User manual XEN XEN 3

[. . . ] Users' Manual Xen v3. 0 DISCLAIMER: This documentation is always under active development and as such there may be mistakes and omissions -- watch out for these and please report any you find to the developers' mailing list, xen-devel@lists. xensource. com. The latest version is always available on-line. Contributions of material, suggestions and corrections are welcome. Xen is Copyright c 2002-2005, University of Cambridge, UK, XenSource Inc. , IBM Corp. , Hewlett-Packard Co. , Intel Corp. , AMD Inc. , and others. Most portions of Xen are licensed for copying under the terms of the GNU General Public License, version 2. [. . . ] you would edit /mnt/etc/fstab instead of /etc/fstab. For this example put /dev/sda1 to root in fstab. Now unmount (this is important!): # umount /mnt In the configuration file set: disk = ['file:/full/path/to/vm1disk, sda1, w'] 32 As the virtual machine writes to its `disk', the sparse file will be filled in and consume more space up to the original 2GB. Note that file-backed VBDs may not be appropriate for backing I/O-intensive domains. File-backed VBDs are known to experience substantial slowdowns under heavy I/O workloads, due to the I/O handling by the loopback block device used to support file-backed VBDs in dom0. Better I/O performance can be achieved by using either LVM-backed VBDs (Section 6. 3) or physical devices as VBDs (Section 6. 1). Linux supports a maximum of eight file-backed VBDs across all domains by default. This limit can be statically increased by using the max loop module parameter if CONFIG BLK DEV LOOP is compiled as a module in the dom0 kernel, or by using the max loop=n boot option if CONFIG BLK DEV LOOP is compiled directly into the dom0 kernel. 6. 3 Using LVM-backed VBDs A particularly appealing solution is to use LVM volumes as backing for domain filesystems since this allows dynamic growing/shrinking of volumes as well as snapshot and other features. To initialize a partition to support LVM volumes: # pvcreate /dev/sda10 Create a volume group named `vg' on the physical partition: # vgcreate vg /dev/sda10 Create a logical volume of size 4GB named `myvmdisk1': # lvcreate -L4096M -n myvmdisk1 vg You should now see that you have a /dev/vg/myvmdisk1 Make a filesystem, mount it and populate it, e. g. : # # # # mkfs -t ext3 /dev/vg/myvmdisk1 mount /dev/vg/myvmdisk1 /mnt cp -ax / /mnt umount /mnt disk = [ 'phy:vg/myvmdisk1, sda1, w' ] LVM enables you to grow the size of logical volumes, but you'll need to resize the corresponding file system to make use of the new space. You can also use LVM for creating copy-on-write (CoW) clones of LVM volumes (known as writable persistent snapshots in LVM terminology). This facility is new in 33 Now configure your VM with the following disk configuration: Linux 2. 6. 8, so isn't as stable as one might hope. In particular, using lots of CoW LVM disks consumes a lot of dom0 memory, and error conditions such as running out of disk space are not handled well. To create two copy-on-write clones of the above file system you would use the following commands: # lvcreate -s -L1024M -n myclonedisk1 /dev/vg/myvmdisk1 # lvcreate -s -L1024M -n myclonedisk2 /dev/vg/myvmdisk1 Each of these can grow to have 1GB of differences from the master volume. You can grow the amount of space for storing the differences using the lvextend command, e. g. : # lvextend +100M /dev/vg/myclonedisk1 Don't let the `differences volume' ever fill up otherwise LVM gets rather confused. It may be possible to automate the growing process by using dmsetup wait to spot the volume getting full and then issue an lvextend. In principle, it is possible to continue writing to the volume that has been cloned (the changes will not be visible to the clones), but we wouldn't recommend this: have the cloned volume as a `pristine' file system install that isn't mounted directly by any of the virtual machines. 6. 4 Using NFS Root First, populate a root filesystem in a directory on the server machine. This can be on a distinct physical machine, or simply run within a virtual machine on the same node. Now configure the NFS server to export this filesystem over the network by adding a line to /etc/exports, for instance: /export/vm1root 1. 2. 3. 4/24 (rw, sync, no_root_squash) Finally, configure the domain to use NFS root. In addition to the normal variables, you should make sure to set the following values in the domain's configuration file: root = '/dev/nfs' nfs_server = '2. 3. 4. 5' # substitute IP address of server nfs_root = '/path/to/root' # path to root FS on the server The domain will need network access at boot time, so either statically configure an IP address using the config variables ip, netmask, gateway, hostname; or enable DHCP (dhcp='dhcp'). Note that the Linux NFS root implementation is known to have stability problems under high load (this is not a Xen-specific problem), so this configuration may not be appropriate for critical servers. 34 Chapter 7 CPU Management Xen allows a domain's virtual CPU(s) to be associated with one or more host CPUs. This can be used to allocate real resources among one or more guests, or to make optimal use of processor resources when utilizing dual-core, hyperthreading, or other advanced CPU technologies. Xen enumerates physical CPUs in a `depth first' fashion. [. . . ] You can see if vnet packets are being sent or received by dumping traffic on the vnet UDP port: # tcpdump udp port 1798 If multicast is not being forwaded between machines you can configure multicast forwarding using vn. Suppose we have machines hostA on 10. 10. 0. 100 and hostB on 61 10. 11. 0. 100 and that multicast is not forwarded between them. We use vn to configure each machine to forward to the other: hostA# vn peer-add hostB hostB# vn peer-add hostA Multicast forwarding needs to be used carefully - you must avoid creating forwarding loops. Typically only one machine on a subnet needs to be configured to forward, as it will forward multicasts received from other machines on the subnet. 62 Appendix C Glossary of Terms BVT The BVT scheduler is used to give proportional fair shares of the CPU to domains. [. . . ]