Database Reference
In-Depth Information
Figure 4.7
New Sample.Basic cache statistics. (From Oracle Essbase Administration Services. With permission.)
for run length encoded. The header for rLE is fixed at approximately 72 bytes. rLE
compression type stores the value and then a count indicating the consecutive blocks
that have the value. rLE works best when there are many missing values or values that
repeat in the data block. According to the documentation, if rLE is specified, Essbase
will analyze the data and choose the appropriate compression. I tried very hard to prove
the documentation correct or otherwise, but I did not see this behavior. There is no sub-
stitute for testing. In general, larger blocks will probably produce smaller PAg files using
rLE than Bitmap and vice versa.
now back to cache settings. As BSo writes a data block, the uncompressed block
(stored cells × 8 bytes) is compressed and moved to the operating system cache. The block
is then written to the PAg file by the operating system. If using direct I/o, the data is
written to the PAg file directly from the Essbase data file cache bypassing the operating
system cache altogether. The data and data file caches give BSo a place to save blocks
in case it needs them again, saving a fetch from the relatively slow PAg disk file. The
Essbase kernel will continue writing blocks to the cache until it is full. once the cache
is full, older blocks are replaced by more recently retrieved data blocks. The object of
sizing the caches is to increase the likelihood that the data block will be in the cache.
The BSo data cache default size is 3072 kb. Dividing 3,072,000 by the block size gives
the number of blocks that fit in the data cache. In most cases, the default is too low. to
size the cache, double it for test, stopping and starting the database after the change.
Then run a full aggregate and note the time. When aggregations times stop improving
or improve only slightly, the cache is too large. The correct value is somewhere between
the data cache setting of the last two tests. Perform several tests adjusting the cache
between the two values until the optimum value is found.
4.6.3
Data File Cache
This cache is only used with Direct I/o. As stated earlier, for Direct I/o, the data cache
and the operating system cache are not used. Direct I/o is better suited to 64-bit operat-
ing systems because they are able to address larger amounts of memory. The data blocks
stored in the data file cache are compressed. Let me say that again. The key concept to
understand about Direct I/o is that the data file cache blocks are the same size as the
