Planning for global-active device
You can prepare your storage systems for global-active device by configuring the primary and secondary storage systems, data paths, pair volumes, and quorum disk.
Storage system preparation
Before you can use global-active device on your storage systems, you must ensure that system requirements, configurations, power and physical cabling, memory requirements, cache requirements, host modes, and system operation modes are configured appropriately.
To prepare the storage systems for global-active device operations:
- Make sure that the primary, secondary, and external storage systems meet the global-active device system requirements described in chapter 2.
- Make sure that the primary storage system is configured to report sense information to the host. The secondary storage system should also be attached to a host server to report sense information in the event of a problem with an S-VOL or the secondary storage system.
- If power sequence control cables are used, set the power source selection switch for the cluster to "Local" to prevent the server from cutting the power supply to the primary storage system. In addition, make sure that the secondary storage system is not powered off during GAD operations.
- Establish the physical paths between the primary and secondary storage systems. Switches and channel extenders can be used. For details, see Planning physical paths.
- Review the shared memory requirements for the primary and secondary storage systems in
Requirements and restrictions. Make sure that the cache in both storage systems works normally. Pairs cannot be created if cache requires maintenance.
Configure the cache on the secondary storage system so that it can adequately support the remote copy workload and all local workload activity. When the cache memory and the shared memory in the storage system become redundant, you can remove them. For instructions on adding and removing cache and shared memory, see Adding and removing cache and shared memory.
When determining the amount of cache required for GAD, consider the amount of Cache Residency Manager data (VSP G1x00 and VSP F1500) that will also be stored in cache.
- Make sure that the appropriate host modes and host mode options (HMOs) are set. For details, see the
Provisioning Guide for the storage system.
- HMO 78, the nonpreferred path option, must be configured to specify nonpreferred paths for HDLM operations.
- HMOs 49, 50, and 51 can be used to improve response time of host I/O for distance direct connections (up to 10 km Long Wave).
- Make sure that the appropriate system option modes (SOMs) are set on your storage systems. For details about SOMs that apply to remote copy operations, contact customer support.
Adding and removing cache and shared memory
You can add cache or shared memory in a storage system in which GAD pairs already exist if additional memory is required. Likewise, you can remove cache or shared memory if it becomes redundant.
Configure cache memory so that it can be used for both the primary and secondary storage systems. Otherwise, creation of GAD pairs fails. Prepare cache memory for the secondary system to support both local and remote copy operations.
For VSP G1x00, VSP F1500, additional shared memory is required for both the primary and secondary storage systems.
For VSP G350, VSP G370, VSP G700, VSP G900, VSP F350, VSP F370, VSP F700, VSP F900, you can use GAD only with shared memory in the basic part. Adding shared memory expands the capacity for creating pairs.
Adding and removing cache memory
You can add cache memory if the size of your cache memory does not meet the requirements. You can remove cache memory if the size of your cache memory becomes redundant.
Identify the status of the GAD volumes in the storage system.
If a GAD volume is in the COPY status, wait until the status changes to PAIR, or suspend the GAD pair.
Do not add or remove cache memory when any volumes are in the COPY status.
When the status of all volumes has been confirmed, cache memory can be added to or removed from the storage system by your service representative. Contact customer support for adding or removing cache memory.
After the addition or removal of cache memory is complete, resynchronize the pairs that you suspended in step 2.
Adding shared memory
You can add shared memory if the size of your shared memory does not meet the requirements.
Identify the status of the GAD volumes in the storage system.
If a GAD volume is in the COPY status, wait until the status changes to PAIR, or suspend the GAD pair.
Do not add shared memory when any volumes are in the COPY status.
When the status of all volumes has been confirmed, shared memory can be added to the storage system by your service representative. Contact customer support for adding shared memory.
After the addition of shared memory is complete, resynchronize the pairs that you suspended in step 2.
