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AW: RES: [Xen-users] Shared volume: Software-ISCSI or GFS or OCFS2?

To: "Bruno Bertechini" <bruno.bertechini@xxxxxxxxxxx>
Subject: AW: RES: [Xen-users] Shared volume: Software-ISCSI or GFS or OCFS2?
From: "Rustedt, Florian" <Florian.Rustedt@xxxxxxxxxxx>
Date: Thu, 20 Nov 2008 13:33:53 +0100
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Thread-topic: RES: [Xen-users] Shared volume: Software-ISCSI or GFS or OCFS2?
 ...my experience with LVM-snapshoting a live-system is, that it freezes with 

While the snapshot is running, the disk is "frozen" for some moments and XFS is 
"intelligent" enough to recognize this and throws an error with the result, 
that the filesystem is shut down. This is normally a good behaviour to prevent 
a system to be corrupted when the disk fails. In this case it prevents 
snapshots from live-systems ;)

This is known on the mailling-lists, btu no near solution in sight.

I don't know, if other filesystems are better for this - better means more 
stupid ;) - but i wouldn't rely on it.

In the moment I am thinking of using
DRBD -> LVM -> (OCFS2 mounted directly) or (XFS distributed via NFS) for our 
solution, because i want to mount some disks multiple times.

As far as i understood now(please correct me), there are two principles to do 

1. file based, distributing file-based disks with some technology
2. blockdevice based, distributing blockdevices with some technology

In any case, you must decide how you want the three storages to be combined 
before you can start.
In case 2, your result could be a single blockdevice on which you could use lvm 
for easy partitioning.
In that case, you could mount the lv's directly with any filesystem on them, as 
long as you don't want to share or snapshot them. Perhaps the commercial DRBD+ 
(linbit.com) is interesting for this with three storages.

Regards, Florian
-----Ursprüngliche Nachricht-----
Von: Bruno Bertechini [mailto:bruno.bertechini@xxxxxxxxxxx] 
Gesendet: Donnerstag, 20. November 2008 12:57
An: 'John Haxby'
Cc: 'Bastian Blank'; Rustedt, Florian; xen-users@xxxxxxxxxxxxxxxxxxx
Betreff: RES: RES: [Xen-users] Shared volume: Software-ISCSI or GFS or OCFS2?

Well.. Let me use this thread and popup my environment and ask for

We have a EMC storage and 03 hosts (all with fibre channel adapters).

Choice 1 :

Regarding Xen backend : Files or LVM ? I'm thinking to use LVM to use online 
resize/snapshots. Are there huge performance differences?

I can't define a infrastructure for EMC/NFS/OCFS/etc without choose before 
select the appropriate backend.

What do you suggest guys?



-----Mensagem original-----
De: John Haxby [mailto:john.haxby@xxxxxxxxxx] Enviada em: quarta-feira, 19 de 
novembro de 2008 11:30
Para: Bruno Bertechini
Cc: 'Bastian Blank'; 'Rustedt, Florian'; xen-users@xxxxxxxxxxxxxxxxxxx
Assunto: Re: RES: [Xen-users] Shared volume: Software-ISCSI or GFS or OCFS2?

Bruno Bertechini wrote:
> Can someone explain why we should use NFS? It is fairly slow and unsecure.
It depends.  If you're using a NetApp server or an EMC Celerra then NFS 
isn't slow and it's secure "enough".   The big advantage of NFS is that 
it is easy to get working and it's well understood by a lot of people.

> Why not use some clustered FS ? 
NFS is a clustered file system -- it's a file system that's visible 
across a cluster.   If you mean an HA cluster, then yes, you can use 
something like ocfs2 or gfs/gfs2.   These things, however, are designed 
when you want concurrent access to individual files within the file system and 
for Xen disk image files you don't normally want that -- unless you're using a 
clustered file system in the guests of course.

The right file system for you depends, in detail, what you want to do.

> Are there alternatives ?

For Xen virtual disk files there are several alternatives: iSCSI and nbd 
both provide access to logical devices across a standard network.   That 
EMC Celerra I mentioned supports iSCSI as well as NFS and, depending on 
what you're doing, you may prefer that.   If you have a fibrechannel SAN 
or even infiniband you can use those as well.

For my non-production purposes I use NFS because it's easy and fast (that is, 
the quickest way to get data between two machines over the 
gigabit LAN is using NFS).   If I were setting up a production cluster 
then I would start with some shared non-local storage and I'd probably be 
looking at NFS again or iSCSI and the choice then depends on what the 
disk array box supports and is good at.   For a system built entirely 
out of stock PCs, well, I wouldn't. I wouldn't build a production cluster of 
any size that way: I want proper storage.

What you do depends on what you're planning to use these machines for and how 
much time and money you're spending.

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