Bitmap space configuration

As part of Copy Services, the system supports policy-based replication. If you are currently using relationships to replicate data between systems, you may need to reallocate bitmap space to use policy-based replication.

Before you begin

The total I/O group memory that can be dedicated to these functions is not defined by the physical memory in the system. The I/O group memory is constrained by the software functions that use the memory.

To use policy-based replication on the system requires a set amount of allocated I/O group memory.

If you plan to use policy-based replication with other functions, use the following table to determine the required allocated memory for other features:
Table 1. Product comparison of required I/O group memory for policy-based replication
Product Total I/O group memory (excluding FlashCopy functions) Required I/O group memory for policy-based replication Available memory for other features (RAID and volume mirroring)
IBM® Storage FlashSystem 9500 5920 MiB 4609 MiB 1311 MiB
IBM Storage FlashSystem 9200 3628 MiB 2561 MiB 1067 MiB
IBM Storage FlashSystem 7300 3628 MiB 2561 MiB 1067 MiB
IBM Storage FlashSystem 7200 3628 MiB 2561 MiB 1067 MiB
IBM SAN Volume Controller 3628 MiB 2561 MiB 1067 MiB
IBM Storage FlashSystem 5045 (requires a minimum of 32 GiB memory in each node canister) 896 MiB 713 MiB 183 MiB
IBM Storage FlashSystem 5200 2088 MiB 1537 MiB 551 MiB
IBM Storage Virtualize for Public Cloud 2088 MiB 1537 MiB 551 MiB
For example, an IBM Storage FlashSystem 9500 system has a maximum of 5920 MiB memory that is used by replication, volume mirroring, RAID, and policy-based replication. If policy-based replication is enabled, it uses 4609 MiB, the remaining 1311 MiB can be allocated between replication, volume mirroring, and RAID with maximum limits as 1024, 512 and 800 MiB each in order. If policy-based replication is disabled, a maximum of 1824 MiB can be allocated between replication, volume mirroring, and RAID with maximum limits as 1024, 512 and 800 MiB each in order.

In planning the installation for a system, consider the future requirements for the advanced functions. Review the following tables to calculate the memory requirements and confirm that your system is able to accommodate the total installation size.

This table describes the configuration of the bitmap space in a system.

Table 2. Bitmap space default configuration
Copy Service Minimum allocated bitmap space Default allocated bitmap space Maximum allocated bitmap space Minimum1 functionality when using the default values
FlashCopy®3 0 20 MiB 20 GiB 40 TiB of FlashCopy target provisioned capacity

5 TiB of incremental FlashCopy target provisioned capacity
Volume mirroring 0 20 MiB 512 MiB 40 TiB of mirrored volumes, or 40 TiB of non-mirrored formatting standard-provisioned volumes, or 20 TiB of mirrored formatting standard-provisioned volumes
RAID 0 40 MiB 800 MiB 80 TiB writable capacity using RAID 0, 1, or 10. Slightly less than 120 TiB writable capacity in five-disk RAID 6 array.
1 The sum of all bitmap memory allocation for all functions except FlashCopy must not exceed 1824 MiB. The actual amount of functions might increase based on settings such as grain size and strip size. RAID is subject to a 15% margin or error.

3 FlashCopy includes the FlashCopy function configured with change volumes, active-active relationships.

Volume Group Snapshots can use up to 20GiB of the FlashCopy bitmap memory, all other features using FlashCopy can use up to 4GiB of the 20GiB.
Note: For IBM Storage FlashSystem 9500, snapshots are supported by the total of 20 GiB bitmap space. FlashCopy functions other than snapshots are limited to a maximum of 4 GiB of bitmap space.

The following tables describe the amount of bitmap space necessary to configure the various Copy Services functions and RAID:

This table provides an example of the amount of memory that is required for remote mirroring functions, FlashCopy functions, and volume mirroring.

Table 3. Examples of memory required
Function Grain size 1 MiB of memory provides the following provisioned capacity for the specified I/O group
FlashCopy 256 KiB 2 TiB of total FlashCopy source provisioned capacity
FlashCopy 64 KiB 512 GiB of total FlashCopy source provisioned capacity
Incremental FlashCopy 256 KiB 1 TiB of total incremental FlashCopy source provisioned capacity
Incremental FlashCopy 64 KiB 256 GiB of total incremental FlashCopy source provisioned capacity
Volume mirroring or format 256 KiB 2 TiB of mirrored provisioned capacity
Notes:
  • For multiple FlashCopy targets, you must consider the number of mappings. For example, for a mapping with a grain size of 256 KiB, 8 KiB of memory allows one mapping between a 16 GiB source volume and a 16 GiB target volume. Alternatively, for a mapping with a 256 KiB grain size, 8 KiB of memory allows two mappings between one 8 GiB source volume and two 8 GiB target volumes.
  • When creating a FlashCopy mapping, if you specify an I/O group other than the I/O group of the source volume, the memory accounting goes toward the specified I/O group, not toward the I/O group of the source volume.
  • For volume mirroring, the 512 MiB of memory space enables 1 PiB of total provisioned capacity.
  • When creating new FlashCopy relationships or mirrored volumes, additional bitmap space is allocated automatically by the system if required.
  • Provisioned capacity and formatting of standard-provisioned volumes both use the mirroring bitmap space.
Before you specify the configuration changes, consider the following factors:
  • For FlashCopy mappings, only one I/O group consumes bitmap space. By default, the I/O group of the source volume is used.
  • When you create a reverse mapping; for example, to run a restore operation from a snapshot to its source volume; a bitmap is also created for this reverse mapping.
  • The smallest possible bitmap is 4 KiB; therefore, a 512 byte volume requires 4 KiB of bitmap space.
On existing systems, also consider these factors:
  • When you create FlashCopy mappings and mirrored volumes, or formatted, standard-provisioned volumes, the system attempts to automatically increase the available bitmap space. You do not need to manually increase this space.
  • If you create and then delete many of these objects, consider using the chiogrp command to reduce the memory that is reserved for these functions and release that memory for another use.
This table shows the distributed RAID requirements for bitmap memory.
Table 4. Distributed RAID requirements
RAID level Approximate required bitmap memory (MiB)
RAID 1 Drive capacity * (count + 2 * stripe) / (stripe * 256 * 8)
RAID 6 Drive capacity * (count + 3 * stripe) / (stripe * 256 * 8)
Note: Drive capacity is in GiB, count = drive count, Stripe = stripe width, strip = strip length (in KiB).
  • For FlashCore Modules, the effective capacity should be used and not the physical capacity

You can also use the lsiogrp and chiogrp command-line interface (CLI) commands to modify the settings.