Fast write cache (FWC) is an optional nonvolatile cache that provides redundancy with the standard adapter cache. The FWC tracks writes that have not been committed to disk.
FWC can significantly improve the response time for write operations. However, care must be taken not to flood the cache with write requests faster than the rate at which the cache can destage its data. It can also adversely affect the maximum I/O rate because additional processing is required in the adapter card to determine if the data that is being transferred is in the cache.
Fast write cache typically provides significant advantages in specialized workloads, for example, copying a database onto a new set of disks. If the fast write cache is spread over multiple adapters, this can multiply the benefit.
The FWC can also reduce JFS log bottlenecks because of three properties of the JFS log, as follows:
When single disk bandwidth becomes the limiting performance factor, one solution is to stripe several RAID 5 devices into a logical volume in conjunction with the FWC option of the SSA adapter. When the adapter is configured for RAID 5, writes equal to or larger than the stripe size bypass the cache. That is why 128 KB writes to a 2+p array with FWC are slower than 127 KB writes, and are equal to 128 KB writes to 2+p without the FWC. This is intended to keep bulk sequential I/O from "washing" the cache. The stripe size is 64 KB times the number of data disks in the RAID 5.