Xen users - Nov 2008 - Shared volume: Software-ISCSI or GFS or OCFS2?

Hello list,

I want to use shared volumes between severall vm''s and defenetly
don''t
want to use NFS or Samba!

So i have three options:
1. simulated(software-) iscsi
2. GFS
3. OCFS2

What do you suggest and why?

Kind regards, Florian
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Well, I''m not sure it''s a matter of iSCSI OR a filesystem, but
that
depends on how you decide to go about setting it up.  In my environment,
I have both iSCSI and FC access to the same volumes.  Some of my servers
have FC HBAs and connect to my XEN volume that way, some of them have
hardware iSCSI HBAs, and some of them use software iSCSI initiators.  I
have three XEN volumes - one for server domUs, one for VDI domUs, and
one for development domUs.  All of these volumes are formatted with
OCFS2 so that I can mount them on all of my servers.  The domUs then use
file-based disks that reside on the shared volumes, so I can migrate
them back and forth between servers relatively easily.

You could use iSCSI to do individual volumes for each of your domUs - I
found it a bit cumbersome to have to go into either EVMS or LVM each
time I wanted to create a domU and create a volume for it, so I''m
sticking with the file-based approach.

-Nick


-----Original Message-----
From: Rustedt, Florian <Florian.Rustedt@smartnet.de>
To: xen-users@lists.xensource.com
Subject: [Xen-users] Shared volume: Software-ISCSI or GFS or OCFS2?
Date: Fri, 14 Nov 2008 11:09:33 +0100


Hello list,

I want to use shared volumes between severall vm''s and defenetly
don''t
want to use NFS or Samba!

So i have three options:
1. simulated(software-) iscsi
2. GFS
3. OCFS2

What do you suggest and why?

Kind regards, Florian
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are intended for the
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If you have received this email in error, please notify the system manager or
the sender immediately and do
not disclose the contents to anyone or make copies thereof.
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Ok?
So what''s the real advantage of using "software-iscsi" then
for me?
And what about GFS instead of OCFS2?
 
Kind regards, Florian

________________________________

Von: Nick Couchman [mailto:Nick.Couchman@seakr.com] 
Gesendet: Freitag, 14. November 2008 17:23
An: Rustedt, Florian
Cc: xen-users@lists.xensource.com
Betreff: Re: [Xen-users] Shared volume: Software-ISCSI or GFS or OCFS2?


Well, I''m not sure it''s a matter of iSCSI OR a filesystem, but
that
depends on how you decide to go about setting it up.  In my environment,
I have both iSCSI and FC access to the same volumes.  Some of my servers
have FC HBAs and connect to my XEN volume that way, some of them have
hardware iSCSI HBAs, and some of them use software iSCSI initiators.  I
have three XEN volumes - one for server domUs, one for VDI domUs, and
one for development domUs.  All of these volumes are formatted with
OCFS2 so that I can mount them on all of my servers.  The domUs then use
file-based disks that reside on the shared volumes, so I can migrate
them back and forth between servers relatively easily.

You could use iSCSI to do individual volumes for each of your domUs - I
found it a bit cumbersome to have to go into either EVMS or LVM each
time I wanted to create a domU and create a volume for it, so I''m
sticking with the file-based approach.

-Nick


-----Original Message-----
From: Rustedt, Florian <Florian.Rustedt@smartnet.de
<mailto:%22Rustedt,%20Florian%22%20%3cFlorian.Rustedt@smartnet.de%3e> >
To: xen-users@lists.xensource.com
Subject: [Xen-users] Shared volume: Software-ISCSI or GFS or OCFS2?
Date: Fri, 14 Nov 2008 11:09:33 +0100


Hello list,

I want to use shared volumes between severall vm''s and defenetly
don''t
want to use NFS or Samba!

So i have three options:
1. simulated(software-) iscsi
2. GFS
3. OCFS2

What do you suggest and why?

Kind regards, Florian
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If you have received this email in error, please notify the system
manager or the sender immediately and do 
not disclose the contents to anyone or make copies thereof.
*** eSafe scanned this email for viruses, vandals, and malicious
content. ***
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**********************




________________________________

This e-mail may contain confidential and privileged material for the
sole use of the intended recipient. If this email is not intended for
you, or you are not responsible for the delivery of this message to the
intended recipient, please note that this message may contain SEAKR
Engineering (SEAKR) Privileged/Proprietary Information. In such a case,
you are strictly prohibited from downloading, photocopying, distributing
or otherwise using this message, its contents or attachments in any way.
If you have received this message in error, please notify us immediately
by replying to this e-mail and delete the message from your mailbox.
Information contained in this message that does not relate to the
business of SEAKR is neither endorsed by nor attributable to SEAKR. 

