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Re: [Xen-devel] Re: [Xen-users] How to share data between guest domains

NFS combined with your solution is definitely a temporary solution. But our ongoing project "Virtual Librarian" should provide better support. VL is designed to allow mutliple VMs to share a base software environment. If a VM needs to modify a shared file, VL can do copy-on-write to create a private copy for this VM. All modifications are therefore visible only to this VM In addition, if multiple private copies are identical, VL can merge them back to a shared copy.

The benefits of VL inlcude:
1. A VM can take advantage of the global disk cache and benefit from previous data accesses from other VMs. We will expect better performance. 2. VL allows finer granularity of sharing, instead of directory level sharing. 3. The shared file system is transparent to guest applications and should be easily adopted. 4. VL allows centralized software updates. These updates can take effect right after the files are updated.

We will put a detailed description of VL soon, if someone is interested in that. :)


Yura Pismerov wrote:
I found that using NFS for things like this makes much more sense.
You can run the domU with NFS root (read-only) and map certain areas you
need read/write
to tmpfs by mounting them with "mount --bind" in Linux). For example, if
I use NFS root and want my /etc
be writable I can always write its content to a tmpfs mounted area and
run "mount --bind /tmpfs/etc /etc".
This also will solve problems with centralized package updates when not
only /usr is being updated, but some other areas (eg. /etc, /var/lib).
You want those areas be shared between domU's as well.

Molle Bestefich wrote:

Todd D. Esposito wrote:
However, on that note, I wonder if you could mount the same file system,
say something like /usr, into multiple domU's READ ONLY.
That works for me.

What doesn't work is mounting that file/device READ/WRITE in one domU
to update the filesystem. For that, I have to take down *all* domUs. Not good...

(When I try I get a vbd: error saying "already in use".)

(I know about caching and that I need eg. a cluster-aware filesystem
to do this.)

I've spent a couple of hours hunting through various Xen source files.
There's a lot of Python functions that are only 3-5 lines long and
which does little else than calling the next function, which makes it
very hard to figure out what's going on :-/.

Could one of you devel guys please let me know where I need to go to
remove this silly limitation? :-)

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