FlashCopy and volume group snapshot capacity configuration

FlashCopy® and volume group snapshot capacity limits are determined by various factors.

  • The physical memory of the system.
  • The platform type in the system.
  • The system's grain size. It can be 256 KiB or 64 KiB.
For volume group snapshots there are two limits, Production Capacity Allowance (PCA) and Snapshot Capacity Allowance (SCA).
  • PCA is the provisioned capacity that is directly accessible from a host.
  • SCA is the provisioned capacity held in snapshots.
The following table shows the maximum values for PCA and SCA.
Table 1. Volume group snapshot capacity limits
Product Node canister memory Grain size Max PCA Max SCA
IBM® Storage FlashSystem 9500 512 GiB 256 KiB 40 PiB 200 PiB
1024 GiB 400 PiB
1536 GiB 800 PiB
512 GiB 64 KiB 10 PiB 50 PiB
1024 GiB 100 PiB
1536 GiB 200 PiB
IBM Storage FlashSystem 9200 128 GiB 256 KiB 4 PiB 4 PiB
384 GiB 200 PiB
768 GiB 300 PiB
128 GiB 64 KiB 1 PiB 1 PiB
384 GiB 50 PiB
768 GiB 75 PiB
IBM Storage FlashSystem 7300 128 GiB 256 KiB 20 PiB 20 PiB
384 GiB 200 PiB
768 GiB 300 PiB
128 GiB 64 KiB 5 PiB 5 PiB
384 GiB 50 PiB
768 GiB 75 PiB
IBM Storage FlashSystem 7200 128 GiB 256 KiB 4 PiB 4 PiB
384 GiB 200 PiB
768 GiB 300 PiB
128 GiB 64 KiB 1 PiB 1 PiB
384 GiB 50 PiB
768 GiB 75 PiB
IBM Storage FlashSystem 5300 32 GiB 256 KiB 4 PiB 4 PiB
128 GiB 4 PiB
256 GiB 200 PiB
32 GiB 64 KiB 1 PiB 1 PiB
128 GiB 1 PiB
256 GiB 50 PiB
IBM Storage FlashSystem 5200 32 GiB 256 KiB 4 PiB 4 PiB
128 GiB 4 PiB
256 GiB 200 PiB
32 GiB 64 KiB 1 PiB 1 PiB
128 GiB 1 PiB
256 GiB 50 PiB
IBM SAN Volume Controller SV3 512 GiB 256 KiB 40 PiB 200 PiB
1024 GiB 400 PiB
1536 GiB 800 PiB
512 GiB 64 KiB 10 PiB 50 PiB
1024 GiB 100 PiB
1536 GiB 200 PiB

Any volume that has snapshots consume from the PCA an amount equal to the provisioned capacity of the volume. When a new snapshot is added additional PCA is consumed temporarily, once the snapshot has been added successfully this additional PCA is freed.

For example, when the first snapshot is added to a volume group, of 100 TiB, 100 TiB of PCA will be consumed and 100 TiB of SCA will be consumed. When a second snapshot is added another 100 TiB of PCA is used along with 100 TiB of SCA. Once the snapshot has been added successfully the additional PCA will be freed, this means these two snapshots consume 100 TiB of PCA and 200 TiB of SCA. Adding more snapshots increase SCA but the PCA remain at 100 TiB provided there is buffer 100 TiB of PCA free.

Volumes of type clone and thin-clone will also consume PCA equal to the volume's provisioned capacity. For a clone, once the clone has completed its background population process the PCA is freed . Similarly, an in-place restore from a snapshot consumes PCA until the restore process has completed.
Note: PCA and SCA are consumed in chunks:
  • If the grain size is 256 KiB, then the chunks are 8 GiB, so a volume of 21 GiB would consume 24 GiB of PCA and SCA.
  • If the grain size is 64 KiB, then the chunks are 2 GiB, so a volume of 21 GiB would consume 22 GiB of PCA and SCA.

If FlashCopy is used it consumes from the PCA. The consumption of the allowance is based on the provisioned capacity of the FlashCopy mapping's target volume.

The following table shows the upper capacity limits that can be achieved for FlashCopy target volumes.
Table 2. FlashCopy capacity limits
Product Grain size Max Target Capacity Allowance (TCA)
IBM Storage FlashSystem 9500 256 KiB 8 PiB
64 KiB 2 PiB
IBM Storage FlashSystem 9200 256 KiB 4 PiB
64 KiB 1 PiB
IBM Storage FlashSystem 7300 256 KiB 4 PiB
64 KiB 1 PiB
IBM Storage FlashSystem 7200 256 KiB 4 PiB
64 KiB 1 PiB
IBM Storage FlashSystem 5300 256 KiB 4 PiB
64 KiB 1 PiB
IBM Storage FlashSystem 5200 256 KiB 4 PiB
64 KiB 1 PiB
IBM SAN Volume Controller SV3 256 KiB 8 PiB
64 KiB 2 PiB
Note:
  1. The TCA is consumed from the PCA, meaning that using FlashCopy reduces the amount of PCA for volume group snapshot.
  2. An incremental FlashCopy mapping consumes twice the amount of TCA than a regular FlashCopy mapping.
  3. If a volume is highly available or in a volume group with policy-based replication, then it consumes PCA equal to the provisioned capacity of the volume.
  4. On the IBM SAN Volume Controller SV3, the PCA is shared with volume mirroring.

Bitmap Space

The PCA, SCA, and TCA are related to the bitmap space available to volume group snapshot and FlashCopy. While it is better to plan volume group snapshot and FlashCopy capacity in terms of PCA, SCA, and TCA, the limits and consumption values comes from the different bitmap limits. The bitmap space is consumed in chunks of 4 KiB pages and each page corresponds to:
  • 8 GiB of capacity if the grain size is 256 KiB.
  • 2 GiB of capacity if the grain size is 64 KiB.
The values for a particular system can be found using the lsiogrp command view. This view has values:
Table 3. FlashCopy capacity limits
Value Description
flash_copy_total_memory The amount of bitmap space in use for PCA.
flash_copy_free_memory The amount of bitmap space reserved but not in use for PCA.
flash_copy_maximum_memory The maximum amount of bitmap space available for PCA.
flash_copy_max_legacy_memory The maximum amount of bitmap space available for TCA.
flash_copy_used_legacy_memory The amount of bitmap space in use for TCA.
snapshot_volatile_total_memory The amount of bitmap space in use for SCA.
snapshot_volatile_free_memory The amount of bitmap space reserved but not in use for SCA.
snapshot_volatile_max_memory The maximum amount of bitmap space available for SCA.
Note: If snapshot_volatile_max_memory is zero then the SCA comes out of the PCA.

The bitmap space is normally managed automatically and objects are created or deleted. However, once space is reserved it is not automatically freed. If the space needs to be freed then chiogrp -feature flash can be used to adjust the amount of bitmap space reserved for PCA(including TCA) and chiogrp -feature snapshot can be used to adjust the amount of bitmap space reserved for SCA.