Removing shared memory used in 64KLDEV Extension (VSP G1x00 and VSP F1500)
You can remove shared memory used in 64KLDEV Extension if it becomes redundant.
Identify the status of all volumes with an LDEV ID of 0x4000 or higher.
If a volume with an LDEV ID of 0x4000 or higher is used by a GAD pair, delete the GAD pair.
Do not remove shared memory used in 64KLDEV Extension when any volume with an LDEV ID of 0x4000 or higher is used by a GAD pair.When the status of all volumes with an LDEV ID of 0x4000 or higher has been confirmed, shared memory can be removed from the storage system by your service representative. Contact customer support for removing shared memory.
Removing shared memory used in TC/UR/GAD (VSP G1x00 and VSP F1500)
You can remove shared memory used by TC/UR/GAD if shared memory is redundant.
Use the following workflow to remove shared memory used in TC/UR/GAD (VSP G1x00 and VSP F1500):
Procedure
Identify the status of all volumes.
If a volume is used by a TC/UR/GAD pair, delete the TC/UR/GAD pair.
Do not remove shared memory used in TC/UR/GAD when any volume is used by a GAD pair.
When the status of all volumes has been confirmed, shared memory can be removed from the storage system by your service representative. Contact customer support for adding or removing cache memory.
Removing shared memory (VSP Gx00 models, VSP Fx00 models)
You can remove shared memory if it is redundant.
Identify the status of all volumes in the storage system.
If a volume is used by a GAD pair, delete the GAD pair.
If you use VSP G350, VSP G370, VSP G700, VSP G900, VSP F350, VSP F370, VSP F700, VSP F900, you can skip this step.
Shared memory can be removed from the storage system by your service representative. Contact customer support for removing shared memory.
Planning system performance
Remote copy operations can affect the I/O performance of host servers and the primary and secondary storage systems. You can minimize the effects of remote copy operations and maximize efficiency and speed by changing your remote connection options and remote replica options.
Your Hitachi Vantara account team can help you analyze your workload and optimize copy operations. Using workload data (MB/s and IOPS), you can determine the appropriate amount of bandwidth, number of physical paths, and number of ports for your global-active device system. When these are properly determined and sized, the data path should operate free of bottlenecks under all workload levels.
Setting preferred and nonpreferred paths
You can improve overall system performance by setting a short-distance straight path as a preferred I/O path if an alternate path that connects a server and a storage system in a GAD configuration contains a short-distance straight path and a long-distance cross path.
By setting the short-distance straight path as a preferred I/O path, you can suppress I/Os to and from the inefficient long-distance cross path. As a result, overall system performance can be improved.

Setting preferred and nonpreferred paths using ALUA
When you perform Asymmetric Logical Unit Access (ALUA) in a cross-path configuration, you can specify the preferred path to use for issuing an I/O request from a server to a storage system.
To specify the preferred path, you must enable the ALUA mode in the storage system, and use the asymmetric access status setting to set the path to use as the preferred path. You might need to restart the server after you make these changes in the storage system for the server to recognize the changes.
Setting preferred and nonpreferred paths using HDLM
You can use Hitachi Dynamic Link Manager (HDLM) to specify alternate paths to be used for normal global-active device operations by using host mode options.
Other paths are used when failures occur in the paths (including alternate paths) that should be used for normal operations. Host mode option (HMO) 78, the nonpreferred path option, must be configured to specify nonpreferred paths, which are used when failures occur.
For example, if servers and storage systems are connected in a cross-path configuration, I/O response is prolonged because the primary-site server is distant from the secondary storage system, and the secondary-site server is distant from the primary storage system. Normally in this case you use paths between the primary server and primary storage system and paths between the secondary server and secondary storage system. If a failure occurs in a path used in normal circumstances, you will use the paths between the primary server and secondary storage system, and paths between the secondary server and primary storage system.

When the settings are applied to HDLM, the attribute of the HDLM path to which HMO 78 was set changes to non-owner path. The attribute of the HDLM path to which HMO 78 was not set changes to owner path. For details, see the documents for HDLM version 8.0.1 or later.
