====== Xen_4.0.1_on_Debian_Squeeze_dom0_and_domU ======

Most of the details on this page are from Debians much improved wiki page at http://wiki.debian.org/Xen

====== Installation of Xen on Debian Squeeze ======
 aptitude -P install xen-linux-system
 <nowiki>//</nowiki>At this point I prematurely rebooted and went
 <nowiki>//</nowiki>xm list
 <nowiki>//</nowiki>WARNING!  Can't find hypervisor information in sysfs!
 <nowiki>//</nowiki>Error: Unable to connect to xend: No such file or directory. Is xend running?
 <nowiki>//</nowiki>don't reboot. Follow the below:
 
 mv /etc/grub.d/10_linux /etc/grub.d/21_linux

===== Additional Tweaks to make dom0 more stable =====
==== Limit RAM available to dom0 ====
 vi /etc/default/grub
 #add the following
 # Disable OS prober to prevent virtual machines on logical volumes from appearing in the boot menu.
 GRUB_DISABLE_OS_PROBER=true
 #limit dom0 to 512MB RAM
 GRUB_CMDLINE_XEN="dom0_mem=512M"
 
 update-grub

==== Disable auto save and restore of domUs on host reboot ====
Apparently in Debians wiki, they say that saving the state of domU's on powerdown doesn't always work. Hence, disable the xen save and restore.
 vi /etc/default/xendomains
 XENDOMAINS_RESTORE=false
 XENDOMAINS_SAVE=""

==== Bridge Networking and disabling Memory Ballooning ====
 vi /etc/xen/xend-config.sxp
 #(vif-script vif-bridge)
 (network-script 'network-bridge antispoof=yes')
 #(enable-dom0-ballooning yes)
 (enable-dom0-ballooning no)

====== Reboot dom0 and bring up Xen ======
 reboot
 xm list
 <nowiki>//</nowiki>outputs:
 Name                                        ID   Mem VCPUs      State   Time(s)
 Domain-0                                     0 15630    12     r-----     10.2

====== Create Virtual Machines - domUs ======
At this stage I used to manually setup lvm volumes and manually debootstrap the VM. See [[Create_DomU#LVM_Based_Setup_of_Virtual_Disk]] and [[Basic_Setup_of_Xen3.2_32bit#Create_DomU]] I'd then copy across resolv.conf, fstab, sources.lst as well as /lib/modules/2.6... etc. However with Etch and Lenny there was additional hassle tweaking inittab and the config to get the xm console to work properly.

With Debian Squeeze, I followed Debians advise on their wiki and started using Xen-Tools. It automates a lot and makes things much easier.

===== Using Xen-Tools to create VMs =====
 apt-get install xen-tools
 vi /etc/xen-tools/xen-tools.conf
 #Below are all the uncommented lines I used:
 lvm = vg1
 install-method = debootstrap
 size   = 20Gb
 memory = 512Mb
 swap   = 1Gb
 fs     = ext3
 dist   = `xt-guess-suite-and-mirror --suite`
 image  = sparse
 gateway    = 10.10.9.126
 netmask    = 255.255.255.192
 bridge=xenbr0
 passwd = 1
 kernel = /boot/vmlinuz-`uname -r`
 initrd = /boot/initrd.img-`uname -r`
 arch = amd64
 mirror = `xt-guess-suite-and-mirror --mirror`
 ext3_options     = noatime,nodiratime,errors=remount-ro
 ext2_options     = noatime,nodiratime,errors=remount-ro
 xfs_options      = defaults
 reiserfs_options = defaults
 btrfs_options    = defaults
 nohosts = 1
 pygrub = 1

==== Set Xen-Tools to use noop Scheduler ====
On the setup here with LVM volumes on top of RAID 1, it is more efficient and better for performance to have VM's use the "noop" I/O scheduler. I've asked people for feedback on this in #Xen on irc.freenode. See chat log below. Also, the following links give reference to this fact also. http://old-list-archives.xen.org/archives/html/xen-users/2010-04/msg00518.html 
http://prefetch.net/blog/index.php/2009/04/23/best-io-scheduler-to-use-with-virtualized-linux-hosts/

 15:01 <@pasik> steviewdr: yes, you need to change the scheduler
 15:01 <@pasik> steviewdr: new enough linux domU kernels automatically use
 15:01 <@pasik> steviewdr: noop for virtual disks
 15:03 < steviewdr> pasik: kernels newer than -> 2.6.32-5-xen-686? So its safe to use noop on domU's on lvm volumes?
 15:03 <@pasik> steviewdr: yes, noop is safe and recommended
 15:03 <@pasik> steviewdr: in domUs

You can check the current scheduler by going:
 cat /sys/block/xvda2/queue/scheduler
You can set the scheduler by going:
 echo "noop" > /sys/block/xvda2/queue/scheduler

We can set this noop scheduler option in the Xen-Tools template so all new VMs created get this noop scheduler.
 **vi /etc/xen-tools/xm.tmpl**
 **#Add in the following:**
 **extra="elevator=noop" **
Note: this line can also be added into a xen vm config in /etc/xen/vm01.cfg

==== Create VM ====
 xen-create-image --hostname walle-vm01 --ip 193.1.99.98 --vcpus 2

==== Set VM to autostart ====
 mkdir /etc/xen/auto
 ln -s /etc/xen/vm01.cfg /etc/xen/auto/

==== Start VM ====
 xm create -c /etc/xen/vm01.cfg

http://old-list-archives.xen.org/archives/html/xen-users/2010-04/msg00518.html
http://prefetch.net/blog/index.php/2009/04/23/best-io-scheduler-to-use-with-virtualized-linux-hosts/