adding file vbd to running domain

adding file vbd to running domain

[Robin Kluit]

Using the current unstable version,

is it possible to add a File-backed virtual block device to a running 
domain?
I've setup a partition with several files on them, as described in the
manual, which i want to mount in running domains on a as-needed base.

Regards, Robin Kluit




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Re: adding file vbd to running domain

[Keir Fraser]

The virtual blkdev driver can only talk to block devices. If there was
some way of 'mounting' a file as a block device then you could connect
your guest to that block-device node, but I don't think there's any
way of doing such a mount.

Probably the most flexible way of managing guest OS disc volumes is to
use LVM2. Then you can create fleible logical volumes on top of one or
more real disc partitions (physical volumes) and thereby get the
manageability you are looking for. 

We currently have no integration of our toolset with LVM2, but there's
no reason for you not to set up some logical volumes, and then specify
those in your guest-OS configuration files. Better integration is
perhaps something we will investigate in future.   

 -- Keir


> Using the current unstable version,
> 
> is it possible to add a File-backed virtual block device to a running 
> domain?
> I've setup a partition with several files on them, as described in the
> manual, which i want to mount in running domains on a as-needed base.
> 
> Regards, Robin Kluit
> 
> 
> 
> 
> -------------------------------------------------------
> SF.Net email is sponsored by Shop4tech.com-Lowest price on Blank Media
> 100pk Sonic DVD-R 4x for only $29 -100pk Sonic DVD+R for only $33
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> Xen-devel@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/xen-devel



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Re: adding file vbd to running domain

[Ian Pratt]

> 
> The virtual blkdev driver can only talk to block devices. If there was
> some way of 'mounting' a file as a block device then you could connect
> your guest to that block-device node, but I don't think there's any
> way of doing such a mount.

Well, if you've got the loop device built into your dom0 kernel
you can use 'losetup' to turn files into block device and then
export them to other domains.

Last month I posted an example of how to create a sparse file
with 'dd' and export it to another domain.

However, by the sounds of it you want to be able to dynamically
add and remove block devices to domains. This used to work under
Xen 1.2, but I'm afraid I think it got broken in the unstable
tree and is yet to be fixed. Although xend and backend blk driver
supports adding and removing blk devices, there's no way to tell
the frontend driver to refresh its view of what block devices are
available. 

Re-writing and simplifying the blk device control message
interface is on the todo list.
 
> Probably the most flexible way of managing guest OS disc volumes is to
> use LVM2. Then you can create fleible logical volumes on top of one or
> more real disc partitions (physical volumes) and thereby get the
> manageability you are looking for. 
> 
> We currently have no integration of our toolset with LVM2, but there's
> no reason for you not to set up some logical volumes, and then specify
> those in your guest-OS configuration files. Better integration is
> perhaps something we will investigate in future.   

I posted another message earlier in the month that gives example
invocations for setting up CoW block devices using LVM2
snapshots.

Ian


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