PVS Default Write Cache Paths Test

The write cache includes data written by the target device. If data is written to the PVS server vDisk in a caching mode, the data is not written back to the base vDisk. Instead, it is written to a write cache file.

If the default path to this write cache file is unavailable, then the client would hang. Similarly if the write cache path is available but cannot be accessed quickly, then target devices may not be able to write changes to the cache as fast as they occur. Naturally therefore, the base vDisk will be updated with the changes only much later. Until then, target devices will be provided with an outdated vDisk upon boot.

To avoid such anomalies, administrators must be able to tell which write cache path is unavailable and which takes a long time to access, well before they impact user experience. This is where the PVS Default Write Cache Paths test helps. For each default write cache path it auto-discovers, this test reports the availability of the path and its access time. In the process, it pinpoints unavailable write cache paths and those that take too long to be accessed. Once the problematic paths are isolated, administrators can rapidly initiate investigations into the root-cause and swiftly resolve it.

Target of the test : Citrix Provisioning server

Agent deploying the test : An internal agent

Outputs of the test : One set of results for the default write cache path of each store assigned to the Provisioning server being monitored

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. Port – Refers to the port used by the Citrix Provisioning server. By default, this is 54321.

  4. mcli path – This test executes commands using the Management Command Line Interface (MCLI) of the Provisioning server to collect the required metrics. To enable the test to execute the commands, the eG agent, by default, auto-discovers the full path MCLI.exe on the target Provisioning server. This is why, the mcli path is set to none by default. If, for some reason, the eG agent is unable to auto-discover the mcli path, then you will have to manually specify the path here using the following pointers:

    • Typically, in a 32-bit Windows system, the MCLI.exe will be available in the following location by default: <System_Root>\Program Files\Citrix\Provisioning Services Console
    • In a 64-bit Windows system on the other hand, the MCLI.exe will be available in the following location by default: <System_Root>\Program Files (x86)\Citrix\Provisioning Services Console

  5. domain name, domain user and domain password – To report farm-related metrics, this test should run using the credentials of a user who fulfills the following requirements:

    • Should belong to the Security group with 'Farm Administrator' access.
    • Should be assigned the Allow log on locally security privilege on the Citrix Provisioning Server host.

    The steps for assigning such privileges to a user are detailed in the Pre-requisites for monitoring the Citrix Provisioning Server topic.

    Once you assigned the aforesaid privileges to the user, then configure this test with the domain name, domain user, and domain password of the same user.
Measurements made by the test
Measurement Description Measurement Unit Interpretation

Write path availability:

Indicates whether/not this default write cache path is available.

Percent

This measure reports the value 100 if the write cache path is available, and 0 if it is not.

Compare the value of this measure across write cache paths to identify those that are currently unavailable.

For high availability of write cache paths, you may want to use the ‘multiple write cache paths’ option that Citrix PVS offers. The multiple write cache paths (for a store) option provides the capability of distributing the write cache files across multiple physical media. When a target device starts the server chooses one of the write cache paths from the list based upon the MAC address of the client. The goal of selecting a path based on the MAC address is to get an even distribution of the clients across the available paths. The algorithm selects the same path for a given client each time that client is booted.

This functionality is needed to ensure that during a High Availability (HA) failover the (new) server would choose the same write cache for the client (otherwise it would not be able to find the write cache file and the client would hang). If the defined write cache path is not available to a server, the server falls back to the standard vDisk path.

Write path access time:

Indicates the time taken to access the write cache file in this path.

Secs

A low value is desired for this measure. A consistent increase in this value is a cause for concern, as it indicates that responsiveness of the default store path is deteriorating. This could be owing to a flaky network connection. It could also be because the path is overloaded with write requests from target devices. In case of the latter, you may want to consider using the ‘multiple write cache paths’ feature offered by Citrix PVS. This capability enables write cache files to be distributed across multiple physical media. This feature helps to improve I/O throughput for heavily loaded servers, and thus improves the responsiveness of write cache paths.