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Re: [Linux-cluster] Snapshotting GFS and freezing



On Wed, Jan 20, 2010 at 09:15:48PM +0200, Pasi Kärkkäinen wrote:
> On Wed, Jan 20, 2010 at 03:14:02PM +0100, Dirk H. Schulz wrote:
> > Pasi Kärkkäinen schrieb:
> >> On Sun, Jan 17, 2010 at 02:47:00PM +0100, Dirk H. Schulz wrote:
> >>   
> >>> Ray Van Dolson schrieb:
> >>>     
> >>>>> Hmm.. so snapshots with CLVM are possible nowadays?
> >>>>>         
> >>>> No....
> >>>>
> >>>> RH has stated (recently on this list) that patches exist to do it, but
> >>>> it hasn't been a high enough priority for them to complete the work to
> >>>> the point where it could be distributed to customers.
> >>>>        
> >>> Okay then, how do you people out there with clusters that run virtual 
> >>> machines with live migration do backups of the virtual machines?
> >>> I surely do not want to shut down the virtual machine to be able to 
> >>> copy the image safely away if I have live migration available.
> >>>
> >>> At the moment there is only one way I can see. PLEASE prove me to be wrong.
> >>> Searching in RedHat's documentation I found that the problem is that 
> >>> lvm snapshots need exclusively allocated logical volumes. So I think 
> >>> the following
> >>> should be technically possible:
> >>> 0. Start environment: the logical volume containing the images of the 
> >>> virtual machines uses gfs and is mounted on all relevant cluster 
> >>> nodes since VMs are running on several cluster nodes.
> >>> 1. All VMs have to be migrated to one of the cluster nodes
> >>> 2. On all other nodes, the gfs volume is unmounted
> >>> 3. On the remaining node (where all VMs now run) the logical volume 
> >>> is bound exclusively with "lvchange -aey LOGICALVOLUME"
> >>>    (I hope this is possible without deactivating it first)
> >>> 4. Now GFS on this volume is frozen: "gfs_tool freeze  
> >>> /mountpoint/of/local/volume"
> >>>     
> >>
> >> Before freezing the GFS you should make sure the VMs are in consistent
> >> state, and the VMs have flushed their caches/buffers/disks.
> >>   
> > That means calling         sync
> >        echo 3 > /proc/sys/vm/drop_caches
> > inside the VM, right? Or is there anything more to do to flush everything?
> >
> 
> And before those commands stop any databases or complex applications.. 
> or make them flush their buffers and lock the tables for writes.
> 

You might also be interested of this thread:
http://lists.xensource.com/archives/html/xen-users/2010-01/msg00393.html

-- Pasi


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