MaxDB/LiveCache 7.7 - Data Cache Size > 32GB

[arno_riegel]

Hi Folks,

For our prod LiveCache DB we've configured an I/O Buffer Cache size of 66GB (CacheMemorySize = 8650752). This results in the following Cache Sizes:

I/O Buffer Cache	67'584.00 MB
Data Cache	32'764.00 MB
Converter	23.42 MB
Misc.	34'796.58 MB
Catalog Cache	820.00 MB
Sequence Cache	0.01 MB

What confuses me is, that the Data Cache size is only ~32GB and that there are 34'796 MB assigned to "Misc". Has anybody an idea what kind of cache "Misc." is or how I can assign more memory to the Data Cache? Is there a maximum limit of 32GB for the Data cache?

Thanks a lot for any help in advance!

Best regards

Arno

[Former Member]

Arno,

Take a look at http://maxdb.sap.com/doc/7_7/44/befc6f33363f47e10000000a114a6b/content.htm.  This explains the breakdown of each of the caches and why some areas cannot be assigned sizes manually.

Regards,

Josh

[arno_riegel]

Josh,

Thanks for the quick response. The question is, why the Data Cache size is "only" 32GB although CacheMemorySize is set to 66GB. In this document (as well as in all others I've seen) the Data Cache is always the biggest part of the I/O Buffer Cache. For my LiveCache DB the "Misc." part is even bigger than the Data Cache size and Data Cache is only about half of the I/O Buffer Cache.

Regards

Arno

[Former Member]

Arno,

The way that the memory gets utilized is a function of the processes accessing livecache, so there's nothing technically "wrong" with how the memory is being allocated.  I've seen customer situations where the Catalog cache is 3x the the data cache regularly for example as well as a current system where the Other or Miscellaneous cache is larger than the data cache.  From a basis standpoint, as long as your hit rates are good (>=96-97%) that's what you're after.

Regards,

Josh

[arno_riegel]

Josh,

The Catalog Cache size can be customized manually (CAT_CACHE_SUPPLY) which may be the reason why you've seen databases where the Catalog Cache is 3x the data cache.

The ammount of memory assigned to "Misc" and the fact that the Data Cache size is not growing beyond 32GB doesn't give me the feeling that there's nothing techincally "wrong". It appears that ~33GB of the I/O Buffer Cache are not used at all...

Regards

Arno

[Former Member]

Arno,

Yes, I was using the catalog cache as an example of overall variability within the different cache areas regardless of parameters so I apologize for not being more specific about that.

What do your cache hit rates and data cache usage look like from transaction LC10 / CCMS?

Regards,

Josh

[arno_riegel]

Josh,

The Data Cache occupancy reaches 100% from time to time and is constantly >95%. So there are good reasons for the DB to occupy more than 32GB Data Cache. The Cache Hit Ratio is ~99.9%, but that's not my question...

The initial question was and still is, why "only" 32GB Data Cache out of 66GB I/O Buffer Size are assigned and what kind of "Misc" Cache(s) are using ~34GB of memory.

Regards

Arno

[Former Member]

Arno,

The best explanation I can find for what occupies the rest of the I/O Buffer Cache is the "Working Memory for the Internal File Directory from - http://maxdb.sap.com/doc/7_8/44/befc6f33363f47e10000000a114a6b/content.htm.

Regards,

Josh