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Low Hit Ratio for Initial Records buffer

28 Nov 2013 3:23 pm || 0

The buffer statistics display (transaction ST02) shows a very low hit ratio for the "Initial Records" buffer. It shows a value lower than 10%, for example. When you go to the detailed display of this buffer (by double-clicking the relevant row), it may look as follows:

Efficiency        HITRATIO          %        8
                   HITS                      423
                  REQUESTS                5.017
                  DB access quality %         8
                  DB access               4.585
                  DB access saved           423
                   Reorgs                      0
Size              Allocated        KB    7.219
                  Available        KB     6.000
                  Used             KB     1.307
                  Free             KB     4.693
Directory entries Available               9.750
                  Used                    4.585
                  Free                    5.165
Swaps            Objects swapped             0
                  Frames swapped              0
Resets            Total                       0

The reason for the low hit ratio may be (as is the case in the above example) the low usage of this buffer. The hit ratio is calculated from the ratio of the number of accesses that could be processed using the buffer (= HITS) to the total number of buffer accesses (= Requests). Therefore, the buffer was accessed only 5.017 times in total (as in the above example). 4.585 of these accesses were not "HITS" because the relevant object had to be read from the database into the buffer first. Since there are currently also 4.585 objects in the buffer ("Directory entries" -> "Used"), it can be assumed that these accesses were initial load accesses. This means that the buffer is still in the "settling phase".
The hit ratio is significant only if the number of requests is many times greater than the capacity of the buffer. A low hit ratio for relatively few accesses to the buffer is therefore normal and does not indicate poor performance.

It is important to observe the number of "Swaps". If the ratio of this number to the number of requests is very high, this indicates that the configuration of the buffer is too small. In this case, you should increase the parameter "rsdb/ntab/entrycount". The maximum number of entries in the "Initial Records" buffer ("Directory Entries Available") is derived from this parameter. It is "rsdb/ntab/entrycount" / 4.

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