Differential data

Differential data is managed with bitmaps in units of tracks. Tracks that receive a write instruction while a pair is being split are managed as differential bitmaps.

With storage systems, data is stored in units of tracks using bitmaps, and is then used to resynchronize the pair.

• If your primary system is other than VSP G1x00, and VSP F1500, and the secondary system is VSP G1x00, or VSP F1500, specify track as the differential data management unit in the primary system. VSP 5000 series support only tracks. Therefore, if you specify cylinders, TCz pairs cannot be created.
• If you use CCI, even though CCI allows you to specify track or cylinder, only track will be used.
• If you are making a TC pair with DP-VOL whose size is larger than 4,194,304 MB (8,589,934,592 blocks), the differential data is managed by the pool to which the TC pair volume is related. In this case, additional pool capacity (up to 4 pages, depending on the software configuration) is required for each 4,123,168,604,160-byte increase in user data.
  Note: The following procedure is to release the differential data (pages) managed by the pool:

  1. Delete all the pairs that use the V-VOL for which you want to release the pages.
  2. Set system option mode 755 to OFF.

     This action enables zero pages to be reclaimed.

  3. Restore the blocked pool.
  4. Release the V-VOL pages.

     For Device Manager - Storage Navigator, use the Reclaim Zero Pages window.

     For CCI, use the raidcom modify ldev command.

  You need to release differential data pages when you downgrade to the firmware version that does not support TC pair creation using DP-VOLs larger than 4,194,304 MB. The amount of time it takes to release differential data pages depends on the number of specified volumes, DP-VOL capacity, the number of allocated pages, and the storage system's workload. It also depends on the type of storage system. In some cases, it could take days to release all the differential data pages.

• After you create a TC pair with DP-VOL whose size is larger than 4,194,304 MB (8,589,934,592 blocks), data management might fail due to the insufficient pool capacity. In this case, all the P-VOL data is copied to the S-VOL in units of tracks when you perform the TC pair resync operation.