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On Fri, Nov 14, 2008 at 11:09:33AM +0100, Rustedt, Florian
wrote:
> I want to use shared volumes between severall vm''s
What is the purpose of this?
>                                                        defenetly
don''t
> want to use NFS or Samba!
Why?
> So i have three options:
> 1. simulated(software-) iscsi
> 2. GFS
> 3. OCFS2
This list shows that you have no clue what you are doing. iSCSI is a
transport protocol. gfs and ocfs2 are filesystems.
> What do you suggest and why?
Stick with NFS. Cluster filesystems are complex and fragile beasts. As
long as you don''t need it for scalability reasons, it does not provide
you much gain.
>
**********************************************************************************************
> IMPORTANT: The contents of this email and any attachments are confidential.
They are intended for the
> named recipient(s) only.
The recipients are always listed in the envelope.
> If you have received this email in error, please notify the system manager
or the sender immediately and do
> not disclose the contents to anyone or make copies thereof.
> *** eSafe scanned this email for viruses, vandals, and malicious content.
***
Do you want to vouch that noone is able to modify an _unsigned_ message
in transit?

Bastian

-- 
Klingon phaser attack from front!!!!!
100% Damage to life support!!!!

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Can someone explain why we should use NFS? It is fairly slow and unsecure.

Why not use some clustered FS ? 

Are there alternatives ?

Bruno

-----Mensagem original-----
De: xen-users-bounces@lists.xensource.com
[mailto:xen-users-bounces@lists.xensource.com] Em nome de Bastian Blank
Enviada em: quarta-feira, 19 de novembro de 2008 09:46
Para: Rustedt, Florian
Cc: xen-users@lists.xensource.com
Assunto: Re: [Xen-users] Shared volume: Software-ISCSI or GFS or OCFS2?

On Fri, Nov 14, 2008 at 11:09:33AM +0100, Rustedt, Florian
wrote:
> I want to use shared volumes between severall vm''s
What is the purpose of this?
>                                                        defenetly
don''t
> want to use NFS or Samba!
Why?
> So i have three options:
> 1. simulated(software-) iscsi
> 2. GFS
> 3. OCFS2
This list shows that you have no clue what you are doing. iSCSI is a
transport protocol. gfs and ocfs2 are filesystems.
> What do you suggest and why?
Stick with NFS. Cluster filesystems are complex and fragile beasts. As
long as you don''t need it for scalability reasons, it does not provide
you much gain.
>
****************************************************************************
******************
> IMPORTANT: The contents of this email and any attachments are
confidential. They are intended for the 
> named recipient(s) only.
The recipients are always listed in the envelope.
> If you have received this email in error, please notify the system manager
or the sender immediately and do 
> not disclose the contents to anyone or make copies thereof.
> *** eSafe scanned this email for viruses, vandals, and malicious content.
***

Do you want to vouch that noone is able to modify an _unsigned_ message
in transit?

Bastian

-- 
Klingon phaser attack from front!!!!!
100% Damage to life support!!!!

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users


_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

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.

jch

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

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

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?

Regards.

Bruno

-----Mensagem original-----
De: John Haxby [mailto:john.haxby@oracle.com] 
Enviada em: quarta-feira, 19 de novembro de 2008 11:30
Para: Bruno Bertechini
Cc: ''Bastian Blank''; ''Rustedt, Florian'';
xen-users@lists.xensource.com
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.

jch


_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

...my experience with LVM-snapshoting a live-system is, that it freezes with
XFS-filesystem.

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
this:

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@acensco.com] 
Gesendet: Donnerstag, 20. November 2008 12:57
An: ''John Haxby''
Cc: ''Bastian Blank''; Rustedt, Florian;
xen-users@lists.xensource.com
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
suggestions:

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?

Regards.

Bruno

-----Mensagem original-----
De: John Haxby [mailto:john.haxby@oracle.com] Enviada em: quarta-feira, 19 de
novembro de 2008 11:30
Para: Bruno Bertechini
Cc: ''Bastian Blank''; ''Rustedt, Florian'';
xen-users@lists.xensource.com
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.

jch
**********************************************************************************************
IMPORTANT: The contents of this email and any attachments are confidential. They
are intended for the
named recipient(s) only.
If you have received this email in error, please notify the system manager or
the sender immediately and do
not disclose the contents to anyone or make copies thereof.
*** eSafe scanned this email for viruses, vandals, and malicious content. ***
**********************************************************************************************


_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

Bruno Bertechini wrote:
> Well.. Let me use this thread and popup my environment and ask for
> suggestions:
>
> 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?
>   
Do you want (live) migration?   If so then you''ll need to be able to 
access the files/LUNs/logical volumes from each machine in the cluster.

That probably constrains your choices somewhat: there is cluster LVM 
(clvm) but I don''t know enough to say anything for or against it; nfs 
and ocfs2 will both work of course; and then there''s the third choice
of
LUNs in the EMC storage being assigned to more than one machine or 
migrated from machine to machine and I have no idea how that is done as 
I don''t have that kind of hardware to hand.

I will say this something resizing and snapshots though.

You don''t need LVM for resizing: you can grow a file by dd''ing
empty
space from /dev/zero on to the end of it.  You''ll need to restart the 
guest that''s using the file to pick up the new file size (although I 
think that might have changed in 3.3, I''m not sure).  Likewise you can 
grow LUNs defined in the array.

Snapshots are not the panacea you might think.   If you just take a 
snapshot of a guest''s virtual disk then what you''ve got is a
corrupt
virtual disk because you haven''t got what was in the guest''s
memory and
on the way to the disk.  If you xm save the guest and copy the memory 
snapshot the disk then what you''ve done is basically a fork(2) of an OS
-- when you restore the backed up copy it''ll carry on doing exactly
what
it was doing at the time of the backup.  My favourite example of the day 
is what happens when that guest is about to change a database over a 
network connection -- that change will happen twice.   Let''s hope
it''s
$salary = $salary * 1.1.

jch

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

Sorry my bad.

I don''t have 03 storages. I have 01 storage and 03 hosts accessing
them.

My goal is (regardless technology): Provide minumum downtime with Xen. I
mean, IF a host has a harwdare malfunction, I can Just start the domU in
another host without do anything else unless mount/Access the path.

Regards

Bruno 

-----Mensagem original-----
De: Rustedt, Florian [mailto:Florian.Rustedt@smartnet.de] 
Enviada em: quinta-feira, 20 de novembro de 2008 10:34
Para: Bruno Bertechini
Cc: xen-users@lists.xensource.com
Assunto: AW: RES: [Xen-users] Shared volume: Software-ISCSI or GFS or OCFS2?

 ...my experience with LVM-snapshoting a live-system is, that it freezes
with XFS-filesystem.

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 this:

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@acensco.com] 
Gesendet: Donnerstag, 20. November 2008 12:57
An: ''John Haxby''
Cc: ''Bastian Blank''; Rustedt, Florian;
xen-users@lists.xensource.com
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
suggestions:

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?

Regards.

Bruno

-----Mensagem original-----
De: John Haxby [mailto:john.haxby@oracle.com] Enviada em: quarta-feira, 19
de novembro de 2008 11:30
Para: Bruno Bertechini
Cc: ''Bastian Blank''; ''Rustedt, Florian'';
xen-users@lists.xensource.com
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.

jch
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I got it.

My current situation is :

We have several Lun''s attached to each hosts. None of them attached to
more
than one at sime time. But we can do it. It''s pretty easy.

As I Said, my goal is the less downtime as possible. I don''t need live
migration I guess. 

Im quite confused on how I can achieve the Best performance and less
downtime with my configuration.

Here it is:

01 EMC Storage (currently with several Lun''s configured).
03 Dell PowerEdge 2900 Hosts with Fibre Channel PCI cards connected to a
fibre switch 
06 domU''s Linux PV 
08 domU''s Windows 2003 R2 64bits
06 domU''s Windows 2008 64bits

Few other domU''s with time.

Im trying to plan a stable backup procedure together with minimum downtime.

I can also use Symantec Backup Exec to take care of Backup stuff directly
for the guests (domU''s)...

What do you guys suggest?

Thanks

Bruno

-----Mensagem original-----
De: John Haxby [mailto:john.haxby@oracle.com] 
Enviada em: quinta-feira, 20 de novembro de 2008 10:37
Para: Bruno Bertechini
Cc: ''Rustedt, Florian''; ''Bastian Blank'';
xen-users@lists.xensource.com
Assunto: Re: RES: RES: [Xen-users] Shared volume: Software-ISCSI or GFS or
OCFS2?

Bruno Bertechini wrote:
> Well.. Let me use this thread and popup my environment and ask for
> suggestions:
>
> 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?
>   
Do you want (live) migration?   If so then you''ll need to be able to 
access the files/LUNs/logical volumes from each machine in the cluster.

That probably constrains your choices somewhat: there is cluster LVM 
(clvm) but I don''t know enough to say anything for or against it; nfs 
and ocfs2 will both work of course; and then there''s the third choice
of
LUNs in the EMC storage being assigned to more than one machine or 
migrated from machine to machine and I have no idea how that is done as 
I don''t have that kind of hardware to hand.

I will say this something resizing and snapshots though.

You don''t need LVM for resizing: you can grow a file by dd''ing
empty
space from /dev/zero on to the end of it.  You''ll need to restart the 
guest that''s using the file to pick up the new file size (although I 
think that might have changed in 3.3, I''m not sure).  Likewise you can 
grow LUNs defined in the array.

Snapshots are not the panacea you might think.   If you just take a 
snapshot of a guest''s virtual disk then what you''ve got is a
corrupt
virtual disk because you haven''t got what was in the guest''s
memory and
on the way to the disk.  If you xm save the guest and copy the memory 
snapshot the disk then what you''ve done is basically a fork(2) of an OS
-- when you restore the backed up copy it''ll carry on doing exactly
what
it was doing at the time of the backup.  My favourite example of the day 
is what happens when that guest is about to change a database over a 
network connection -- that change will happen twice.   Let''s hope
it''s
$salary = $salary * 1.1.

jch


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