Bitmap space configuration
Copy Services functions 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.
Before you begin
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.
This table describes the configuration of the bitmap space in a system.
|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 provisioned capacity|
|FlashCopy®3||0||20 MiB||2 GiB||10 TiB of FlashCopy source
5 TiB of incremental FlashCopy source provisioned capacity
|Volume mirroring||0||20 MiB||512 MiB||40 TiB of mirrored volumes|
|RAID||0||40 MiB||512 MiB||80 TiB
writable capacity using RAID 0,
1, or 10
80 TiB writable capacity in three-disk RAID 5 array
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 552 MiB.
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 Table 3.
2 Remote copy includes Metro Mirror, Global Mirror, and active-active relationships.
3 FlashCopy 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.
|Function||Grain size||1 MiB of memory provides the following provisioned capacity for the specified I/O group|
|Remote copy||256 KiB||2 TiB of total Metro Mirror, Global Mirror, or HyperSwap® provisioned capacity|
|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||256 KiB||2 TiB of mirrored provisioned capacity|
- 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 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.
- When you create FlashCopy mappings and mirrored volumes, HyperSwap 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.
- 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.
|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 writable 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.|
|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.
|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.