Hitachi Dynamic Link Manager
Hitachi Dynamic Link Manager (HDLM) allows you to specify alternate paths to be used for normal global-active device operations. Other paths are used when failures occur in the paths (including alternate paths) that should be used for normal operations. Host mode option (HMO) 78, the nonpreferred path option, must be configured to specify nonpreferred paths, which are used when failures occur.
For example, if servers and storage systems are connected in a cross-path configuration, I/O response is prolonged because the primary-site server is distant from the secondary storage system, and the secondary-site server is distant from the primary storage system. Normally in this case you use paths between the primary server and primary storage system and paths between the secondary server and secondary storage system. If a failure occurs in a path used in normal circumstances, you will use the paths between the primary server and secondary storage system, and paths between the secondary server and primary storage system.

After you incorporate the storage system settings to HDLM, the attribute of the HDLM path to which HMO 78 was set changes to non-owner path. If HMO 78 is not set to the path, the HDLM path attribute changes to owner path.
Planning physical paths
When configuring physical paths to connect the storage systems at the primary and secondary sites, make sure that the paths can handle all of the data that could be transferred to the primary and secondary volumes under all circumstances.
When you plan physical paths, keep in mind the required bandwidth, Fibre Channel or iSCSI data path requirements, and whether you plan a direct connection, a connection using switches, or a connection using channel extenders.
Determining the required bandwidth
You must have sufficient bandwidth to handle all data transfers at all workload levels. The amount of required bandwidth depends on the amount of server I/O to the primary volumes.
To identify the required bandwidth, you must first collect the write workload data under all workload conditions, including peak write workload, and then measure and analyze the data. You can use performance-monitoring software such as Hitachi Tuning Manager or Hitachi Performance Monitor to collect the workload data.
Fibre Channel connections
You can use Fibre Channel connections for direct connections, switch connections, and extender connections.
Use short-wave (optical multi-mode) or long-wave (optical single-mode) optical fiber cables to connect the storage systems at the primary and secondary sites. The required cables and network relay devices differ depending on the distance between the primary and secondary storage systems, as described in the following table.

Distance between storage systems |
Cable type |
Network relay device |
Up to 1.5 km |
Short wave (optical multi-mode) |
Switches are required if the distance is 0.5 to 1.5 km. |
1.5 to 10 km |
Long wave (optical single-mode)* |
Not required. |
10 to 30 km |
Long wave (optical single-mode)* |
Switches must be used. |
30 km or longer |
Communication line |
An authorized third-party channel extender is required. |
* Long wave cannot be used for FCoE (VSP G1x00 and VSP F1500). |
No special settings are required for the storage system if switches are used in a Fibre Channel environment.
Long-wave (optical single-mode) cables can be used for direct connection at a maximum distance of 10 km. The maximum distance that might result in the best performance differs depending on the link speed, as shown in the following table. For details about the availability of serial-channel GAD connections, contact customer support.
Link speed |
Maximum distance for best performance |
1 Gbps (VSP G1x00 and VSP F1500) |
10 km |
2 Gbps (VSP G1x00 and VSP F1500) |
6 km |
4 Gbps |
3 km |
8 Gbps |
2 km |
16 Gbps |
1 km |
32 Gbps (VSP Gx00 models) |
0.6 km |
iSCSI data path requirements
You can use iSCSI connections for direct connections, switch connections, and extender connections.
The following table lists the requirements and cautions for systems using iSCSI data paths. For details about the iSCSI interface, see the Provisioning Guide.
Item |
Requirement |
Remote paths |
Add only remote paths of the same protocol to a single path group. Make sure that Fibre Channel and iSCSI remote paths are not mixed in a path group. |
Physical paths |
|
Ports |
|
Network setting |
|
Connection types
Three types of connections are supported for GAD physical paths: direct, switch, and channel extenders.
