Re: [Xen-users] Xen 4.0 feature request

[Tim Post <echo@xxxxxxxxxxxx>]

>  The admin program could then signal a guest to go into a 
> 'clean' state (and wait for it to reach that state), make an LVM 
> snapshot of it's volume, and then signal the guest to resume. Your 
> LVM snapshot would then represent the guests view of the volume at 
> that instant - but it still would not be clean in the way that an 
> unmounted filesystem with no open files is clean.
> 

We have actually developed tools that do this for xen, dom-0 is able to
obtain the guest's cooperation via simple writes to xenstore, which the
guest watches and acts upon.

The problem is (and this holds true for VMWare too) is that some user
space programs do their own buffering, relational database servers being
a good example. So, the kernel being in a clean state is just 2/3 of the
battle.

Of course its possible to add guest side hooks that would handle telling
DB servers to write everything in memory (and block until all
transactions are closed) prior to telling dom-0 to 'go ahead' .. but
that just gets icky.

There's also the issue of files in mid transfer (ftp uploads, etc). In
some cases, snapshots work very well .. in other cases not so well, no
matter what virtualization technology is being used.

I think it goes to show, if the UI is aesthetically pleasing, many
people just don't consider the underlying mechanisms .. why would
they ? :)

Cheers,
--Tim



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