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Cache hit rate and performance

hujirong
hujirong asked
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Last Modified: 2013-11-17
Hi

Take the following data for example, basically:
1. Is it good if cache hit rate is high? How high should be normally?
2. Miss rate is high means the cache size is not big enough, right?

Jirong

mvfsstat -c
----------------- Wed Nov  1 11:37:55 2006 ---------------------
dnlc:   7342936  7260055( 98.9%) hit    54331 dot 6364243 dir 746652 reg 94829 noent
                82881 (  1.1%) miss     15113 events
                114575(  1.6%) add      18633 dir 64784 reg 31158 noent
attr:   8693955     8598294( 98.9%) hit  183 lvut
                     95661(  1.1%) miss (53565cto+0gen+102timo+415new,41579ev;2839lvut)
                    329505 updates      51269unexp+32108exp mod, 3817 vmod
rddir:   64770       54001( 83.4%) hit  10769 miss
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Commented:
You are starting from the wrong point. What is the problem that the user is experiencing? If none then stop.

Author

Commented:
Thank you. I guess you mean nothing is wrong with that cache setting.

Jirong
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Top Expert 2014
Commented:
I don't think sjm_ee is saying that nothing is wrong with your cache stats.  He is trying to figure out what your problem is and then make suggestions as to what you should look at to help resolve the problem.

A high cache hit ratio means either:  You are reading the same data over and over and over .... and over again, you have the right amount of cache for you environment, or you have too much cache .

Miss rate just means that when the system went to read the data, it was not in cache and a "real" I/O had to be done.  Miss rates are not bad.  The first time you go to read data it will not be in cache, so it will be a miss.

If your system is meeting your SLA's, then what you should do is just keep an eye on your cache stats, along with memory usage, paging/swap rates, I/O rates, transaction rates, file/data base size, and CPU utilization.  Then if (when) your system does not meet your SLA', look at what is different from the stats and try and find out why it changed.

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Author

Commented:
Thank you very much. It's very well explained.

Jirong
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