[Xen-users] Re: [rhelv5-list] shared storage manual remount ...

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Thu, Feb 04, 2010 at 01:40:26AM +0100, Zoran Popovi? wrote:
>    I am wondering if there is a way to solve the following problem: I suppose
>    that the usual way is to establish distributed file system with locking
>    mechanisms like it is possible with GFS and Red Hat Cluster Suite or
>    similar, but I am interested in doing some of this manually and ONLY with
>    raw devices (no file system), or simply in knowing some general
>    principles. The case: I have a VLUN (on FC SAN) presented on two servers,
>    but mounted only on one host - to be more precise, used by a Xen HVM guest
>    system as a raw physical phy:// drive. Then, I put this guest down, and
>    bring it manually up on second host - it can see changed images, and make
>    changes to the presented disks. Then I put it down there, and bring it up
>    again on the first host - BUT THEN, this guest (or host) doesn't see
>    changes made by the second system, it still sees the picture as it was the
>    way it left it.
>    Or even better, if I bring HVM guest on a host, then put it down, make
>    restore of his disks on the storage (I am using HP EVA8400, restoring
>    original disk from a snapshot - it does have redundant controllers but
>    their cache must be in sync for sure), and then bring it up - it still
>    sees things on the disks as they were before restore. But if I _RESTART_
>    the host, it can see restored disks correctly. Now, I am wondering why is
>    this happening, and if it is possible somehow to resync with the storage
>    without restart (I wouldn't like that on production ! and on our windows
>    systems this is possible) ... I've tried sync (but that is like flushing
>    buffer cache), and I didn't try echo 3 > /proc/sys/vm/drop_caches after
>    that (I've just come upon some articles about that), and I am not sure if
>    that would really invalidate cache and help me. What is the right way of
>    dong this ? Please, help ...
>    ZP.

Exactly what changes in the guest are you talking about? (that are not visible 
after
switching hosts).

There was a pygrub caching bug in Xen in EL5, but that shouldn't affect HVM 
guests,
since they don't use pygrub.

If you use phy: backend for the disks, then there should be no caching in dom0.
Please paste your /etc/xen/hvmguest config file.

-- Pasi


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