OS Cache Test

This test reveals whether or not the operating system's cache has been effectively utilized. This test is disabled by default.

Target of the test : A Windows host

Agent deploying the test : An internal agent

Outputs of the test : One set of qresults for every Windows host monitored

Configurable parameters for this test
  1. Test period - How often should the test be executed
  2. Host - The host for which the test is to be configured.
Measurements made by the test
Measurement Description Measurement Unit Interpretation

Copy read hits:

Indicates the percentage of cache copy read requests that hit the cache - i.e., they did not require a disk read in order to provide access to the page in the cache.

Percent

A copy read is a file read operation that is satisfied by a memory copy from a page in the cache to the application's buffer. The LAN Redirector uses this method for retrieving information from the cache, as does the LAN Server for small transfers. This is a method used by the disk file systems as well. Ideally, the value of this measure should be high. A very low value could indicate an increase in disk accesses and related processing overheads.

Copy reads:

Indicates the frequency of reads from pages of the file system cache that involve a memory copy of the data from the cache to the application's buffer.

Reads/Sec

The LAN Redirector uses this method for retrieving information from the file system cache, as does the LAN Server for small transfers. This is a method used by the disk file systems as well.

Data flushes:

Indicates the rate at which the file system cache has flushed its contents to disk as the result of a request to flush or to satisfy a write-through file write request. More than one page can be transferred on each flush operation.

Flushes/Sec

 

Data map hits:

Indicates the percentage of data maps in the file system cache that could be resolved without having to retrieve a page from the disk, because the page was already in physical memory.

Percent

A high value of this measure is desirable.

Data maps:

Indicates the frequency with which a file system such as NTFS, maps a page of a file into the file system cache to read the page.

Maps/Sec

 

Fast reads:

Indicates the frequency of reads from the file system cache that bypass the installed file system and retrieve the data directly from the cache.

Reads/Sec

Normally, file I/O requests invoke the appropriate file system to retrieve data from a file, but this path permits direct retrieval of data from the cache without file system involvement if the data is in the cache. Even if the data is not in the cache, one invocation of the file system is avoided and processing overheads are reduced.

Lazy write flushes:

Indicates the rate at which the Lazy Writer thread has written to disk.

Flushes/Sec

Lazy Writing is the process of updating the disk after the page has been changed in memory, so that the application that changed the file does not have to wait for the disk write to be complete before proceeding. More than one page can be transferred by each write operation.

Lazy write pages:

Indicates the rate at which the Lazy Writer thread has written to disk.

Pages/Sec

MDL read hits:

Indicates the percentage of Memory Descriptor List (MDL) Read requests to the file system cache that hit the cache, i.e., did not require disk accesses in order to provide memory access to the page(s) in the cache.

Percent

Ideally, this percentage should be high.

MDL reads:

Indicates the frequency of reads from the file system cache that use a Memory Descriptor List (MDL) to access the data.

Reads/Sec

The MDL contains the physical address of each page involved in the transfer, and thus can employ a hardware Direct Memory Access (DMA) device to effect the copy. The LAN Server uses this method for large transfers out of the server.

Pin read hits:

Indicates the percentage of pin read requests that hit the file system cache, i.e., did not require a disk read in order to provide access to the page in the file system cache.

Percent

While pinned, a page's physical address in the file system cache will not be altered. The LAN Redirector uses this method for retrieving data from the cache, as does the LAN Server for small transfers. This is usually the method used by the disk file systems as well.

Read aheads:

Indicates the frequency of reads from the file system cache in which the Cache detects sequential access to a file.

Reads/Sec

The read aheads permit the data to be transferred in larger blocks than those being requested by the application, reducing the overhead per access.