Xen IntelliCache Test

IntelliCache is a XenServer feature that can be used in a XenDesktop deployment to cache temporary and non-persistent operating-system data on the local XenServer host. IntelliCache is available for Machine Creation Services (MCS)-based desktop workloads that use NFS storage.

In a typical XenDesktop configuration (without IntelliCache), desktop VMs read the operating-system data from a master image on a costly shared storage array. When IntelliCache is enabled, a portion of the virtual-machine runtime reads and writes occur on low-cost local storage: XenServer caches the operating-system files on its local hard drive in a Read Cache.

Likewise, when IntelliCache is enabled, each desktop VM writes to its own Write Cache on the local host, preventing writes to shared storage. The read and write caches, if adequately sized and effectively used, can thus go a long way in reducing the load on the remote storage and the amount of network traffic. But, how does an administrator determine whether these caches are sized and used right? This can be achieved using the Xen IntelliCache test. This test continuously tracks the usage of the read and write caches in each of the local storage repositories and reports whether/not the caches are able to service most of the I/O requests to the shared storage, without actually accessing the shared storage! This way, administrators will be able to figure out whether/not the caches effectively serve the purpose they were intended for. In addition, the test also checks and reports the current cache size, so that administrators can judge its adequacy and make changes, if required.

Note:

The performance metrics reported by this test are enabled by default in the XenServer 6.1.0 Performance and Monitoring Supplemental Pack. In XenServer 6.2.0 however, these metrics, though part of the core product, are disabled by default, owing to performance reasons related to XenCenter. This means that, when monitoring XenServer 6.2.0, this test will not report any metrics by default. In such cases, to make sure that the test reports metrics, do the following:

  • Login to the XenServer host as root user.
  • Enable the metrics by issuing the following command from the CLI:

    xe-enable-all-plugin-metrics true

Target of the test : A XenServer host

Agent deploying the test : An internal/remote agent

Outputs of the test : One set of results for each local SR on the monitored XenServer host

Configurable parameters for the test

  1. Test period - How often should the test be executed
  2. Host - The host for which the test is to be configured.
  3. XEN user - To enable the eG agent to connect to the XenServer API for collecting statistics of interest, this test should login to the XenServer as a root user. Provide the name of the root user in the XEN USER text box. Root user privileges are mandatory when monitoring a XenServer 5.5 (or below). However, if you are monitoring XenServer 5.6 (or above) and you prefer not to expose the credentials of the root user, then, you have the option of configuring a user with pool-admin privileges as the xen user. If you do not want to expose the credentials of a root/pool-admin user, then you can configure the tests with the credentials of a xen user with Read-only privileges to the XenServer. However, if this is done, then the Xen Uptime test will not run, and the Xen CPU and Xen Memory tests will not be able to report metrics for the control domain descriptor. To avoid such an outcome, do the following before attempting to configure the eG tests with a xen user who has Read-only privileges to the XenServer:

    • Modify the target XenServer’s configuration in the eG Enterprise system. For this, follow the Infrastructure -> Components -> Add/Modify menu sequence, pick Citrix XenServer as the Component type, and click the Modify button corresponding to the target XenServer.
    • In the modify component details page that then appears, make sure that the os is set to Xen and the Mode is set to ssh.
    • Then, in the same page, proceed to provide the User and Password of a user who has the right to connect to the XenServer console via SSH.
    • Then, click the Update button to save the changes.
  4. Once this is done, you can configure the eG tests with the credentials of a xen user with Read-only privileges.   

  5. xen password - The password of the specified xen user needs to be mentioned here.
  6. confirm password - Confirm the xen password by retyping it here.
  7. ssl - By default, the Xen Server is not SSL-enabled. This indicates that by default, the eG agent communicates with the XenServer using HTTP. Accordingly, the ssl flag is set to No by default. If you configure the XenServer to use SSL, then make sure that the SSL flag is set to Yes, so that the eG agent communicates with the XenServer using HTTPS. Note that a default SSL certificate comes bundled with every XenServer installation. If you want the eG agent to use this default certificate for communicating with an SSL-enabled XenServer, then no additional configuration is required. However, if you do not want to use the default certificate, then you can generate a self-signed certificate for use by the XenServer. In such a case, you need to explicitly follow the broad steps given below to enable the eG agent to communicate with the XenServer via HTTPS:

    • Obtain the server-certificate for the XenServer
    • Import the server-certificate into the local certificate store of the eG agent

    For a detailed discussion on each of these steps, refer to the Troubleshooting section of this document.

  8. webport - By default, in most virtualized environments, the XenServer listens on port 80 (if not SSL-enabled) or on port 443 (if SSL-enabled). This implies that while monitoring an SSL-enabled XenServer, the eG agent, by default, connects to port 443 of the server to pull out metrics, and while monitoring a non-SSL-enabled XenServer, the eG agent connects to port 80. Accordingly, the webport parameter is set to 80 or 443 depending upon the status of the ssl flag.  In some environments however, the default ports 80 or 443 might not apply. In such a case, against the webport parameter, you can specify the exact port at which the XenServer in your environment listens so that the eG agent communicates with that port.
Measurements made by the test
Measurement Description Measurement Unit Interpretation

IntelliCache hits

Indicates the rate at which the caches on this SR serviced I/O requests.

Hits/Sec

A high value is desired for this measure. A low value is indicative of ineffective cache usage, which is typically caused if the cache is unable to serve most I/O requests. This in turn increases the load on the shared storage. 

IntelliCache misses

Indicates the number of requests per second that the caches on this SR did not service.

Misses/Sec

A low value is desired for this measure. A high value is indicative of ineffective cache usage, which is typically caused if the cache is unable to serve most I/O requests. This in turn increases the load on the shared storage. 

IntelliCache hit ratio

Indicates the percentage of I/O requests that were serviced by the caches on this SR.

Percent

A value over 80% for this measure is a sign of a healthy cache. A very low value indicates that the cache was unable to service most of the I/O requests to the SR. One of the common reasons for this is the lack of adequate space in the SR to store cached data. If the caches are not adequately sized, the read cache will not be able to hold frequently-referenced data and the write caches will not be able to accept data written to it. Check the value of the IntelliCache size measure to know how much space has been allocated to the caches on an SR.

Should the local storage reach capacity, IntelliCache will transparently “fall back” to shared storage, increasing the processing overheads in the bargain. Forecasting your local disk space requirements helps prevent XenServer from having to fall back to shared storage to handle the IOPS demand. Depending on variables in your environment, like patterns of user activity, you may need to plan more space for your Read Cache size.

In addition, your disk‐space requirements could increase any time multiple catalogs are present, such as during an upgrade rollout. For example, if virtual machines use multiple versions of the same catalog, Read Cache space usage will increase proportionately.

From a planning perspective, you should assume all of the master image could potentially be stored in the Read Cache. Consequently, if you have multiple catalogs on a host, you should assume that each catalog’s master image could be stored in the Read Cache. For example, if you have two catalogs each with different versions of applications in them, both master images could potentially be stored.

Likewise, if you are rolling out an operating system update, you may have two catalogs before users reboot and switch over to the new image.

IntelliCache size

Indicates the size of the caches on this SR.

MB