qcow and updates

qcow and updates

[Bill Davidsen]

This question came out of a discussion of nesting (or stringing if you 
like) disk images using qcow.

If I have an unpatched install image, say CentOS-5.2 (box "A" below). 
and I make a copy of that with qcow and apply patches to that copy (box 
"B" below), everything is just fine. But if I should make a working qcow 
image from the patched version (box "C" below), which works fine 
initially, what would happen if I applied patches to the "current" 
version? I assume that would mess up the application version, since it's 
a physical copy, but I'd like to be sure.

	 ______________
	| [A]          |
	| orig release |
	|______________|
		||
	   {qcow copy|
		||
	 ____________________
	| [B]                |
	| patched to current |
	|____________________|
		||
	   {qcow copy}
		||
	 _________________________
	| [C]                     |
	| fully config. app. svr. |
	|_________________________|

I could play with union filesystem, network mounting of /var and /usr, 
and other tricks, but I thought I'd check that this really is a problem, 
and the current version needs a "real" copy to the app server, and then 
the app server needs to be patched after that.

-- 
Bill Davidsen <davidsen@tmr.com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

Re: qcow and updates

[Avi Kivity]

Bill Davidsen wrote:
> This question came out of a discussion of nesting (or stringing if you 
> like) disk images using qcow.
>
> If I have an unpatched install image, say CentOS-5.2 (box "A" below). 
> and I make a copy of that with qcow and apply patches to that copy 
> (box "B" below), everything is just fine. But if I should make a 
> working qcow image from the patched version (box "C" below), which 
> works fine initially, what would happen if I applied patches to the 
> "current" version? I assume that would mess up the application 
> version, since it's a physical copy, but I'd like to be sure.
>
>      ______________
>     | [A]          |
>     | orig release |
>     |______________|
>         ||
>        {qcow copy|
>         ||
>      ____________________
>     | [B]                |
>     | patched to current |
>     |____________________|
>         ||
>        {qcow copy}
>         ||
>      _________________________
>     | [C]                     |
>     | fully config. app. svr. |
>     |_________________________|
>
> I could play with union filesystem, network mounting of /var and /usr, 
> and other tricks, but I thought I'd check that this really is a 
> problem, and the current version needs a "real" copy to the app 
> server, and then the app server needs to be patched after that.
>


Changing anything except the last image in the chain is liable to 
break.  In other words: once you use an image as a base for another 
image, this first image must never change.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.