Home > Articles > Operating Systems, Server

The Xen Hypervisor

  • Print
  • + Share This
This chapter is from the book

XenStore

Another important part of the administrative architecture for a Xen system is XenStore. Earlier in this chapter, we saw the XenStore daemon, xenstored, in the process listing of a running Xen system. In this section, we discuss XenStore in more detail.

XenStore is a database of configuration information shared between domains. Domains read and write the XenStore database to communicate with each other. This database is maintained by Domain0 on behalf of all the domains. XenStore supports atomic operations such as reading a key or writing a key. When new values are written into XenStore, the affected domains are notified.

XenStore is often used as a mechanism for controlling devices in guest domains. XenStore can be accessed in a number of different ways, such as a UNIX socket in Domain0, a kernel-level API, or an ioctl interface. Device drivers write request or completion information into XenStore. Although drivers can write anything they want, XenStore is designed for small pieces of information, such as configuration information or status, and not for large pieces of information, such as in bulk data transfer.

XenStore is actually located in a single file, /var/lib/xenstored/tdb, on Domain0. (tdb stands for Tree Database.) Listing 3.7 shows the tdb file located in the /var/lib/xenstored directory on Domain0 and some attributes of this file.

Listing 3.7. Contents of /var/lib/xenstored/

[root@dom0]:/var/lib/xenstored# ls -lah
total 112K
drwxr-xr-x  2 root root 4.0K Feb 24 13:38 .
drwxr-xr-x 25 root root 4.0K Oct 16 11:40 ..
-rw-r-----  1 root root  40K Feb 24 13:38 tdb
root@dom0]:/var/lib/xenstored#
[root@dom0]:/var/lib/xenstored# file tdb
tdb: TDB database version 6, little-endian hash size 7919 bytes
root@dom0]:/var/lib/xenstored#

Viewed another way, XenStore is similar in flavor to the /proc or sysfs virtual file system in Linux and some UNIX variants. Internal to the XenStore database file is a logical file system tree with three main paths: /vm, /local/domain, and /tool. The /vm and /local/domain directory paths have subareas dedicated to individual domains, while the /tool path stores general information about various tools and is not indexed by domain. The second directory level in /local/domain might seem somewhat unnecessary given that /local contains only the domain subdirectory.

Each domain has two identification numbers. The universal unique identifier (UUID) is an identifying number that remains the same even if the guest is migrated to another machine. The domain identifier (DOMID) is an identifying number that refers to a particular running instance. The DOMID typically changes when the guest is migrated to another machine.

The /vm path is indexed by the UUID of each domain and stores configuration information such as the number of virtual CPUs and the amount of memory allocated to the domain. For each domain, there is a /vm/<uuid> directory. Table 3.5 explains the contents of the /vm/<uuid> directory.

Table 3.5. Contents of the /vm/<uuid> Directory

Entry

Description

uuid

UUID of the domain. The UUID of a domain does not change during migration, but the domainId does. Because the /vm directory is indexed by UUID, this entry is somewhat redundant.

ssidref

SSID reference for the domain.

on_reboot

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

on_poweroff

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

on_crash

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

vcpus

Number of virtual CPUs allocated to this domain.

vcpu_avail

Number of active virtual CPUs for this domain.

Note: Number of disabled virtual CPUs is given by vcpus minus vcpu_avail.

memory

Amount of memory in megabytes allocated to the domain.

name

Name of the domain.

Regular guest domains (DomUs) also have a /vm/<uuid>/image directory. Table 3.6 explains the contents of the /vm/<uuid>/image directory.

Table 3.6. Contents of the /vm/<uuid>/image Directory

Entry

Description

ostype

linux or vmx to identify the builder type

kernel

Filename path on Domain0 to the kernel for this domain

cmdline

Command line to pass to the kernel for this domain when booting

ramdisk

Filename path on Domain0 to the ramdisk for this domain

The /local/domain is indexed by the DOMID and contains information about the running domain such as the current CPU the domain is pinned to or the current tty on which the console data from this domain is exposed. For each domain, there is a /local/domain/<domId> directory. Note that the UUID used when indexing in the /vm directory is not the same as the DOMID used when indexing in the /local/domain directory. The UUID does not change during migration, but the DOMID does. This enables localhost-to-localhost migration. Some of the information in /local/domain is also found in /vm, but /local/domain contains significantly more information, and the version in /vm does not change. The /local/domain directory for a domain also contains a pointer to the /vm directory for the same domain. Table 3.7 explains the contents of the /local/domain/<domId> directory.

