.49g ubuntu_usr   25.40
>  ubuntu_usr-20120220  rootvol_tmp swi-a-   2.49g ubuntu_usr   14.20
>  ubuntu_var           rootvol_tmp owi-ao   1.99g
>  ubuntu_var-20111213  rootvol_tmp swi-a-   1.99g ubuntu_var   60.55
>  ubuntu_var-20120209  rootvol_tmp swi-a-   1.99g ubuntu_var   47.15
>  ubuntu_var-20120214  rootvol_tmp swi-a-   2.49g ubuntu_var   30.73
>  ubuntu_var-20120220  rootvol_tmp swi-a-   2.49g ubuntu_var   22.28
>
> [ Please nobody lecture me on the performance impacts of the above
> number of snapshots.  I fully understand, and yes, probably do need
> to do some snapshot pruning ].
>
>> lvcreate -L8G -s -n rootsnapshot /dev/vol00/root ( in my case )
>
> Yup, repeat for as many mount points as you normally have per above.
>
>> And to back out if needed -
>>
>> lvconvert --merge /dev/vol00/rootsnapshot ; init 6
>
> It's even easier than that.  Simply boot the snapshot (assuming you
> upgraded the origin).  To be more clear, add a new grub entry and
> change the root= on the kernel command line in the new entry to
> point to your snapshot.  If you do as I do and have a separate /usr
> and /var, you of course need to make sure you update /etc/fstab on
> the snapshot to boot the snapshotted version of those.
>
>> Granted I've had a back end running for 4 years now.  Never really had
>> a problem doing updates other then when changing MythtV releases
>> (tweaking etc...).
>
> Well, I run master (for the moment, will switch to 0.25-fixes when 0.25
> is released) so I like having the backout plan.
>
>> Don't know how much simpler it can get; to get back to go.
>
> Well, per above, it is even simpler if you simply boot the snapshot.
> I've never used the lvconvert --merge facility.  Does it have a time
> penalty while it merges the snapshot back up into the origin?

It can't merge an open lvm so it does it on the reboot.  It finishes
the convert after the reboot in a couple of minutes.

lvs shows status

I think the --merge is fairly new.  You can test your kernel to see if
it's supported -

dmsetup targets

>
> I'd just as soon keep my forks open though so that I can do a port-
> mortem should I need to.
>
> Cheers,
> b.
>
>
>
>
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Just thought I'd pass it on and get some feed back. Takes the nail biting away.