You can use Hitachi Command Suite or CCI to configure ports and topologies.
Establish bidirectional physical path connections from the primary to the secondary storage system and from the secondary to the primary storage system.
Direct connection
You can connect two storage systems directly to each other.

You can use the following host mode options (HMOs) to improve response time of host I/O by improving response time between the storage systems for distance direct connections (up to 10 km Long Wave) when the open package is used.
- HMO 49 (BB Credit Set Up Option1) (VSP G1x00 and VSP F1500)
- HMO 50 (BB Credit Set Up Option2) (VSP G1x00 and VSP F1500)
- HMO 51 (Round Trip Set Up Option)
For more information about HMOs, see the Provisioning Guide for your storage system.
The fabric and topology settings depend on the settings of packages, the protocol used for the connection between the storage systems, and the setting of HMO 51. The link speed that can be specified differs for each condition.
Package name |
Protocol |
HMO 51 setting |
Fabric setting |
Topology: remote replication ports |
Link speed that can be specified |
16FC8 (VSP G1x00 and VSP F1500) |
8 Gbps FC |
OFF |
OFF |
FC-AL |
|
8 Gbps FC |
ON |
OFF |
Point-to-Point |
| |
8 Gbps FC |
OFF |
OFF |
Point-to-Point |
| |
16FC16 (VSP G1x00 and VSP F1500) |
16 Gbps FC |
OFF |
OFF |
FC-AL |
|
16 Gbps FC |
ON |
OFF |
Point-to-Point |
| |
OFF |
OFF |
Point-to-Point |
| ||
8FC16 (VSP G1x00 and VSP F1500) |
16 Gbps FC |
OFF |
OFF |
FC-AL |
|
16 Gbps FC |
ON |
OFF |
Point-to-Point |
| |
OFF |
OFF |
Point-to-Point |
| ||
8IS10 (VSP G1x00 and VSP F1500) |
10 Gbps iSCSI |
N/A |
N/A |
N/A |
10 Gbps |
CHB(FC32G) (VSP G350, VSP G370, VSP G700, VSP G900, VSP F350, VSP F370, VSP F700, VSP F900) |
32GbpsFC |
OFF |
OFF |
FC-AL |
|
ON |
OFF |
FC-AL |
| ||
OFF |
OFF |
Point-to-Point |
| ||
OFF |
ON |
Point-to-Point |
|
Connection using switches
You can use host mode options to improve response times when switches are used for distance connections.

Switches from some vendors (for example, McData ED5000) require F_port.
You can use the following host mode options (HMOs) to improve response time of host I/O by improving response time between the storage systems when switches are used for distance connections (up to approximately 500 km with a round-trip response of 20 ms or less) and the open package is used.
- HMO 49 (BB Credit Set Up Option1) (VSP G1x00 and VSP F1500)
- HMO 50 (BB Credit Set Up Option2) (VSP G1x00 and VSP F1500)
- HMO 51 (Round Trip Set Up Option)
For details about HMOs, see the Provisioning Guide for the storage system.
The fabric and topology settings depend on the settings of packages, and protocol used for the connection between storage systems, and the HMO 51 setting. The link speed that can be specified differs on each condition.
