Cache Server Administration
When installing or operating a cache server, there are certain things you should take into consideration to ensure that you get the most from the cache server.
Effective Cache Size with Pre-Set Database Size
The Cache Server is at its most efficient when its memory use is ca. 70% of the specified database size.
We recommend that you set the database to be ca. 50% larger than the actual cache size.
Example calculation
If the effective cache size is 100 MB, the database instance should be 150 MB.
This apparent loss of disk memory is due to the fact that the disk space is assigned in chunks to the stored documents. The last chunk is usually only partly used up. In the worst case, only one byte of it is used.
How do you recognize that the cache is not performing optimally, and what can you do about it?
a) Check the ratio of cache hits to cache misses.
The cache first tries to deal with incoming client requests on its own, without getting the content from another cache server or content server. If it succeeds, this is known as a cache hit. If, on the other hand, the cache does have to request the object from another server, this is known as a cache miss.
The number of cache hits should be much higher than the number of cache misses. Well-tuned caches have hit rates of over 80%.
b) Check the fill level of the cache
The closer the cache gets to its maximum fill level, the more likely it is that cache displacements will occur. These displacements cost time, as the cache logic first has to calculate the required space, then check which objects can be displaced, and then displace these objects.
c) Check the correlation of cache misses to fill level
Displacement occurs in the case of a cache miss only if the cache also becomes full at the same time. If a high number of cache misses in relation to the total number of requests now occurs, the cache is then in what is known as a "thrashing" state.
You should try to avoid this state at all costs, as it has a very negative effect on performance. The only solution to thrashing is to extend the amount of free memory available, or to reset the cache. (See also:
d) Increase the cache area by adding extra devspaces
To ensure that the previous cache content is not lost when you re-start the cache, you need to take the following steps when extending the database:
If the database is not stopped at this point, you can carry out the step above after changing the INI file. This causes the database to stop briefly.
Where can you find statistical information on the cache server?
Open transaction CSADMIN. Instead of a content server name, enter a cache server name. Statistics for this cache server are then displayed under "Statistics".
The following values are of interest here:
The relationship between these two values tells you the relative fill level.
cacheMaxSize is specified in the file CSProxyCache.INI. Compare cacheMaxSize with the actual size of the database (using DBMGUI). cacheCurrentSize is always smaller or equal to cacheMaxSize.Related Notes
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