VNX: The effect of performance when enabling or disabling SP Cache (User Correctable)

When read cache is enabled: 
      A read request can be serviced directly from cache if the requested data is found in the cache (read hit).
      This provides a fast response time to the compute system (about a millisecond) as data is sent directly to the compute system, without any back-end storage operation.
      Read cache can identify trends and patterns in data access, and pre-fetch data that may satisfy future read requests.
 
When write cache is enabled: 
      Write data is first placed in cache and then written to the storage.
      When an I/O is written to cache and acknowledged, it is completed in less time (from the compute system s perspective) than it would take to write directly to storage.
      Write cache can immediately acknowledge write requests, and then asynchronously process that data and write to the drive.
      Write response times are much faster because the write operations are isolated from the storage devices.
 
When read cache is disabled: 
      A read request must be read from the storage. 
      The back end accesses the appropriate storage device and retrieves the requested data. 
      Data is then placed in cache and finally sent to the compute system through the front end. 
      Therefore, disabling read cache increases the I/O response time.
 
When write cache is disabled: 
      It acts like a write-through cache, and all writes are committed to disk before an acknowledgement is sent to the host. 
      Write data will not be mirrored to the peer SP. Subsequent reads may benefit from the write data held in cache, but write speed will be limited to the speed of the underlying disks.
      Because data is committed to storage as it arrives, the write-response time is longer because of the storage operations.
      Therefore, Disabling Write Cache causes I/O to be written directly to disk, there will be a significantly negative impact on performance.
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How to disable write cache?
From Unisphere:
Disable write cache: System --> Under System Management click on System Properties --> Go to SP Cache --> uncheck write cache (disable)

How to enable write cache?
From Unisphere:
Enable write cache: System  --> Under System Management click on System Properties -->  Go to SP Cache --> check write cache (enable)

Note: 
It is recommended that the default setting of write cache:  Enabled  remain in place.

Fix: Write cache disablement cannot be overridden by customers or support. In most cases when it occurs, it is in response to a serious hardware fault. If the system cannot safely hold the non-persistent memory, it will be disabled and IO will go directly to disk. This results in significantly high response times and can result in Data Unavailable. Help resolve write cache disabled states as soon as possible.
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