Re: [Xen-users] XCP - GFS - ISCSI

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Mon, Jan 18, 2010 at 10:36:29AM +0000, Julian Chesterfield wrote:
> Pasi Kärkkäinen wrote:
> >On Fri, Jan 15, 2010 at 02:06:30PM +0200, Pasi Kärkkäinen wrote:
> >  
> >>On Fri, Jan 15, 2010 at 11:56:46AM +0000, Julian Chesterfield wrote:
> >>    
> >>>Dave Scott wrote:
> >>>      
> >>>>Hi Pasi,
> >>>>
> >>>>[cc:d Julian who is responsible for storage in XCP]
> >>>>
> >>>> 
> >>>>        
> >>>>>I haven't looked at the XCP code yet, but are there some special
> >>>>>patches for
> >>>>>LVM to make it work in a shared environment on multiple hosts?
> >>>>>
> >>>>>I guess it's not CLVM, since you support snapshots.. so xapi is doing
> >>>>>some coordination of management commands and making sure only one LVM
> >>>>>command is issued at a time?
> >>>>>   
> >>>>>          
> >>>>Julian could describe the detail better than me but my high-level 
> >>>>understanding is:
> >>>>
> >>>>* xapi nominates one host to be the 'SR master': all LVM 
> >>>>metadata-changing commands are run here
> >>>>
> >>>>* all hosts are allowed to map/unmap LVs so the LVM commands were 
> >>>>patched to make absolutely sure they didn't attempt to change any 
> >>>>metadata
> >>>>
> >>>>* unless you request a special "raw" LV, vhd metadata is added to the 
> >>>>LV: this is how we handle snapshots
> >>>> 
> >>>>        
> >>>Yep, this is correct. We use XAPI as the "Cluster lock manager" 
> >>>essentially. There is a strict notion of ordering of events, and XAPI 
> >>>always ensures that there is a single SRMaster for any shared SR. The SR 
> >>>master is the only entity that modifies LVM metadata, and it 
> >>>strategically refreshes slaves as necessary. Typically slaves only 
> >>>operate in an LVM Read-only mode, so the LVM metadata is refreshed when 
> >>>a slave needs to access a new logical volume, and the slave is only 
> >>>allowed to create device-mapper nodes, never to modify the LVM metadata. 
> >>>There are patches to LVM to add an explicit 'master' flag, this ensures 
> >>>that non-masters never attempt to repair LVM metadata if ever it is read 
> >>>and found to be inconsistent. In practice this would never happen due to 
> >>>the way LVM updates its metadata and the fact that we do not allow 
> >>>shared Volume Groups that span more than one LUN, however it's an 
> >>>important safety catch.
> >>>
> >>>      
> >>Thank you both for answers! This is good explanation of how it works.
> >>
> >>    
> >>>Snapshot and clone support is provided via the VHD layer that resides 
> >>>above raw Logical Volumes. i.e. we create VHD Copy-on-write instances in 
> >>>the same way as the file-based VHD support (e.g. NFS or local Ext3 
> >>>partitions).
> >>>
> >>>      
> >>Oh, so XenServer/XCP doesn't use LVM snapshots at all? That's good to 
> >>know.
> >>
> >>    
> >
> >So where/how does it store the deltas between the original volume and
> >the snapshot? 
> >  
> A VHD format disk is created on top of the raw Logical Volume. VHD has a 
> metadata format that provides sparseness and also allows chains of 
> dependencies to be created as differencing Copy-on-write disks via the 
> parent locator fields.
>

Ok. 


> >  
> >>Is there some commandline tool to control the VHD snapshots? 
> >>
> >>    
> >
> >Other than 'xe', of course.
> >  
> Yes. There is a vhd-util commandline tool that allows you to query VHD 
> headers, generate new VHD images, create CoW VHD dependency images etc...
> 

Great. I have to try these some day.

Thanks!

-- Pasi


_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users