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

[Xin Zhao <zhaoxin@eecs.umich.edu>]

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. :)

Xin

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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>>
>>     
>
>
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