Write cache and auxiliary write cache IOA

Write cache

The write cache provides greater data integrity and improved performance. When the iSeries(TM) server sends a write operation, the data is written to the cache. Then, a write-completion message is sent back to the server. Later, the data is written to the disk. The cache provides a faster write capability and ensures data integrity.

The following actions occur during a write request from the server:

1. Data is committed to a nonvolatile battery-backed cache in the IOA.
2. A write completion message is sent from the server.
3. The following actions occur after the write completion message is sent.
   1. A write operation is sent from the IOA cache to the disk unit:
      • A write operation is sent from the IOA cache to the disk unit:
        • Reads the original data.
        • Calculates delta parity by comparing new and original data.
        • Writes the new data.
      • Write operations for parity data:
        • Reads the original parity information.
        • Calculates the new parity by comparing the delta parity and the original parity.
        • Writes the new parity information.
   2. Data is marked as committed data when it is successfully written to both the data disk unit and the parity disk unit.

The performance for this type of write operation is dependent on disk contention and the time that is needed to calculate the parity information.

Auxiliary cache IOA

The auxiliary cache IOA mirrors the write cache on a storage IOA. Protection is enhanced because two copies of data are stored onto two separate IOAs. If a failure occurs to the write cache, then the auxiliary cache IOA serves as a back up during the recovery of the failed IOA.

When an iSeries(TM) server sends a write operation, the data is written to the write cache on the storage IOA. The storage IOA mirrors the write cache data to the auxiliary write cache IOA. Then, a write-completion message is sent back to the server and the data is then written to a disk.

The auxiliary write cache is an additional IOA that has a one-to-one relationship with a disk IOA. The auxiliary write cache protects against extended outages due to the failure of a disk IOA or its cache by providing a copy of the write cache which can be recovered following the repair of the disk IOA. This avoids a potential system reload and gets the system back on line as soon as the disk IOA is replaced and the recovery procedure completes. Note, however, that this IOA is not a failover device that can keep the system operational if the disk IOA, or it’s cache, fails.

For more information about auxiliary write cache see the Planning for IBM^® i5 Data Protection with Auxiliary Write Cache Solutions redbook.