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openSUSE Leap Documentation › Virtualization Guide › Managing Virtual Machines with Xen › XenStore: Configuration Database Shared between Domains
ContentsContents
Virtualization Guide
  1. About This Manual
  2. I Introduction
    1. 1 Virtualization Technology
    2. 2 Introduction to Xen Virtualization
    3. 3 Introduction to KVM Virtualization
    4. 4 Virtualization Tools
    5. 5 Installation of Virtualization Components
  3. II Managing Virtual Machines with libvirt
    1. 6 Starting and Stopping libvirtd
    2. 7 Guest Installation
    3. 8 Basic VM Guest Management
    4. 9 Connecting and Authorizing
    5. 10 Managing Storage
    6. 11 Managing Networks
    7. 12 Configuring Virtual Machines with Virtual Machine Manager
    8. 13 Configuring Virtual Machines with virsh
    9. 14 Managing Virtual Machines with Vagrant
  4. III Hypervisor-Independent Features
    1. 15 Disk Cache Modes
    2. 16 VM Guest Clock Settings
    3. 17 libguestfs
    4. 18 QEMU Guest Agent
  5. IV Managing Virtual Machines with Xen
    1. 19 Setting Up a Virtual Machine Host
    2. 20 Virtual Networking
    3. 21 Managing a Virtualization Environment
    4. 22 Block Devices in Xen
    5. 23 Virtualization: Configuration Options and Settings
    6. 24 Administrative Tasks
    7. 25 XenStore: Configuration Database Shared between Domains
    8. 26 Xen as a High-Availability Virtualization Host
  6. V Managing Virtual Machines with QEMU
    1. 27 QEMU Overview
    2. 28 Setting Up a KVM VM Host Server
    3. 29 Guest Installation
    4. 30 Running Virtual Machines with qemu-system-ARCH
    5. 31 Virtual Machine Administration Using QEMU Monitor
  7. VI Managing Virtual Machines with LXC
    1. 32 Linux Containers
    2. 33 Migration from LXC to libvirt-lxc
  8. Glossary
  9. A Appendix
  10. B XM, XL Toolstacks and Libvirt framework
  11. C GNU Licenses
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25 XenStore: Configuration Database Shared between Domains Edit source

25.1 Introduction
25.2 File System Interface

This section introduces basic information about XenStore, its role in the Xen environment, the directory structure of files used by XenStore, and the description of XenStore's commands.

25.1 Introduction Edit source

XenStore is a database of configuration and status information shared between VM Guests and the management tools running in Dom0. VM Guests and the management tools read and write to XenStore to convey configuration information, status updates, and state changes. The XenStore database is managed by Dom0 and supports simple operations such as reading and writing a key. VM Guests and management tools can be notified of any changes in XenStore by watching entries of interest. Note that the xenstored daemon is managed by the xencommons service.

XenStore is located on Dom0 in a single database file /var/lib/xenstored/tdb (tdb represents tree database).

25.2 File System Interface Edit source

XenStore database content is represented by a virtual file system similar to /proc (for more information on /proc, see Book “System Analysis and Tuning Guide”, Chapter 2 “System Monitoring Utilities”, Section 2.6 “The /proc File System”). The tree has three main paths: /vm, /local/domain, and /tool.

  • /vm - stores information about the VM Guest configuration.

  • /local/domain - stores information about VM Guest on the local node.

  • /tool - stores general information about various tools.

Tip
Tip

Each VM Guest has two different ID numbers. The universal unique identifier (UUID) remains the same even if the VM Guest is migrated to another machine. The domain identifier (DOMID) is an identification number that represents a particular running instance. It typically changes when the VM Guest is migrated to another machine.

25.2.1 XenStore Commands Edit source

The file system structure of the XenStore database can be operated with the following commands:

xenstore-ls

Displays the full dump of the XenStore database.

xenstore-readpath_to_xenstore_entry

Displays the value of the specified XenStore entry.

xenstore-existsxenstore_path

Reports whether the specified XenStore path exists.

xenstore-listxenstore_path

Displays all the children entries of the specified XenStore path.

xenstore-writepath_to_xenstore_entry

Updates the value of the specified XenStore entry.

xenstore-rmxenstore_path

Removes the specified XenStore entry or directory.

xenstore-chmodxenstore_pathmode

Updates the read/write permission on the specified XenStore path.

xenstore-control

Sends a command to the xenstored back-end, such as triggering an integrity check.

