Datastore cache performance

The datastore makes use of an in-memory cache that is used for all read and write operations. The datastore stores data on disk in pages, and any system or user request to the datastore can require data from many pages. Any page not currently in the in-memory cache must be retrieved from the disk. Every datastore request can result in a number of cache page hits or misses. In addition, if the data on a page has been modified since being read into the cache, it is termed dirty and must be written to the disk before the page is removed from the cache. If a page is unchanged since being loaded into the cache, it is termed clean and can be unloaded without writing to disk.

If the datastore spends a lot of time swapping pages in and out of the cache, it can cause performance problems.

Datastore cache performance factors

The Datastore cache performance page enables you to determine whether the cache is performing effectively. It shows the following data at 10-minute intervals:

• Cache hits
• Cache misses
• Clean pages evicted
• Dirty pages evicted

Cache hits and cache misses are the most important indicators of cache performance.

Expected datastore performance depends a great deal on the environment being discovered, the performance of the appliance hardware and disk subsystem, and how heavily the system is used for data browsing and reporting. The following figures are representative of a typical installation in a typical environment while actively scanning or consolidating:

• A datastore cache that is performing well can be expected to have cache hits on the order of tens of millions, and misses on the order of tens of thousands or fewer, per 10 minutes.
• A datastore cache that is experiencing on the order of hundreds of thousands of misses per 10 minutes might have adverse effects on the system as a whole.

Examples

This graph shows three discovery runs in which a datastore cache is performing well with cache hits between 75,000,000 and 95,000,000 per 10 minutes and cache misses below 45,000 per ten minutes.

This graph shows a discovery run from midnight to 1:00 A.M. in which the datastore cache experiences just under 50,000,000 cache hits per 10 minutes, but with 375,000 cache misses per 10 minutes. The rate of cache hits is typical, although the rate at which cache misses are occurring is high and might be causing adverse effects to the system as a whole.

To view the datastore cache performance page

1. From the main menu, click the Administration Settings icon.
2. In the Appliance section, click Performance.
3. Click the Datastore tab.
4. Select the date for which you want to view the performance data, from the Date drop-down selector.
5. If you are viewing a cluster, you can also select individual machines or the whole cluster from the View drop-down selector.
   
   

Related topics

Monitoring appliance performance