Disk Space

Job Design and Disk Space Utilization

As mentioned in EVM Job Solutions, a good approach is to design a group of small, interrelated jobs rather than a few large jobs. This approach provides the greatest flexibility in running jobs and optimizing the utilization of disk space on the StorageWorks subsystem.

 

Required Free Disk Space

EVM can only run a job when there are sufficient free disks for the replication of source units. At a minimum, the free capacity that is required must equal the disk capacity of the source units. EVM selects free disks for the BCV unit on a disk-to-disk basis, not on the basis of overall unit capacity.

 

Actual Free Space Assigned

In many cases, EVM will have to select free disks whose total capacity exceeds that of the source units. These assignments can vary each time the same job is run.

For example, assume that the job includes three storage units that are comprised of 9 GB disks. When you create the job there are sufficient resources and the job is validated and saved.

When the job is first run, all of the 9 GB disks in the subsystem are in use and only 18 GB disks are free. In this case, each of the three BCV units will use twice as much disk space as its source unit.

The second time the job is run there are enough 9 GB disks free to assign them to two BCV units. The third BCV unit uses free 18 GB disks. During this run, the disk space used for two BCV units is the same as the source units, but the disk space for the third BCV unit is twice the size of its source.