25.2.2 /vm Edit source

The /vm path is indexed by the UUID of each VM Guest, and stores configuration information such as the number of virtual CPUs and the amount of allocated memory. There is a /vm/<uuid> directory for each VM Guest. To list the directory content, use xenstore-list. 

tux > sudo xenstore-list /vm
00000000-0000-0000-0000-000000000000
9b30841b-43bc-2af9-2ed3-5a649f466d79-1

The first line of the output belongs to Dom0, and the second one to a running VM Guest. The following command lists all the entries related to the VM Guest: 

tux > sudo xenstore-list /vm/9b30841b-43bc-2af9-2ed3-5a649f466d79-1
image
rtc
device
pool_name
shadow_memory
uuid
on_reboot
start_time
on_poweroff
bootloader_args
on_crash
vcpus
vcpu_avail
bootloader
name

To read a value of an entry, for example the number of virtual CPUs dedicated to the VM Guest, use xenstore-read: 

tux > sudo xenstore-read /vm/9b30841b-43bc-2af9-2ed3-5a649f466d79-1/vcpus
1

A list of selected /vm/<uuid> entries follows:

uuid

UUID of the VM Guest. It does not change during the migration process.

on_reboot

Specifies whether to destroy or restart the VM Guest in response to a reboot request.

on_poweroff

Specifies whether to destroy or restart the VM Guest in response to a halt request.

on_crash

Specifies whether to destroy or restart the VM Guest in response to a crash.

vcpus

Number of virtual CPUs allocated to the VM Guest.

vcpu_avail

Bitmask of active virtual CPUs for the VM Guest. The bitmask has several bits equal to the value of vcpus, with a bit set for each online virtual CPU.

name

The name of the VM Guest.

Regular VM Guests (not Dom0) use the /vm/<uuid>/image path: 

tux > sudo xenstore-list /vm/9b30841b-43bc-2af9-2ed3-5a649f466d79-1/image
ostype
kernel
cmdline
ramdisk
dmargs
device-model
display

An explanation of the used entries follows:

ostype

The OS type of the VM Guest.

kernel

The path on Dom0 to the kernel for the VM Guest.

cmdline

The kernel command line for the VM Guest used when booting.

ramdisk

The path on Dom0 to the RAM disk for the VM Guest.

dmargs

Shows arguments passed to the QEMU process. If you look at the QEMU process with ps, you should see the same arguments as in /vm/<uuid>/image/dmargs.

25.2.3 /local/domain/<domid> Edit source

This path is indexed by the running domain (VM Guest) ID, and contains information about the running VM Guest. Remember that the domain ID changes during VM Guest migration. The following entries are available:

vm

The path of the /vm directory for this VM Guest.

on_reboot, on_poweroff, on_crash, name

See identical options in Section 25.2.2, “/vm”

domid

Domain identifier for the VM Guest.

cpu

The current CPU to which the VM Guest is pinned.

cpu_weight

The weight assigned to the VM Guest for scheduling purposes. Higher weights use the physical CPUs more often.

Apart from the individual entries described above, there are also several subdirectories under /local/domain/<domid>, containing specific entries. To see all entries available, refer to XenStore Reference.

/local/domain/<domid>/memory

Contains memory information. /local/domain/<domid>/memory/target contains target memory size for the VM Guest (in kilobytes).

/local/domain/<domid>/console

Contains information about a console used by the VM Guest.

/local/domain/<domid>/backend

Contains information about all back-end devices used by the VM Guest. The path has subdirectories of its own.

/local/domain/<domid>/device

Contains information about the front-end devices for the VM Guest.

/local/domain/<domid>/device-misc

Contains miscellaneous information about devices.

/local/domain/<domid>/store

Contains information about the VM Guest's store.

→Chapter 26 Xen as a High-Availability Virtualization Host←Chapter 24 Administrative Tasks
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