Re: Feedback and errors

[Avi Kivity <avi@qumranet.com>]

Anthony Liguori wrote:
>>>
>>> 2/ two instances of kvm can be passed the same -hda. There is no 
>>> locking whatsoever. This messes up things seriously.
>>>
>>>       
>>
>> These two are upstream qemu problems. Copying qemu-devel.
>>
>> I guess using file locking by default would improve the situation, 
>> and we can add a -drive ...,exclusive=no option for people playing 
>> with cluster filesystems.
>>   
>
> This is not a situation where the user has a reasonable expectation of 
> what will happen that we violate.  If the user is unhappy with the 
> results, it's because the user made a mistake.  

Well, one user (me) has made this mistake, several times.

> FWIW, the whole override thing for Xen has been an endless source of 
> pain.  It's very difficult (if not impossible) to accurately determine 
> if someone else is using the disk.  

What's wrong with the standard file locking API?  Of course it won't 
stop non-qemu apps from accessing it, but that's unlikely anyway.

> Also, it tends to confuse people trying to do something legitimate 
> more often than helping someone doing something stupid.

-drive exclusive=off (or share=yes)

>
> I very frequently run multiple VMs with the same disk.  I do it 
> strictly for the purposes of benchmarking.  There are ways to share a 
> disk without using a clustered filesystem.

I imagine only raw format disks, and only as non-root filesystems (or 
with -shapshot, which should automatically set exclusive=off)?

>
> If a higher level management tool wants to enforce a policy (like 
> libvirt), then let it.  We should not be enforcing policies within 
> QEMU though.

I agree that qemu is not the place to enforce policies, but covering a 
hole that users are likely to step into, while allowing its explicit 
uncovering, is a good thing.  We're not enforcing the policy, only hinting.


-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.


-------------------------------------------------------------------------
This SF.net email is sponsored by the 2008 JavaOne(SM) Conference 
Don't miss this year's exciting event. There's still time to save $100. 
Use priority code J8TL2D2. 
http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone