RE: [Xen-users] problem with extra storage via iscsi

["Artur Linhart - Linux communication" <AL.LINUX@xxxxxxxxxxx>]

Hello,

        From my point of view, Tammo is right...

You write:
Why would performance be better with an extra layer (dom0)? 

- because in DomU You can have as far as I know in maximum 100MBit network
device, in Dom0 You can use the gigabit (or possible 10Gbit - I did not
tried it) connection without problems, so any network device should have
then much more better performance if mapped as block device through Dom0...


You write:
Connecting the LUN to dom0 and assigning it as a regular blockdevice to domU
has a large drawback: there is no protection against using the same 
blockdevice in multiple domU's.

- if You have some device already mounted in some DomU or in Dom0 as
read-write, then the start of the next guest which is trying to mount such a
device as read-write too results into the error by start of such a domain
and the DomU is not started then - at least on my Xen boxes it behaves so...

        With regards, Archie

-----Original Message-----
From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
[mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Gertjan Oude
Lohuis
Sent: Wednesday, August 27, 2008 7:14 PM
To: Tammo Schuelke
Cc: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] problem with extra storage via iscsi

Tammo Schuelke wrote:
>> open-iscsi assigns the discoverd LUN also to /dev/sda
>>     
>
> So the domU does stop working since it's losing its root partition?
> If not, what's the output of udevinfo -q symlink -n /dev/sda?
>   

Thanks for your reply!

No, the domU keeps working, I think because all partitions are already 
mounted when open-iscsi starts. Output of udevinfo:

root@guest1 ~ # udevinfo -q symlink -n /dev/sda
disk/by-id/scsi-36006048c630e7b571c1ed50a0956f129 
disk/by-path/ip-10.4.1.201:3260-iscsi-iqn.1992-05.com.emc:ck2000809002410000
-7-lun-1

> In general, iSCSI from within a domU should be working since it's plain
TCP/IP.
Yes, that is exactly what I thought.

> Performance wise it would probably be better to connect to the targets in
dom0 and assign the LUN to domU as a regular block device.
>   

Why would performance be better with an extra layer (dom0)? Connecting 
the LUN to dom0 and assigning it as a regular blockdevice to domU has a 
large drawback: there is no protection against using the same 
blockdevice in multiple domU's. Normally LUN masking would protect 
against that, but a LUN mask is based on IQN, and the IQN belongs to the 
dom0.


Regards,
Gertjan Oude Lohuis

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