[libvirt] [PATCH] Allow use of file images for LXC container filesystems

[Eric Blake]

On 08/04/2011 09:41 AM, Daniel P. Berrange wrote:
> From: "Daniel P. Berrange"<berrange at redhat.com>
>
> A previous commit gave the LXC driver the ability to mount
> block devices for the container filesystem. Through use of
> the loopback device functionality, we can build on this to
> support use of plain file images for LXC filesytems.
>
> By setting the LO_FLAGS_AUTOCLEAR flag we can ensure that
> the loop device automatically disappears when the container
> dies / shuts down
>
> * src/lxc/lxc_container.c: Raise error if we see a file
>    based filesystem, since it should have been turned into
>    a loopback device already
> * src/lxc/lxc_controller.c: Rewrite any filesystems of
>    type=file, into type=block, by binding the file image
>    to a free loop device

Neat trick.

If I understand correctly, 'dumpxml' for an lxc domain using one of 
these loopback files will have the live xml showing the loopback device 
created, while the configured xml still shows the original file.  But I 
don't see anything that reverts type=block back into type=file when 
doing a VIR_DOMAIN_XML_INACTIVE, other than the fact that live and 
configured xml are stored in separate domain def objects.

I'm worried that you might be missing some changes to domain_conf.[hc] 
to correctly manipulate actual vs. configured setup all within the 
context of live xml.  This is a similar problem to the recent <actual> 
for networks - we must track which loopback block device was tied to the 
the filesystem, in case libvirtd gets restarted before the lxc domain 
goes away, but we also don't want to tie ourselves to that device when 
the lxc domain reboots (rather, each boot should create a new loopback 
device, opened on the original file).

On the other hand, VIR_DOMAIN_XML_INACTIVE normally means you will only 
see the configured setup, and is only ever used on active configuration 
by virDomain[Managed]Save, which are currently unsupported with lxc. 
And the LO_FLAGS_AUTOCLEAR ends the loopback device as soon as the lxc 
domain quits, which is a convenient way to handle a lot of cleanup.

So while I didn't find any coding bugs in this patch, I need some 
reassurance that we are handling things correctly from the 
VIR_DOMAIN_XML_INACTIVE aspect before I give an ack.

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org