Automatic deletion of history blocks

History blocks in the circular and alternate storage locations may be automatically deleted to make room for new history blocks. Whether or not the blocks are deleted is determined by the minimum threshold and the maximum size and/or age specified for the storage location.

The history block management system will check for available space in the circular and alternate locations if it detects any changes made by other subsystems or the user in controlled directories.

The history block management system computes the sum of the sizes of all history blocks (including the current one) in the circular storage location and determines if there is enough space on the drive to hold all of the blocks.

If the space available on the storage location drive is below a certain threshold, the Storage subsystem will delete enough of the oldest history blocks to bring the available disk drive space back to a positive value and then move the new history block in.

If an alternate storage location exists, the older block(s) will be moved there instead of being deleted. The alternate storage location functions exactly like the circular storage location. However, when the blocks exceed the set limits (minimum threshold, maximum size, or maximum age), the oldest blocks will be deleted from disk.

To avoid this loss of data, it is important that the system administrator regularly monitors the disk availability and periodically backs up old blocks to long term storage before they are deleted.

For example, a history block is stored in the circular storage location. The maximum size of the disk drive for circular storage is 120 MB. In addition to the circular storage location, an alternate location with a maximum disk drive size of 120 MB is defined. For both locations, the minimum threshold value is 50 MB. Essentially, this means that there is 70 MB of actual storage space.

Note: The sizes in this example are purposely small; the disk drives for storage locations should be much larger.

You should typically set the minimum threshold to a value that is 3 times larger than the size of the biggest history block. This will provide the history block management system enough time to copy oldest history block from the circular location to the alternate, and then delete block from the circular location.

If you monitor the disk drive space available in the circular or alternate storage location over time, the value will fluctuate between the threshold value and the maximum size of the location, with sharp increases when blocks are moved out. While the system is moving a block(s) out, the space available will dip just below the threshold value before the increase.

If the maximum threshold is reached before the age of the block reaches the specified limit, the block is moved or deleted. A block will be moved or deleted within one history block duration of it reaching the age limit. If, for any reason, the system is unable to move a block that is past the age limit, the block will not be deleted until the size or space limit is reached.