Re: [Xen-users] High availability and live migration with just LVMand hardware fencing

[Bart Coninckx <bart.coninckx@xxxxxxxxxx>]

On Friday 21 May 2010 15:32:22 John Madden wrote:
> >  > What's wrong with backing up from the clients?
> >
> > You mean the guests? Well, it could be they get corrupted while for
> > instance failing over because a xen host dies. Quickest restore path is
> > then a snapshot restore and then a guest data restore.
> 
> If you're doing guest data restore once the guest is up, why bother with
> snapshots at all?  (In other words, you can deploy VMs all day from a
> vanilla install process and then restore data, so snapshots don't really
> buy you anything.)

John,

Basically to safe time. Your reasoning makes sense for Linux systems where 
restore is just a copy of files, but for Windows HVM guests the restore 
process could be really lon because of installation of individual programs, 
settings, mailboxes etc. 

> > Wait a sec, so i still would have no snapshots? How is this achieved
> > then with hvm guests?
> 
> Correct.  I don't know how anyone accomplishes this or thinks that they
> do, none of it adds up as far as I'm concerned.  If you want consistent
> backups, they have to happen either FROM the client or somehow in
> concert with it.
> 
> You wouldn't run fsck on a mounted filesystem, would you?  Or assume
> that a 'dd if=/dev/sda' backup would be consistent while you were
> writing data to the disk, right?  Well, taking snapshots "from the san"
> or "from dom0" without "letting the client know" is the same thing.

Well, maybe I'm a bit influenced by my Vmware past, where it is possible to 
put a Windows guest in a suspended state, having it flushed all data to disk 
and remaining in a frozen state, where it is possbile to then take a quick LVM 
snaphost, start the guest again and copy the LVM snapshot for possible future 
restore. I guess I am looking for the same thing with Xen. But it would 
require something like passing a ACPI suspend command to the guest; I believe 
Vmware does this, either by using Vmware tools, either by sending the ACPI 
command.

> > Do they need to be stopped for backups? Live migration still works,
> > right? (Provided i have clvm)
> 
> LVM+cLVM is only one way to accomplish live migration.  You can also do
> it with disk images stored on NFS or a clustered filesystem like OCFS2,
> for example.  I happen to like LVM+cLVM because I prefer the nature and
> performance advantage of phy: disk assignment.

I recently read that Suse changed the Xen scripts to be able to use an iSCSI 
target directly as vdb in the Xen config file, allowing live migration without 
cLVM. One user reported that this worked without problems for him. What are 
your thoughts about that?

Cheers,


Bart

> John
> 

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