Look-aside is a term used to describe the process of searching for a particular record in storage buffers. Finding the desired CI in a virtual storage buffer reduces the overall access time and helps reduce the CPU time it takes to process the I/O operations to disk. The major objective when tuning I/O is to find the data in virtual storage and avoid a physical I/O operation. A good look-aside hit ratio can improve online response times. The look-aside hit ratio is one of the ways you can measure the effectiveness of the assigned buffering to a file. The higher the percentage of times you find the data in virtual storage, the better the response time. The look-aside formula is:
% HR = Total # of buffer hits* 100 (Total # of misses + total # of hits)
- Hit: when the desired CI is found in the virtual storage buffer
- Miss: when the desired CI is not found in the buffer requiring an I/O
- HR: look-aside hit ratio.
When processing direct KSDS files, it’s important to determine the number of index buffers to be allocated to the file; this ensures the best look-aside hit ratio for the high-level index records (i.e., second level and higher indices). These indices are called the Index Set (IS) to differentiate them from the lowest index level called the Sequence Set Index (SSI). A VSAM LISTCAT provides us with the necessary information so that we could determine how many IS indices are on a file (Figures 2, 3, and 4). Before entering into the formula, look at the number of index levels in the file to avoid unneeded computations. The number of index levels can be found on the second LISTCAT page at the bottom right-hand side (Figure 3). If the file has two index levels, then there’s only one IS record. So only one additional index buffer is required. If the index level is equal to one, then there are no IS records and no additional BUFNIs are required. In this case, there’s only one SSI record.
The information required from the LISTCAT is as follows:
- Total number of index records (Figure 2)
- Data CISZ (Figure 2)
- Number of data CIs per CA (Figure 2)