Purpose
Determines the number of free cylinders below which cylinder defragmentation can begin. The system dynamically keeps cylinders defragmented.
If the system has less than the specified number of free cylinders, defragmentation occurs on cylinders with at least 25% free space, but not enough contiguous sectors to allocate a data block.
Field Group
File System
Valid Range
0 through 100 free cylinders
Default
100 free cylinders
Changes Take Effect
After the DBS Control Record has been written.
Usage Notes
A cylinder becomes fragmented as a result of modifications to table data. As data blocks (DBs) are created, deleted, or change size as a result of these modifications, what started out as a small number of large free spaces on the newly allocated cylinder becomes a large number of small free spaces. Although there may still be a large total number of free sectors on the cylinder, with time there are fewer ranges of contiguous free sectors that are large enough to store new DBs. When new DBs cannot be stored on the cylinder, new free cylinders must be found and allocated.
The defragmentation process moves the data blocks closer together, coalescing the free space into areas that are large enough that new DBs can be allocated on the cylinder. This can reduce the need to allocate free cylinders for new data storage. The defragmentation process actually moves DBs from the fragmented cylinder to a new cylinder, where the DBs can be packed together, leaving a single, large set of consecutive free sectors. The cylinder from which the DBs were moved is freed, and made available for new allocations.
To keep costs to a minimum, background defragmentation is enabled only when the free cylinder count falls below the current value in the DefragLowCylProd field.
Setting this field to a value of 0 disables defragmentation. In this case, the lack of enough adjacent sectors to allocate a data block causes a new cylinder to be allocated.
The disadvantage of this approach is that free cylinders are a critical resource, and when no free cylinders remain, MiniCylPacks begin.
The operation is performed in the background.
Performance Implications
A low value in this field reduces the performance impact of defragmentation. However, setting the value extremely low might cause cylinders to be consumed more quickly, which could cause more MiniCylPacks to run. Set DefragLowCylProd higher than MiniCylPackLowCylProd because defragmentation has a smaller performance impact than cylinder pack.