Impact of disabled write cache

Impact of disabled write cache is felt on performance (mostly the writes will be bit slow). No dirty pages can exist when Write cache is disabled. All the data from write cache will be flushed to disk (either vault or data as per the situation) before the write cache can be disabled.

All the writes will go to the disks directly and only then the acknowledgement to the host can be sent. Hence, the writes will be slower. Reads may be slower as the read-hit possibility from write cache portion is not available.

As suhasbhide72 mentioned, you will begin performing at disk speeds if write cache is disabled, which will cause slower writes on systems that were using cache in a nice way before.

However, if you have a Clariion where write cache is constantly getting forced flushes, you will actually see better performance by turning the cache off. If you have applications that are doing a lot of bulk writes and eating up cache, you are better off running directly to disk rather than filling the cache and waiting for forced flushes.

The operation of the write cache is highly dependent on the I/O. Any discussion of its operation would require understanding the if the I/O was random or sequential, the percentage of writes versus reads, the block-size of the I/O, and if it is steady or 'bursty'.

Also, are you disabling at the LUN or the storage system level?

You may want to read the Memory section of the EMC CLARiiON Storage System Fundamentals for Performance and Availability. This includes a detailed discussion on cache operation including "write through" operation and enabling and disabling.

The EMC CLARiiON Best Practices for Fibre Channel Storage: CLARiiON Release 26 Firmware Update likewise contains information on CX3 write cache operation.

Both of these documents are available on PowerLink.