Cache % Hit on Symm 8830

[villavic]

I have an EMC Symm 8830 with 32GB cache. 13 Tru64 clusters are connected (2 nodes per cluster).

The cache %hit is between 60 to 80%, according to april average WLA statistics. My support told me this is good, since for open systems typical behavior is between 40 to 80%. Is that correct? Or should I be worry?

I always knew that a good %hit is over 90%, but don't know if Symmetrix cache design is different. When I saw the 60-80% chart, I thought my cache was saturated

 

[comtec17]

The cache hit rate is supposed to be high. A good rate is in the high 90's with 100% being the best.

 

[culainn]

with regard cache hit%, obviously the higher the better. But cache is only effective if your workload is "cache friendly". The most common cause of low cache hits is probably due to alot of random read taking place. Random reads negate the symmetrix prefetch alogrithms and thus reads are requested from disk thus lowering the cache hit %. In write terms you could be hitting a device write pending limit, this can be rectified with a little thought.
So to answer your question of what is good and what is bad, it depends on the environment. But in open systems terms I would say anything above 60% is good.

 

[dmayes]

%Utilization is not the best indicator of saturation (IMHO).

You need to ask your EMC CE about the "cache fall-through" time. In other words how long is data being held in cache before it is flushed/forced out. Two minutes would be the minimum time (general rule-of-thumb).

HTH...Dale

 

[culainn]

Fall through time is not a measure of cache residency, as the data within cache, for example, can get rewritten, a write on WP thus the data has changed and been promoted again to the top of the LRU. But yes if your fall through time is low and you have a low cache hit rate chances are you have a cache unfriendly workload, but if you have a fall through time which is low and a high cahe hit rate, don't worry about your fall through, your getting the hits.

 

[dmayes]

Well that's a fascinating answer but not true in my case.

I had a cache hit ratio around 80% but a fall-through of less than 60 seconds. My busiest server was seeing an average response time of 12ms.

The cache was increased from 16 to 24...the cache hit ratio remained the same...the fall-through increased dramatically (in the 3-4 min range) AND my busiest server was seeing an average response time of 9ms.

HTH...Dale