A Write Hit, also called Write Pending Write which happens when there is sufficient room in cache to store a requested write IO. The host director finds an available cache slot and pipes the host data to it. The host director immediately finishes the operation with the host, signaling that the data was successfully written. The cache slot used for the write is marked with a special “write pending” code to indicate that it cannot be reused until the data is safely stored on disk.
A Write Miss happens when Symmetrix cache slots in which to store writes, run out, a new write request cannot be serviced as fast as a regular write hit. This operation is commonly called a Delayed Fast Write or Write Miss.
Based on above description, the Write Miss requires one more step than Write Hit, which is cache allocation. But this cache allocation process is very quick. It means there is not any performance difference between Write Hit and Write Miss. In an extreme case, the Write Miss has negative performance is once the Writing data reach the system level cache limitation. (75% of available cache) So when a request for a write occurs that cannot be serviced because of lack of cache, existing write pending data is immediately destaged by disk directors. When the host director observes that there is space in cache, the write is completed as normal. This is where the term Delayed Fast Write originates: it is a normal Fast Write that must be delayed while destaging is done.
Following formulas illustrate the time structure of those three Symmetrix write operations:
- Write Hit = Write data the cache slots (64KB tracks) where is allocated and data existed.
- Write Miss = Allocate cache slots + Writing data
- Delayed Write = Destage data into disk + Free up cache slots + Writing Data
As the cache slots allocation is very fast. From performance perspective, usually we don’t care about the “Write Hit” and “Write Miss”. But we have to consider if write IOs are “Delayed Write”. That’s why we should check if the System Write Pending value (75% system cache) is reached. Or if the Write Pending limit of individual device (5% system write pending) is reached.
from:
https://www.dell.com/community/Symmetrix/Understand-the-Write-Hit-and-Write-Miss-in-Symmetrix/td-p/7063670