You can reclaim zero pages on each V-VOL. If you stop the reclaim zero pages operation, the zero pages that have already been reclaimed cannot be restored.
Logically, there is no difference between a page with zero data and the area of a V-VOL without a page allotted. However, a page with zero data uses pool capacity, whereas a V-VOL without a page allotted does not use pool capacity.
Zero pages can be reclaimed when all these conditions are satisfied:
- The V-VOL is not used in conjunction with another storage system software product that does not support reclaiming zero pages.
- LDEV formatting is not being performed on the V-VOL.
- The V-VOL is not blocked.
- The V-VOL does not have the data direct mapping attribute enabled.
- The V-VOL is not a deduplication system data volume.
- The V-VOL is associated with a pool.
- The pool associated with the V-VOL is not blocked, or is full and blocked.
- The LUN path is not defined between V-VOL and the host group enabled with host mode option 97.
If a V-VOL is used as a pair volume in one of the following products, reclaiming page is disabled. Therefore, the used capacity of the V-VOL is not reduced.
- ShadowImage
- Volume Migration
- Thin Image Advanced
- TrueCopy
- Universal Replicator
- Global-active device
If the capacity saving status is one of the following, reclaiming page is not enabled even when the Write Same/UNMAP command is received. Therefore, the used capacity of the V-VOL is not reduced.
- Deleting Volume or Deleting Volume (nn%). Reclaiming a page becomes enabled after deleting the V-VOL is complete.
- Failed. See Troubleshooting for provisioning.
If capacity saving is enabled after receiving the Write Same/UNMAP command, the zero data page reclaiming process might be retried internally. It might take longer to reclaim zero data pages, however retrying the Write Same/UNMAP command is not required.
Reclaiming zero data pages of a V-VOL with capacity saving disabled takes about one minute per 4 GB to complete. However, reclaiming zero data pages of a V-VOL with capacity saving enabled takes about one through five minutes per 4 GB to complete. For a volume with capacity saving enabled, it takes time to initialize the control information in addition to reclaiming zero data pages of the V-VOL, and therefore, it might take longer than a volume with capacity saving disabled.
One page at the end of a DRS-VOL might not be reclaimed.
Issues to consider when reclaiming zero pages
- Pages that include control cylinders are not processed when releasing pages in a V-VOL.
- Pages that include file system metadata cannot be reclaimed.
- While releasing pages from a V-VOL, performance of the host I/O to the V-VOL might temporarily decrease due to scanning for non-zero data.
- If you stop an operation to reclaim zero pages in mid-stream, the pages that have been released will remain as free pool capacity.
- After reclaiming zero pages, Dynamic Provisioning automatically balances usage levels among DDP groups in the pool. This rebalancing is performed on DDP groups related to the pool. If you do not want automatic balancing of the usage levels of DDP groups, contact customer support to change your configuration.
- Dynamic Provisioning does not automatically balance the usage levels among DDP groups if the cache memory is not redundant or if the pool usage reaches the threshold.
Operations to be performed
The following table shows the operation to be performed when a zero data page reclaimed for a V-VOL for which capacity saving is enabled. Notes:
- Reclaiming zero pages takes about one minute per 4 GB to complete.
- An I/O from the host is accepted during the zero pages reclaiming process. The workload for reclaiming zero pages is low and does not affect the I/O processing.
| Reclaiming page | V-VOL used capacity | Pool used capacity |
|---|---|---|
| Disabled | Not reduced | Reduced |
The following table shows the operation to be performed when the Write Same/UNMAP command is received for a V-VOL for which capacity saving is enabled. A page in a V-VOL is reclaimed even when the Write Same/UNMAP command is received.
| Reclaiming page | V-VOL used capacity | Pool used capacity |
|---|---|---|
| Enabled | Reduced | Reduced |
Additional notes
If all the tracks that belong to a page assigned to a V-VOL have no records written, you can reclaim the page and return it to the pool's available capacity.
- In these cases, reclaiming zero pages stops, and V-VOL pages might not be released:
- The pool-VOL accessed by the target V-VOL is blocked.
- The pool associated with the target V-VOL is blocked while the operation to reclaim zero pages is in progress.
- Cache memory failure occurs while the operation to reclaim zero pages is in progress.
- While the operation to reclaim zero pages is in progress, the initial copy operation of the TrueCopy pair, the Universal Replicator pair, the global-active device pair, or the ShadowImage pair is performed on the V-VOL.
- The LUN path is defined between V-VOL and the host group enabled with host mode option 97 while the operation to reclaim zero pages is in progress.
- The pages containing control information cannot be reclaimed. See Pool-VOL requirements.
- Reclaiming zero pages is performed per page (42 MB).
- The pages containing data cannot be reclaimed, and therefore the amount of deleted data might not match the used capacity of V-VOLs reduced by reclaiming pages.