[libvirt] [PATCH] vz: set mount point for container image-based disks

Maxim Nestratov mnestratov at virtuozzo.com
Mon Sep 21 09:35:41 UTC 2015

21.09.2015 12:23, Daniel P. Berrange пишет:
> On Mon, Sep 21, 2015 at 12:14:57PM +0300, Maxim Nestratov wrote:
>> 21.09.2015 11:44, Daniel P. Berrange пишет:
>>> On Sun, Sep 20, 2015 at 10:17:51PM +0300, Maxim Nestratov wrote:
>>>> From: Maxim Nestratov <mnestratov at virtuozzo.com>
>>>> In order to support not only root disks with type=file for containers,
>>>> we need to specify mount points for them.
>>>> For instance, if a secondary disk is added by the following record in
>>>> xml:
>>>>      <disk type='file' device='disk'>
>>>>        <driver type='ploop' cache='writeback'/>
>>>>        <source file='/vz/some_path_to_image_dir'/>
>>>>        <target bus='sata' dev='sdb'/>
>>>>      </disk>
>>>> we are going to add it to container mounted to '/mnt/sdb' path.
>>> That's not what the <disk> element is for.  <disk> is about exposing
>>> block device nodes to the container. It shouldn't try todo anything
>>> with this device nodes. They might be used as raw data storage by
>>> an application, so we can't assume they should be mounted.
>>> If you want to mount things then you should be using <filesystem>
>>> instead.
>> Hm. It actually means that any disks with type file shouldn't work in
>> containers. Right?
> No, the disk source on the host is not correlated to how it is
> exposed to the guest.
> With <disk type="file"> it means you have to setup some kind of
> loop device in the host OS, and then expose the block device to
> the guest with that. The same if you have <disk type="network">
> then you have to use host kernel or qemu-nbd, for example, to
> setup a block device that you can then expose to the guest.
>> And working root disks like this is a mistake? But why?
> Yes, that would be a mistake too. The root filesystem should
> be exposed using <filesystem> with a <target> of /
>> In vz, any images plugged into containers are also treated as disks. The
>> only difference between 'filesystem' and 'disk' is whether we should mount
>> it or not. That's all. While from point of view of a container user it is
>> just another storage. Why not  just mount it automatically?
> The <disk> element is intended to expose raw block devices to the guest.
> The <filesystem> is intended to expose mounted volumed to the guest.
> A <disk> can support both type=block and type=file as sources on the
> host side, as well as others like type=network.
> A <filesystem> can support both type=block and type=file as sources
> on the host, as well as a few others.
> If you make <disk> automatically mount volumes, then you've just
> discarded the only semantic difference between <filesystem> and
> <disk>, which is not only wrong but also pointless. If you want
> to mount it, you should just use <filesystem>, and not try to
> make <disk> do the same as <filesystem>.
> It is key that <disk> does *not* try to interpret what todo with
> the storage - it is entirely upto the guest to decide what todo
> with it. For example, an oracle database might decide to use the
> block device directly as data storage. Or you might be running an
> application that wants to manipulate the filesystem (eg a fsck
> tool) inside the block dev, so again it would be inappropriate to
> mount it.
> Regards,
> Daniel
Ok. I see. Thank you for the explanation.
Please, disregard the patch then.

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