Xen by default just uses the dom0's clock, which isn't updated within the domU's. Either set /proc/xen/independent_wallclock to 1 (so that this domU has an indepedent clock from the host dom0, or set the clock in the dom0.
This is specifically for Citrix XenServer, although the principles will of course work in other Xen implementations
I recently had a scenario where I was replacing two Windows servers with XenServer guests. This was fine, but we needed a way to backup to the existing SCSI DDS4 DAT drive. After failing to make PCI passthrough work, I settled on the much nicer method of providing the tape drive via an iSCSI target on the XenServer Host (Dom0). Here is how I achieved this.
NOTE WELL: This method replaces the old one using a patched ietd. It is STILL unsupported
Download the following packages to the DDK from 
http://scst.sourceforge.net/downloads.html
Here's where it gets a little ugly, and I'd welcome suggestions (just edit this wiki) for how to do this the "right" way. This is necessary because the make install part of the build depends on having the kernel-dev skeleton installed, and this can't be installed on a XenServer Host (to my knowledge).
cd /usr/src/kernels/2.6.18-128.1.6.el5.xs5.5.0.505.1024xen-i686 tar -czvpf /opt/build/build.tar.gz . cd /opt/build
OK, you're now ready to install on your target XenServer? host
yum --enablerepo=base install gcc make patch m4 openssl-devel yum-utils
. /etc/xensource-inventory
if "$PRODUCT_VERSION" == "5.6.0"? ; then
yum -y --enablerepo=base install yum-utils
yumdownloader e2fsprogs-devel
rpm --nodeps -Uvh e2fsprogs-devel*.rpm
rm -f e2fsprogs-devel*.rpm
fi
mkdir /lib/modules/$kern/build
cd /lib/modules/$kern/build
tar -zxvf /opt/build/build.tar.gz
cd /opt/build
cd scst-1.0.1.1
make install
cd ../scstadmin-1.0.6
make
make install
cd ../iscsi-scst-1.0.1.1
make
make install
Now have a look in /proc/scsi/scsi (or use lsscsi) to get your HCIL info for your tape drive and create /etc/scst.conf using THAT info, NOT the example info given here...
[HANDLER tape] DEVICE 6:0:3:0 [ASSIGNMENT Default] DEVICE 6:0:3:0,0
here)Target iqn.2005-03.org.open-iscsi:76cf8781117f:storage
You should now be able to discover and connect to the target.
Tested using MS iSCSI initiator on Windows Server 2008 against the above config on XenServer 5.5.
I note this page is referenced A LOT from Citrix and other sites. It is very important to note this is NOT supported by Citrix. It would be great if they provided this ability natively, to save me the ton of work I've had to do retesting the old solution and finally developing this one (well, I developed nothing, just put it all together, but still...). If this is useful to anyone out there, I'm a huge beer nerd, and love great craft beer from all over the world. If you want to say thanks, you can send beer to:
Greig McGill?
23 Saxbys Road
Hamilton 3206
New Zealand
If you can't be arsed sending me a nice beer, well, I never thought you would anyway! :)
The old method below works on XenServer? pre 5.0
Note 1: This is totally unsupported by Citrix
Note 2: I've used the XenServer terminology "host" instead of Dom0, as this applies to the Citrix commercial implementation of Xen. It will probably work fine on OSS Xen, but you can just install the normal kernel dev packages and ignore the DDK stuff.
Note 3: This is for XenServer 4.1.0, but the principles are the same for previous versions. Just ensure you understand each step rather than following blindly.
Note 4: You'll need to enable yum repositories. Do this by editing /etc/yum.repos.d/CentOS-Base.repo, and set "enabled=1" for the Base, Updates and Addons repositories
Note 5: Thanks to the wonderful work of Blake-r the rawio patch has now been updated to work against iscsitarget-0.4.17 - when using this against a kernel newer than 2.6.22, you'll need to edit kernel/target_raw.c and replace all the psg.page occurrences with psg.page_link due to changes in the scatterlist struct. To take advantage of this, just substitute the newer versions of iscsitarget and the rawio patch in these instructions. You should be able to keep all the instructions the same, but I've not tested this yet.
http://optusnet.dl.sourceforge.net/sourceforge/iscsitarget/iscsitarget-0.4.14.tar.gz into your DDK VM.http://sourceforge.net/mailarchive/attachment.php?list_name=iscsitarget-devel&message_id=1170171101.2822.23.camel@localhost.localdomain&counter=1 into your DDK VM (assuming you're saving it in /tmp)yum install kernel-devel bison flex tar -zxvf iscsitarget-0.4.14.tar.gz cd iscsitarget-0.4.14 patch -p0 < /tmp/raw.p make
yum install make gcc cd iscsitarget-0.4.14 make install mkdir /lib/modules/`uname -r`/kernel/iscsi cp kernel/iscsi_trgt.ko /lib/modules/`uname -r`/kernel/iscsi depmod -aq
The last three steps are required because make install will not copy the kernel module correctly outside the target environment.
Target iqn.2007-04.com.example:tape0
Lun 0 H=1,C=0,I=6,L=0,Type=rawio
Type 1
See the following, requested by Simon Larsen at http://fragged.nu/2009/05/making-a-tape-drive-available-to-a-guest-via-iscsi/
Simon, if you read this, your captcha on your blog is broken, preventing comments, and there's no contact email address. Hope you find this, and find it useful. ;)
Just bung it in your rc.local.
#!/bin/bash
#
# This script is a (very) primitive method of determining the current HCIL info
# for /etc/ietd.conf (config file for iscsi-target) to work around some
# controllers which change this ID at boot time. You'll probably want to add
# some data validation of those vars as this is purely for my environments.
# Feel free to adapt or use it in any way you see fit.
# Greig McGill. August, 2009
# First set some vars
ietd="/etc/ietd.conf"
iqn="iqn.2009-08.nz.org.aol.internal:tape0"
# Get the whole HCIL string representing the tape drive.
# IMPORTANT NOTE: I am assuming there is only one sequential access device.
# If I am wrong, you WILL need to rewrite this, or badness will happen.
# You have been warned.
HCIL="`cat /proc/scsi/scsi | grep -B2 Sequential | head -1`"
# Now get each component
H="`echo $HCIL | cut -c 11`"
C="`echo $HCIL | cut -c 23`"
I="`echo $HCIL | cut -c 30`"
L="`echo $HCIL | cut -c 38`"
# Got all that, now generate ietd.conf and restart iscsi-target
cat <<EOT >$ietd
Target $iqn
Lun 0 H=$H,C=$C,I=$I,L=$L,Type=rawio
Type 1
EOT
/etc/init.d/iscsi-target restart
# exit with no error
exit 0
See iSCSINotes as well for more information
No page links to XenNotes.
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