Bypassing cache during write operations

Database, recovery log, and storage pool volumes are opened with the appropriate operating system settings to require data write requests to bypass any cache and be written directly to the device.

By bypassing cache during write operations, IBM Storage Protect maintains the integrity of client attributes and data. Bypassing the cache is required. If an external event, such as a power failure, causes the server or the computer where the server is installed to halt or break while the server is running, the data in the cache might or might not be written to the disk. If the IBM Storage Protect data in the disk cache is not successfully written to the disk, information in the server database or recovery log might not be complete. Also, data that was supposed to be written to the storage pool volumes might be missing.

Hard disk drives installed on the computer where the server is installed and running have less of an issue with bypassing cache. In this case, the operating system settings that are used when IBM Storage Protect opens volumes on that hard disk drive generally manage the cache behavior appropriately and honor the request to prevent caching of write operations.

Typically, the use and configuration of caching for disk subsystems is a greater issue because disk subsystems often do not receive information from the operating system about bypassing cache for write operations. Disk subsystems also might ignore this information when a volume opens. Therefore, the caching of data write operations might result in corruption of the server database or loss of client data, or both. The problems are dependent upon which IBM Storage Protect volumes are defined on the disk subsystem and the amount of data lost in the cache. Disk subsystems should be configured to not cache write operations when an IBM Storage Protect database, recovery log, or storage pool volume is defined on that disk. Another alternative is to use nonvolatile cache for the disk subsystem. Nonvolatile cache employs a battery backup or some other sort of scheme to allow the contents of the cache to be written to the disk if a failure occurs.