Package name |
Protocol |
HMO 51 setting |
Fabric setting |
Topology: Initiator and RCU Target |
Link speed that can be specified |
16FC8 (VSP G1x00 and VSP F1500) |
8 Gbps FC |
OFF |
ON |
Point-to-Point |
|
8 Gbps FC |
ON |
ON |
Point-to-Point |
| |
8 Gbps FC |
OFF |
OFF |
Point-to-Point |
| |
8FC16 (VSP G1x00 and VSP F1500) |
16 Gbps FC |
OFF |
ON |
Point-to-Point |
|
16 Gbps FC |
ON |
ON |
Point-to-Point |
| |
16 Gbps FC |
OFF |
OFF |
Point-to-Point |
| |
16FC16 (VSP G1x00 and VSP F1500) |
16 Gbps FC |
OFF |
ON |
Point-to-Point |
|
16 Gbps FC |
ON |
ON |
Point-to-Point |
| |
16 Gbps FC |
OFF |
OFF |
Point-to-Point |
| |
16FE10 (VSP G1x00 and VSP F1500) |
10 Gbps FCoE |
OFF |
ON |
Point-to-Point |
10 Gbps |
10 Gbps FCoE |
ON |
ON |
Point-to-Point |
10 Gbps | |
8IS10 (VSP G1x00 and VSP F1500) |
10 Gbps iSCSI |
N/A |
N/A |
N/A |
10 Gbps |
CHB(FC32G) VSP G350, VSP G370, VSP G700, VSP G900, VSP F350, VSP F370, VSP F700, VSP F900 |
32GbpsFC |
OFF |
ON |
Point-to-Point |
|
ON |
ON |
Point-to-Point |
| ||
* 4HF32R (4 ports, FC 32 Gbps Ready Package) supports multiple transfer speed protocol. Depending on the mounted SFP parts, you can use either 16 Gbps or 32 Gbps protocol. |
Connection using channel extenders
You should use channel extenders and switches for long-distance connections (up to 500 km and the round trip time is 20 ms or less).
Set Fabric to ON and topology to Point-to-Point for the remote replication ports (Initiator and RCU Target).

- When the primary and secondary storage systems are connected using switches with a channel extender, and multiple data paths are configured, the capacity of data to be transmitted might concentrate on particular switches, depending on the configuration and the settings of switch routing. Contact customer support for more information.
- Make sure that your channel extenders can support remote I/O. For details, contact customer support.
- Create at least two independent physical paths (one per cluster) between the primary and secondary storage systems for hardware redundancy for this critical element.
- If you plan to use more than 4,000 pairs, when creating pairs you should restrict the number of pairs to 4,000 or less per physical path to distribute the load across multiple physical paths.
Planning ports (VSP G1x00 and VSP F1500)
Data is transferred from Initiator ports in one storage system to RCU Target ports in the other system. After identifying the peak write workload, which is the amount of data transferred during peak periods, you can determine the amount of bandwidth and the number of Initiator and RCU Target ports required.
The following describes the port attributes that you must set on the VSP G1x00 and VSP F1500.
- Initiator ports: Send remote copy commands and data to the RCU Target ports on a connected storage system. One Initiator port can be connected to a maximum of 64 RCU Target ports.
CautionDo not add or delete a remote connection or add a remote path at the same time that the SCSI path definition function is in use.
- RCU Target ports: Receive remote copy commands and data from the Initiator ports on a connected storage system. One RCU Target port can be connected to a maximum of 16 Initiator ports.
The number of remote paths that can be specified does not depend on the number of ports. The number of remote paths can be specified for each remote connection.
- Target ports: Connect the storage system to the host servers. When a server issues a write request, the request is sent from a Target port on the storage system to a VSP G1x00 and VSP F1500 volume.
- External ports: Connect the storage system to external storage systems or iSCSI-attached servers configured using Universal Volume Manager. The external storage system or iSCSI-attached server for the GAD quorum disk is connected to an external port on the primary and secondary storage systems.
Fibre Channel used as remote paths
Before configuring a system using Fibre Channel, there are restrictions that you need to consider.
For details about Fibre Channel, see the Provisioning Guide for your system.
- When you use Fibre Channel as a remote path, if you specify Auto for Port Speed, specify 10 seconds or more for Blocked Path Monitoring. If you want to specify 9 seconds or less, do not set Auto for Port Speed.
- If the time specified for Blocked Path Monitoring is not long enough, the network speed might be slowed down or the period for speed negotiation might be exceeded. As a result, paths might be blocked.
Planning the quorum disk
If you use an external storage system, it must be prepared for the GAD quorum disk. If you use a disk in a server as the quorum disk, you do not need to prepare the external storage system for the quorum disk.
