Bitmap space configuration for Copy Services, volume mirroring, or RAID

Copy Services features and RAID require that small amounts of volume cache be converted from cache memory into bitmap memory to allow the functions to operate. If you do not have enough bitmap space allocated when you try to use one of the functions, you will not be able to complete the configuration.

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

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.

Bitmap space default configuration describes the configuration of the bitmap space in a system.

Table 1. 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
Remote copy²	0	20 MiB	512 MiB	40 TiB of remote mirroring volume capacity
FlashCopy^®³	0	20 MiB	2 GiB	10 TiB of FlashCopy^® source volume capacity 5 TiB of incremental FlashCopy^® source volume capacity
Volume mirroring	0	20 MiB	512 MiB	40 TiB of mirrored volumes
RAID	0	40 MiB	512 MiB	80 TiB array capacity using RAID 0, 1, or 10 80 TiB array capacity in three-disk RAID 5 array Slightly less than 120 TiB array capacity in five-disk RAID 6 array
The sum of all bitmap memory allocation for all functions except FlashCopy^® must not exceed 552 MiB. ¹ The actual amount of functionality might increase based on settings such as grain size and strip size. RAID is subject to a 15% margin or error. For more details, see Traditional RAID requirements. ² Remote copy includes Metro Mirror, Global Mirror, and active-active relationships. ³FlashCopy^® includes the FlashCopy^® function, Metro Mirror and Global Mirror configured with change volumes, and active-active relationships.

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 2. Examples of memory required
Function	Grain size	1 MiB of memory provides the following volume capacity for the specified I/O group
Remote copy	256 KiB	2 TiB of total Metro Mirror, Global Mirror, or HyperSwap^® volume capacity
FlashCopy^®	256 KiB	2 TiB of total FlashCopy^® source volume capacity
FlashCopy^®	64 KiB	512 GiB of total FlashCopy^® source volume capacity
Incremental FlashCopy^®	256 KiB	1 TiB of total incremental FlashCopy^® source volume capacity
Incremental FlashCopy^®	64 KiB	256 GiB of total incremental FlashCopy^® source volume capacity
Volume mirroring	256 KiB	2 TiB of mirrored volume capacity
Note: 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 full 512 MiB of memory space enables 1 PiB of total volume mirroring capacity. When creating new FlashCopy relationships or mirrored volumes, additional bitmap space is allocated automatically by the system if required.

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.
For Metro Mirror, Global Mirror, and HyperSwap^® active-active relationships, two bitmaps exist. For Metro Mirror or Global Mirror relationships, one is used for the master clustered system and one is used for the auxiliary system because the direction of the relationship can be reversed. For active-active relationships, which are configured automatically when HyperSwap^® volumes are created, one bitmap is used for the volume copy on each site because the direction of these relationships can be reversed.
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.
When you configure change volumes for use with Global Mirror or Metro Mirror, two internal FlashCopy^® mappings are created for each change volume.
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, HyperSwap^® volumes, or formatted, fully allocated volumes, the system attempts to automatically increase the available bitmap space. You do not need to manually increase this space.
Metro Mirror and Global Mirror relationships do not automatically increase the available bitmap space. You might need to use the chiogrp command to manually increase the space in one or both of the master and auxiliary systems.
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 traditional RAID requirements for bitmap memory.

Table 3. Traditional RAID requirements
RAID level	Strip size	Approximate required bitmap memory
RAID 0, 1, or 10	Not applicable	1 MB of bitmap space for every 2 TiB of array capacity.
RAID 5 and RAID 6	128 KiB	1 MiB of bitmap space for every 1 TiB of capacity on the smallest drive in the array.
RAID 5 and RAID 6	256 KiB	1 MiB of bitmap space for every 2 TiB of capacity on the smallest drive in the array.
Note: There is a margin of error on the approximate bitmap memory usage of approximately 15%. For example, the usage for a 256 KiB RAID 5 array is about 1.15 MiB for the first 2 TiB of drive capacity.

The following table shows the distributed RAID requirements for bitmap memory.

Table 4. Distributed RAID requirements
RAID level	Approximate required bitmap memory
RAID 5	Drive capacity * (count + 2 * stripe) / (strip * stripe * 8)
RAID 6	Drive capacity * (count + 3 * stripe) / (strip * stripe * 8)
Note: Drive capacity is in GB, count = drive count, stripe = stripe width, strip = strip length (in KB).

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