eG Monitoring
 

Measures reported by PVSWritePathTest

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 Until then, target devices will be provided with an outdated vDisk upon boot.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 PVSWritePathTest 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.

The measures made by this test are as follows:

Measurement Description Measurement Unit Interpretation
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.

Response_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.