Installation of the external storage system
Where you install the external storage system depends on the number of sites in your configuration.
In a three-site configuration, you install the external storage system in a third site away from the primary and secondary sites. I/O from servers continues if any failure occurs at the primary site, the secondary site, or the site where the external storage system is installed.
In a two-site configuration, you install the external storage system at the primary site. If failure occurs at the secondary site, I/O from servers will continue. However, if a failure occurs at the primary site, I/O from servers will stop.
At the secondary site, you cannot install any external storage system for quorum disks.
Relationship between the quorum disk and number of remote connections
When you use multiple remote connections, you should prepare as many quorum disks as remote connections to avoid the possibility of a single remote connection failure causing the suspension of the GAD pairs that are using the other normal remote connections.
Simultaneously, you must make a combination of one quorum disk, one remote connection from the primary storage system to the secondary storage system, and one remote connection from the secondary storage system to the primary storage system.

When all paths used in the remote connection are blocked, the GAD pairs will be suspended in units of quorum disks. In the configuration shown below, the GAD pairs that are using remote connection 1 will be suspended even if the failure occurred at remote connection 2. Also, when a failure occurs at the path from the volume at the primary site or the secondary site to the quorum disk, the GAD pairs that are using the same quorum disk will be suspended.

Suspended pairs depending on failure location (quorum disk not shared)
When the same number of quorum disks as the remote connections are used, only GAD pair that uses the failed remote connection, a quorum disk or a path to the quorum disk, is suspended.
The GAD pair that uses the normal remote connection, quorum disk and path to the quorum disk, can keep the status being mirrored. The following figure shows the relationship between the failure locations and the GAD pair suspended by the failure.

# |
Failure locations |
GAD pair 1 |
GAD pair 2 |
1 |
Remote connection 1 from the primary site to the secondary site |
Suspended |
Not suspended |
2 |
Remote connection 1 from the secondary site to the primary site |
Suspended |
Not suspended |
3 |
Remote connection 2 from the primary site to the secondary site |
Not suspended |
Suspended |
4 |
Remote connection 2 from the secondary site to the primary site |
Not suspended |
Suspended |
5 |
Path to the quorum disk 1 |
Not suspended* |
Not suspended |
6 |
Quorum disk 1 |
Not suspended* |
Not suspended |
7 |
Path to the quorum disk 2 |
Not suspended |
Not suspended* |
8 |
Quorum disk 2 |
Not suspended |
Not suspended* |
* The GAD pair is not suspended, but I/O mode of the S-VOL changes to Block for pairs created, resynchronized, or swap resynchronized on 80-04-2x or earlier for VSP G1x00 and VSP F1500 and 83-03-3x or earlier for VSP Gx00 models). |
Suspended pairs depending on failure location (quorum disk shared)
When a quorum disk is shared by more than one connections, all GAD pairs which share a quorum disk are suspended, regardless of the failure locations, as shown below.

# |
Failure locations |
GAD pair 1 |
GAD pair 2 |
1 |
Remote connection 1 from the primary site to the secondary site |
Suspended |
Suspended |
2 |
Remote connection 1 from the secondary site to the primary site |
Suspended |
Suspended |
3 |
Remote connection 2 from the primary site to the secondary site |
Suspended |
Suspended |
4 |
Remote connection 2 from the secondary site to the primary site |
Suspended |
Suspended |
5 |
Path to the quorum disk 1 |
Not suspended* |
Not suspended* |
6 |
Quorum disk 1 |
Not suspended* |
Not suspended* |
* The GAD pair is not suspended, but I/O mode of the S-VOL changes to Block for pairs created, resynchronized, or swap resynchronized on 80-04-2x or earlier for VSP G1x00 and VSP F1500 and 83-03-3x or earlier for VSP Gx00 models). |
Relationship between quorum disks and consistency groups
A single quorum disk can be shared by multiple consistency groups.
