Flash Cache Test

Exadata Smart Flash Cache provides an automated caching mechanism for frequently-accessed data in the ExadataDatabase Machine. It is a write-backcache which can service extremely large numbers of random reads and writes and improvesthe responsiveness of OLTP applications deployed on the Database Machine.

When the database sends a read or write request to Exadata Storage Server, it includes additional information in the request about whether the data is likely to be read again and therefore whether it should be cached. Based on the information the database sends, the Exadata Storage Server Software intelligently decides which data is likely to be re-read, and is worth caching, versus data that would just waste cache. Random reads and writes against tables and indexes are likely to have subsequent reads and normally will be cached and have their data delivered from the flash cache. In addition, Exadata Smart Flash Cache is persistent across Exadata Storage Serverrestartsand will not require any warm up period. If the Flash cache does not contain the data that the read I/O references and does not have the space to accommodate the data blocks written to it by the write I/O, direct disk accesses are bound to increase. Reading from and writing directly to disk are expensive operations that consume time and resources. If this is to be avoided, the Flash cache has to be optimally sized and utilized. But, how does one know how well the cache is being used? For this, you can use the Flash Cache test.

This test monitors the space utilization and I/O load on the Flash cache, reports how well the cache services the read and write I/O load, and in the process, indicates whether/not the cache has been effectively utilized. I/O processing bottlenecks (if any) in the flash cache and inoptimal flash cache usage can thus be detected and the reasons for the same investigated.

Target of the test : Oracle Exadata Storage Server

Agent deploying the test : A remote agent

Outputs of the test : One set of the results for the target Oracle Exadata Storage Server that is being monitored

Configurable parameters for the test
Parameter Description

Test period

How often should the test be executed

Host

The IP address of the host for which this test is to be configured.

Port

The port number at which the specified host listens. By default, this is NULL.

Username, Password and Confirm Password

By default, this test uses the Cell Control Command-Line Interface (CellCLI) to pull out the required metrics. To use the CLI, the test first needs to connect to the target storage server via SSH, and then run commands using CLI. For running the commands, this test requires the credentials of a cellmonitor user. Specify the login credentials of such a user in the User and Password text boxes and confirm the Password by retyping it in the Confirm Password text box.

SSH Port

This test uses the Cell CLI to pull metrics from the target Oracle Exadata Storage Server. To run the CLI commands, this test first needs to establish an SSH connection with the target storage server. To enable the test to establish this connection, specify the SSH Port here.

Timeout

 Specify the time duration for which this test should wait for a response from the storage system in the Timeout text box. By default, this is 120 seconds.

Measurements made by the test
Measurement Description Measurement Unit Interpretation

Flash cache size allocated

Indicates the total size allocated to the flash cache.

GB

 

Used flash cache size

Indicates the amount of space utilized by the flash cache.

GB

A value close to Flash cache size allocated measure indicates that the flash cache is running out of space.

Usable flash cache size

Indicates the amount of space available for use in the flash cache.

GB

A high value is desired for this measure.

Flash cache usage

Indicates the percentage of flash cache space that is already utilized.

Percent

A value close to 100 indicates that the flash cache is being utilized to its maximum capacity. Administrators may need to free up space in the flash cache or allocated additional resources.

Flash cache read hits

Indicates the rate at which data was read from the flash cache.

MB/sec

Ideally, the value of these measures should be high. A consistent drop in this value is a cause for concern, as it could indicate that data is not read from / written to the flash cache frequently. This in turn implies low flash cache usage and high disk usage.

Flash cache write hits

Indicates the rate at which data was written to the flash cache.

MB/sec

Read request hits

Indicates the rate at which read requests were serviced by the flash cache.

Requests/sec

Ideally, the value of these measures should be high. A consistent drop in this value is a cause for concern, as it could indicate that read/write I/O requests are not being frequently serviced by the flash cache. This in turn implies low flash cache usage and high disk usage.

 

Write request hits

Indicates the rate at which write requests were serviced by the flash cache.

Requests/sec

Flash cache read Misses

Indicates the rate at which data was read from the disks since data could not be serviced from the flash cache.

MB/sec

A low value is desired for these measures. A high value indicates that the cache is used ineffectively meaning that the cache is unable to serve data for most I/O requests thus increasing the load on the disks.

Flash cache write Misses

Indicates the rate at which data was written to the disks directly instead of being written to the flash cache.

MB/sec

Read request misses

Indicates the rate at which read requests were serviced by the disks since the requests could not be serviced by the flash cache.

Requests/sec

A low value is desired for these measures. A high value indicates that the cache is used ineffectively meaning that the cache is unable to serve most read/write I/O requests thus increasing the load on the disks.

 

Write request misses

Indicates the rate at which write requests were directly serviced by the disks instead of the flash cache.

Requests/sec

Skipped reads

Indicates the rate at which data was read from the disks bypassing the flash cache.

MB/sec

A low value is desired for this measure. A high value may indicate that the flash cache is not used or the requests sent to the flash cache bypasses without hitting the cache, and accesses the disk directly.

Skipped reads due to heavy load on flash cache

Indicates the rate at which data was read directly from the disks due to heavy load on the flash cache.

MB/sec

 

Skipped reads due to large IO request

Indicates the rate at which data was read from the disks for the I/O requests that bypassed the flash cache due to large I/O request size.

MB/sec

 

Skipped writes

Indicates the rate at which data was written to the disks bypassing the flash cache.

MB/sec

A low value is desired for this measure. A high value may indicate that the flash cache is not used or the requests sent to the flash cache bypasses without hitting the cache, and accesses the disk directly.

Skipped writes due to heavy load on flash cache

Indicates the rate at which data was written directly to the disks due to heavy load on the flash cache.

MB/sec

 

Skipped writes due to large IO request

Indicates the rate at which data was written to the disks for the I/O requests that bypassed the flash cache due to large I/O request size.

MB/sec

 

Skipped read requests

Indicates the rate at which read I/O requests bypassed the flash cache.

Requests/sec

A low value is desired for this measure. A high value may indicate that the flash cache is not used or the read I/O requests sent to the flash cache bypasses without hitting the cache, and accesses the disk directly.

Skipped read requests due to heavy load on flash cache

Indicates the rate at which read I/O requests bypassed the flash cache due to heavy load.

Requests/sec

A low value is desired for this measure. A high value may indicate that the flash cache is experiencing heavy load. Administrators should analyze the reason behind such heavy load on the flash cache and optimize the load.

Skipped read requests due to Large IO Size

Indicates the rate at which read I/O requests bypassed the flash cache due to large I/O requests size.

Requests/sec

 

Skipped Write requests

Indicates the rate at which write I/O requests bypassed the flash cache.

Requests/sec

A low value is desired for this measure. A high value may indicate that the flash cache is not used or the write I/O requests sent to the flash cache bypasses without hitting the cache, and accesses the disk directly.

Skipped Write requests due to heavy load on flash cache

Indicates the rate at which write I/O requests bypassed the flash cache due to heavy load.

Requests/sec

A low value is desired for this measure. A high value may indicate that the flash cache is experiencing heavy load. Administrators should analyze the reason behind such heavy load on the flash cache and optimize the load.

Skipped Write requests due to Large IO Size

Indicates the rate at which write I/O requests bypassed the flash cache due to large I/O request size.

Requests/sec