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Re: [Xen-users] Sharing file/folder

Subject: Re: [Xen-users] Sharing file/folder
From: "lucianobarreto@xxxxxxxxx" <lucianobarreto@xxxxxxxxx>
Date: Mon, 14 Feb 2011 18:41:55 -0200
Cc: xen-users@xxxxxxxxxxxxxxxxxxx
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Sorry if duplicated...
Thanks guys, let me expose my problem. I'm prototyping a fault tolerance server (byzantine fault tolerance). Vms need do comunicate each order for make decisions about requests. My host will be out of network (cant access anything) and my Vms can comunicate to the external world, but the decision need to be local. Yes, I could comunicate via network inside VM to exchange information but my propose is to have a reliable local share to do it. In another work a friend used a Virtualbox folder share to do it, but now I need increase performance and use Xen to do it.

About clustered filesystems, any options to use it in a normal block device like Sata HD or SCSI? I understand..."clustered"... but any option?

Thanks and I will get more information about clustered filesystems

2011/2/14 Javier Guerra Giraldez <javier@xxxxxxxxxxx>
On Mon, Feb 14, 2011 at 1:37 PM, lucianobarreto@xxxxxxxxx
<lucianobarreto@xxxxxxxxx> wrote:
> I need to share some files between VMs. This files will be used to transfer
> some information (read/write). But I need do it without any network resource
> (NFS or others). I've tried to do it sharing a partition just for test
> proposes, but i see that when i create a file on one VM another can't see it
> and there isnt any concurrence in this approach.
> Anyone can help me??

to share files, you need a shared filesystem.  there are two main
classes of these:

- network filesystems: NFS, Samba, 9p, etc.  these work really well;
you shouldn't reject them without good reason.

- clustered filesystems: GFS, OCFS2, CXFS, etc. they're designed for
SAN systems where several hosts access the same storage box.  in VM
case, if you create a single partition accessible from several VMs you
get exactly the same situation, (shared block device) and need the
same solution.

what definitely won't work is to use a 'normal' filesystem (ext3/4,
XFS, ReiserFS, FAT, HPFS, NTFS, etc) on a shared partition (just like
it won't work in a shared block device).  Since every filesystem
aggressively caches metadata to avoid rereading the disk for every
access, a VM won't be 'notified' if another one modifies a directory,
so it won't 'notice' any change.  and worse, since now the cached
metadata isn't consistent with the content of the disk, any write will
result in a heavily corrupted filesystem.

better go with NFS


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Luciano Barreto
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