When creating GAD pairs to be registered to different consistency groups, you can specify the same quorum disk ID.

Pairs registered to the same consistency group must use the same quorum disk. When creating pairs in a single consistency group, you cannot specify multiple quorum disk IDs.

Response time from the external storage system
You should monitor the response time of the quorum disks regularly using Performance Monitor on the primary or secondary storage system to detect possible issues.
If the response time from the external storage system for quorum disks is delayed for more than one second, GAD pairs might be suspended by some failures. Specify
on the monitoring objects. If the response time exceeds 100 ms, review the configuration and consider the following actions:- Lower the I/O load, if the I/O load of volumes other than the quorum disk is high in the external storage system.
- Remove the causes of the high cache load, if the cache load is high in the external storage system.
- Lower the I/O load of the entire external storage system, when you perform maintenance of the external storage system. Alternatively, perform maintenance on the external storage system with settings that will minimize the impact to the I/O, referring to the documentation for the external storage system.
Cache pending rate of the CLPR to which the quorum disk is assigned
If the write-pending rate of the CLPR to which the quorum disk (external volume) on the primary or secondary storage systems is assigned is high, the I/O performance of the GAD pair volumes might decrease or the GAD pairs might be suspended by some failure.
To address this situation:
- Use Performance Monitor on the primary or secondary storage system to perform regular monitoring of the write-pending rate of the CLPR to which the quorum disks are assigned (specify on the monitoring objects). For details, see the Performance Guide for the storage system.
- If the write-pending rate exceeds 70%, review your configuration and consider the following actions:
- Lower the I/O load in the storage system.
- If the cache load is high:
- Lower the I/O load.
- Migrate the quorum disk to a CLPR for which the cache load is low.
- Add cache memory to increase the cache capacity of the storage system.
- The cache pending rate might exceed 70% temporarily due to failures on the primary and secondary storage systems. To prevent the I/O performance of the GAD pair volumes from decreasing or the GAD pairs from being suspended by failures related to this situation, the write-pending rate should be below 35% under normal conditions.
Planning GAD pairs and pair volumes
This section describes planning for differential data management, calculating the maximum number of GAD pairs, and the requirements for primary and secondary volumes related to the GAD configuration.
Differential data
Differential data is managed by the bitmap in units of tracks. A track that receives a write command while the pair is split is managed as differential data in the bitmap. When the pair is resynchronized, the differential data is copied to the S-VOL in units of tracks.
When a GAD pair contains a DP-VOL that is larger than 4,194,304 MB (8,589,934,592 blocks), the differential data is managed by the pool to which the GAD pair volume is related.
In this case, additional pool capacity (up to 4 pages, depending on the software configuration) is required for each increase of user data size by 4,123,168,604,160 bytes (~4 TB). For a GAD pair with a DP-VOL that is larger than 4,194,304 MB (8,589,934,592 blocks), data management might fail due to insufficient pool capacity. If this occurs, all of the P-VOL data (all tracks) is copied to the S-VOL when the pair is resynchronized.
For instructions on releasing the differential data (pages) managed in a pool, see Releasing the differential data managed in a pool.
Maximum number of GAD pairs
The maximum number of GAD pairs per storage system is specified in Requirements and restrictions. The maximum number of pairs per storage system is subject to restrictions, such as the number of cylinders used in volumes or the number of bitmap areas used in volumes.
When you create all pairs with DP-VOLs or external volumes, the maximum number of pairs is calculated by subtracting the number of quorum disks (at least one) from the maximum number of virtual volumes that can be defined in a storage system (total number of DP-VOLs plus external volumes: 63,231 for VSP G1x00 and VSP F1500).
In the calculation formulas below, "ceiling" is the function that rounds up the value inside the parentheses to the next integer. "Floor" is the function that rounds down the value inside the parentheses to the next integer.
