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

[Pasi Kärkkäinen <pasik@xxxxxx>]

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? 

> Is there some commandline tool to control the VHD snapshots? 
> 

Other than 'xe', of course.

-- Pasi


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