Am looking for difference between cache usage and cache write pending?
The cache usage : is the amount of cache which is used by the immediate read/write IO on the storage array. When a host reads/writes , the idea is to have max reads from host, fulfilled from the cache for better performance.
Cache Write Pending - Is percentage of write I/Os which are in cache and haven't been destaged to disk. Most storage Arrays , store immediate write I/Os in cache and acknowledge successful write completion to the host for the next write I/O. The writes are then destaged to disk asynchronously in the background transparent to host. When you reach a certain limit where your host is writing faster than the back-end can destage you reach a write pending stage where another write I/O will have to wait unless destaging,which increases the write time. This results in performance issues since the host has to wait for IO processing. More drives or back-end disk adapter addition can increase the cache destaging as well.
Cache usage % : Percentage of cache being used for read and write operations, a good indicator is 100% usage. An average below 80% means that your array is being underutilized.
Cache Write Pending % : It is the percentage of cache being dynamically allocated for write operations. An ideal value is below 30%. A slow back-end combined with high IO can result in high CWP. If the CWP hits 70% it starts to priority destage the data and can also apply inflow control generating high response time. One of the most common cause for high CWP is turning ON of cache for external virtualized storage.
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