Calculating the number of cylinders
To calculate the number of cylinders, start by calculating the number of logical blocks, which indicates volume capacity measured in blocks.
number-of-logical-blocks =
volume-capacity-in-bytes / 512
Then use the following formula to calculate the number of cylinders:
number-of-cylinders = ceiling(ceiling(number-of-logical-blocks / 512) / 15)
Calculating the number of bitmap areas
Calculate the number of bitmap areas using the number of cylinders.
number-of-bitmap-areas = ceiling((number-of-cylinders × 15) / 122,752)
122,752 is the differential quantity per bitmap area. The unit is bits.
The following are examples of correct and incorrect calculations, assuming that one volume has 10,017 cylinders and another volume has 32,760 cylinders.
- Correct:
ceiling((10,017 × 15) / 122,752) = 2
ceiling((32,760 × 15) / 122,752) = 5
The calculation result is seven bitmap areas in total.
- Incorrect:
10,017 + 32,760 = 42,777 cylinders
ceiling((42,777 × 15) / 122,752) = 6
The calculation result is six bitmap areas in total.
Calculating the number of available bitmap areas
The total number of bitmap areas available in the storage system is:
- VSP G200, VSP G350, VSP F350: 36,000
- VSP G400, VSP G600, VSP G800, VSP F400, F600, F800, VSP G370, VSP G700, VSP G900, VSP F370, VSP F700, VSP F900, VSP G1000, VSP G1500, VSP F1500: 65,536
The number of bitmap areas is shared by TrueCopy, TrueCopy for Mainframe, Universal Replicator, Universal Replicator for Mainframe, and GAD. If you use these software products, subtract the number of bitmap areas required for these products from the total number of bitmap areas in the storage system, and then use the formula in the next section to calculate the maximum number of GAD pairs. For details about calculating the number of bitmap areas required for the other software products, see the appropriate user guide.
Calculating the maximum number of pairs
Use the following values to calculate the maximum number of pairs:
- The number of bitmap areas required for pair creation.
- The total number of available bitmap areas in the storage system, or the number of available bitmap areas calculated in
Calculating the number of available bitmap areas.
Calculate the maximum number of pairs using the following formula with the total number of bitmap areas in the storage system (or the number of available bitmap areas) and the number of required bitmap areas, as follows:
maximum-number-of-pairs-that-can-be-created = floor(total-number-of-bitmap-areas-in-storage-system / number-of-required-bitmap-areas)
Calculate the maximum number of pairs using the already calculated necessary number of bitmap areas, and the number of bitmap areas in storage systems listed in the following table. The number of bitmap areas in a storage system is determined by the availability of shared memory extended for GAD and the storage system model.
Extension status of shared memory for GAD |
Number of bitmap areas in storage systems |
Base (no extension) |
Varies depending on the model:
|
With extension |
Varies depending on the model:
|
S-VOL resource group and storage system: same serial number and model
You can create GAD pairs specifying a volume in a resource group that has the same serial number and model as the storage system for the S-VOL.
In this case, you must specify a volume in the resource group (virtual storage machine) whose serial number and model are same as the secondary storage system for the P-VOL.
When you create GAD pairs, the virtual LDEV ID of the P-VOL is copied to the virtual LDEV ID of the S-VOL. In the following figure, the copied virtual LDEV ID of the P-VOL is equal to the original virtual LDEV ID of the S-VOL. The volume in a resource group that has the same serial number and the same model as the storage system and whose original LDEV ID is equal to the virtual LDEV ID will be treated as a normal volume, not as a virtualized volume by the global storage virtualization function.

When virtual information is copied from the P-VOL to the S-VOL and a normal volume requirement is not met, you cannot create GAD pairs. For example, when the copied virtual emulation type of the P-VOL is not the same as the original emulation type of the S-VOL. The virtual emulation type includes the virtual CVS attribute (-CVS). The storage system does not support LUSE, so LUSE configuration (*n) volumes are not supported as P-VOLs.