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The hardware mapping is not going to be deterministic between
servers. So you need to rely on some unique identifier stored in
the contents of the virtual disks. Which one you can use depends on
how you backup and restore your virtual disks. Where does your DR
restore run? In domU or in dom0? And how exactly do you backup and
restore the virtual disks?<br>
<br>
Thanks,<br>
Herbert.<br>
<br>
<div class="moz-cite-prefix">On 10/29/12 12:19 AM, neo3 matrix
wrote:<br>
</div>
<blockquote
cite="mid:CAGF-398QHXDD0E+tjmkBp75wk9Q932m25g0g-WsrU2Fj_8-N2A@mail.gmail.com"
type="cite">Thank you John and Grzegorz for your quick replies.
<div>Sorry for late reply (as I was not in town).</div>
<div><br>
</div>
<div>Basically, I am working on a Disaster Recovery software whose
disk logic is totally dependent on disk mapping .</div>
<div><br>
</div>
<div>Even in my project, I should be able to map a disk from one
OS on Xenserver to other OS on other Xenserver.</div>
<div><br>
</div>
<div>So, as John suggested, </div>
<div>>>> "The good news with Xen disks is that they
really do have deterministic slots. The virtual disk in slot
xvdb will >>> always be xvdb (the "vbd-NNN" numbers
simply refer to event channels."</div>
<div>Here, I can't use vbd-XXX names as they differ from one guest
OS to other and one Xenserver to other.</div>
<div>Also, changing the Xenserver setting in VM definition file is
in customer's hand which I cannot control.</div>
<div><br>
</div>
<div>
<div>I can't try UUID as well for disk mapping because the
problem is during disaster recovery process we used to
repartition disks and due to that UUID of disk and partitions
gets changed.</div>
</div>
<div><br>
</div>
<div>So, to map disks from one xenserver to other, I need some
mapping mechanism like host:channel:id:lun </div>
<div>OR </div>
<div>the way "lscsci" command gives output which is similar in
other Xenserver as well and I can be able to map disks.</div>
<div><br>
</div>
<div>Can you please help me out in the same?</div>
<div><br>
</div>
<div>Thank you for your valuable suggestions.</div>
<div><br>
</div>
<div>Regards,</div>
<div>Neo</div>
<div><br>
</div>
<div><br>
</div>
<div><br>
<br>
<div class="gmail_quote">On Fri, Oct 19, 2012 at 1:08 PM, John
Haxby <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:john.haxby@gmail.com" target="_blank">john.haxby@gmail.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
<br>
<div class="gmail_quote">
<div class="im">On 19 October 2012 07:32, neo3 matrix <span
dir="ltr"><<a moz-do-not-send="true"
href="mailto:neo3matrix@gmail.com" target="_blank">neo3matrix@gmail.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div>Hi all,</div>
<div><br>
</div>
<div>I have installed RHEL6 as guest OS on Citrix Xen
Server. After installation of OS, I can see disk
names as /dev/xvda, /dev/xvdb instead of traditional
convention like /dev/sda, /dev/sdb on guest OS.</div>
<div><br>
</div>
<div>Generally, on physical machines, in
/proc/scsi/scsi file, we get a unique entry for
every disk connected to the system. For e.g. string
"scsi02:00:00:01" indicates that this disk is
connected to the machine via Host=2, Channel=00,
Id=00 Lun=01. This helps me in my project to
uniquely identify each and every disk in scenarios
where many times after reboot OR in SAN boot cases
OR in some Disaster Recovery procedures, disk names
might change from say /dev/sda to /dev/sdb after
reboot. But, this Host:Channel:ID:Lun combination
remains same for every disk and I can uniquely
identify the disks though their /dev/sd* names have
changed.</div>
<div><br>
</div>
<div>For my project, on Citrix Xenserver, I need to
know the unique disk location for such Xen guest OS
devices by which I can easily identify disks across
the reboots for the above mentioned cases. </div>
<br>
</blockquote>
</div>
<div><br>
<br>
Oooh. Complicated question.<br>
<br>
To start with, those SCSI-type disk identifiers are not
as deterministic as you might hope. "Host=2" merely
refers to the SCSI controller that was discovered at
position number 2. These days, discovery order counts
for nothing: devices are discovered and initialised
concurrently, more or less, so while you might get
devices discovered in the "right" order almost all the
time, you might find that powering on a machine on a
particularly cold morning changes the order in which
things are discovered. Even worse, if what used to be
/dev/sdb goes AWOL then you're not left with a hole in
the sequence, everything from what used to be /dev/sdc
onwards gets renamed.<br>
<br>
The good news with Xen disks is that they really do have
deterministic slots. The virtual disk in slot xvdb
will always be xvdb (the "vbd-NNN" numbers simply refer
to event channels, they aren't random but they might as
well be). You would need to edit the VM definition in
the host to change the virtual disks.<br>
<br>
Even more good news: you don't need to use the /dev/sdX,
/dev/xvdX, etc names at all.<br>
<br>
If you look in /dev/disk you'll find several
directories. The one you're probably most interested in
is /dev/disk/by-uuid: entries in /etc/fstab should be
using the UUID= format to identify things that aren't
logical volumes (logical volumes are named within their
volume group and the physical volumes that make up a
volume group aren't tied to specific devices although
they do have specific UUIDs).<br>
<br>
Running "blkid" (as root) is quite useful as well.<br>
<br>
jch<br>
</div>
</div>
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