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AW: [Xen-users] Live migration and drbd

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Subject: AW: [Xen-users] Live migration and drbd
From: "Rustedt, Florian" <Florian.Rustedt@xxxxxxxxxxx>
Date: Mon, 10 Nov 2008 11:07:24 +0100
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Thread-topic: [Xen-users] Live migration and drbd
 Well, ok, if you're only running images with one side and have no shared 
virtual disks, then you only need write access from one side...

But the cute advantage of live-migration & drbd  & paravirtualization is, that 
you can do load-balancing with two active xen-hosts, so running several 
machines on both sides with several shared partitions, p.e. "/usr" or 
"/lib/modules", "/boot", "/usr/src", "/var/log" etc..

And in that moment, you've got concurrent access to the same file(system) and 
need an additional secutiry layer between.

So, as far as i can see, we're both right, depending on the eye of the beholder 

Regards, Florian

-----Ursprüngliche Nachricht-----
Von: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
[mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] Im Auftrag von Rafal Kupka
Gesendet: Montag, 10. November 2008 10:41
An: xen-users@xxxxxxxxxxxxxxxxxxx
Betreff: Re: [Xen-users] Live migration and drbd

On Mon, Nov 10, 2008 at 10:07:48AM +0100, Rustedt, Florian wrote:

> No no,
> that's not right!!
> You definetly need an network-filesystem for drbd primary/primary!

Not for Xen live migration.

> There are TWO systems, that access files for writing and both are not 
> running the same kernel ergo doesn't "know", that the other side is 
> actually locking/writing a file!

It is explained in http://www.drbd.org/users-guide/ch-xen.html. Xen needs 
primary/primary for block device write test during live migration only.

>From that manual:
"Enabling dual-primary mode is necessary because Xen, before initiating live 
migration, checks for write access on all VBDs a resource is configured to use 
on both the source and the destination host for the migration."

> So you need OCFS & co. For doing the file-locking....

If you want concurrent file access from two DomUs -- yes, otherwise no.

Great software without the knowledge to run it is pretty useless.
(Linux Gazette #1)

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