Table 3.7. Contents of the /local/domain/<domId> Directory

Entry

Description

domId

Domain identifier of the domain. The domain ID changes during migration, but the UUID does not. Because the /local/domain directory is indexed by domId, this entry is somewhat redundant.

/vm Related Entries

on_reboot

Refer to Table 3.5.

on_poweroff

Refer to Table 3.5.

on_crash

Refer to Table 3.5.

name

Refer to Table 3.5.

vm

Pathname of the VM directory for this same domain.

Scheduling Related Entries

running

If present, indicates that the domain is currently running.

cpu

Current CPU to which this domain is pinned.

cpu_weight

The weight assigned to this domain for scheduling purposes. Domains with higher weights use the physical CPUs more often.

xend Related Entries

cpu_time

You might think this would be related to the cpu and cpu_weight entries, but it actually refers to the xend start time. Used for Domain0 only.

handle

Private handle for xend.

image

Private xend information.

Under /local/domain/<domId> for each domain, there are also several subdirectories including memory, console, and store. Table 3.8 describes the contents of these subdirectories.

Table 3.8. Subdirectories of the /local/domain/<domId> Directory

Entry

Description

/local/domain/<domId>/console

ring-ref

Grant table entry of the console ring queue.

port

Event channel used for the console ring queue.

tty

tty on which the console data is currently being exposed.

limit

Limit in bytes of console data to be buffered.

/local/domain/<domId>/store

ring-ref

Grant table entry of the store ring queue.

port

Event channel used for the store ring queue.

/local/domain/<domId>/memory

target

Target memory size in kilobytes for this domain.

Three additional subdirectories under /local/domain/<domId> are related to device management—backend, device, and device-misc. These all have subdirectories of their own.

Earlier in this chapter, we discussed how device management in Xen is divided into backend drivers running in privileged domains with direct access to the physical hardware and frontend drivers that give unprivileged domains the illusion of a generic and dedicated version of that resource. The backend subdirectory provides information about all physical devices managed by this domain and exported to other domains. The device subdirectory provides information about all frontend devices used by this domain. Finally, the device-misc directory provides information for other devices. In each case, there can be a vif and vbd subdirectory. Virtual Interface (vif) devices are for network interfaces, and Virtual Block Devices (vbd) are for block devices such as disks or CD-ROMs. In Chapter 9, "Device Virtualization and Management," we discuss in more detail how XenStore enables communication between backend and frontend drivers.

Table 3.9 describes a number of tools that can be used to explore and manipulate XenStore. They are often located in /usr/bin. These commands allow the XenStore database, which is stored as a file (/var/lib/xenstored/tdb), to be viewed as a logical file system.

Table 3.9. XenStore Commands

Command

Description

xenstore-read <Path to
XenStore Entry>

Displays the value of a XenStore entry

xenstore-exists <XenStore Path>

Reports whether a particular XenStore Path exists

xenstore-list <XenStore Path>
xenstore-ls <XenStore Path>

Shows all the children entries or directories of a specific XenStore path

xenstore-write <Path to
XenStore Entry> <value>

Updates the value of a XenStore entry

xenstore-rm <XenStore Path>

Removes XenStore entry or directory

xenstore-chmod <XenStore
Path> <mode.

Updates the permission on a XenStore entry to allow read or write

xenstore-control

Sends commands to xenstored, such as check to trigger an integrity check.

xsls <Xenstore path>

Recursively shows the contents of a specified XenStore path; equivalent of a recursive xenstore-list plus a xenstore-read to display the values

For example, Listing 3.8 shows an example of a xenstore-list on /local/domain/0.

Listing 3.8. xenstore-list

[user@dom0]#xenstore-list /local/domain/0
cpu
memory
name
console
vm
domid
backend
[user@dom0]

Listing 3.9 shows a shell script from the XenWiki used to dump the entire contents of XenStore. Notice the calls to xenstore-list and xenstore-read. This would be similar to an xsls on the XenStore root.

Listing 3.9. Shell Script for Dumping the Contents of XenStore from http://wiki.xensource.com/xenwiki/XenStore

#!/bin/sh

function dumpkey() {
   local param=${1}
   local key
   local result
   result=$(xenstore-list ${param})
   if [ "${result}" != "" ] ; then
      for key in ${result} ; do dumpkey ${param}/${key} ; done
     else
      echo -n ${param}'='
      xenstore-read ${param}
   fi
}

for key in /vm /local/domain /tool ; do dumpkey ${key} ; done
  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020