There is NOT a fixed amount of cache allocated per device.  There is a maximum WP count per device, but nothing is allocated.  One device can consume all of read cache if that was the only active device.  Striped metas can give better performance but it has less to do with cache and more to do with the concurrency.   Without any performance data to go on, my guess is you are limited by your FA (front-end) if the WP counts are not high.
All:  I'm testing SRDF/A between two sites 18ms apart.  The device I'm testing on is a thin meta with 4 members.  The performance seen on that device is terrible.  I see read and write latency in the 80+ ms range.     My implementation engineer told me that I should try striped metas, as opposed to concatenated b/c they'd have more cache, and more spindles to work with.  I've tested with both striped, and concatenated, and I cannot see a significant difference in speed between them.  I am new to Symmetrix, and I've taken the Config Management course, but I'm not very familiar with how the cache works.  My engineer tells me that each device gets a set amount of cache, and SRDF/A is filling that up b/c it's waiting to send, which hurts performance on that LUN.  If that was true, then the striped meta should theoretically show much better performance than a concatenated, simply b/c it's using the cache for all the devices on a write, versus just the head.  But again, I don't see much difference in speed either way.    So my question is, does Symmetrix really only dedicate a single, fixed slice of cache to each device?  When that's full, it can't borrow cache from somewhere else?    There's a setting in there for Dynamic Cache Partitioning.  The approximate number of documents on Powerlink referencing this is 0.  What is Dynamic Cache Partitioning?  Will it help my situation by providing a larger pool of cache to the SRDF/A devices?  What are the recommended settings for this?  By the way, this is a new implementation, and nothing else was going on during my tests.  I had a single ESXi 5 host running 2x 4Gbps FC links into 2 FE directors for this test.    Thanks for the information.  Brandon
Trouble with SRDF/A filling up cache. . .