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Configurations with TrueCopy

You can combine Universal Replicator and TrueCopy operations to a third location. For details about TrueCopy, see the Hitachi TrueCopy® User Guide.

Overview of UR/TC configurations

Universal Replicator (UR) and TrueCopy (TC) can share the same data volumes. By combining UR and TC you can extend disaster recovery options to a third data center.

The terms "P-VOL" (primary volume) and "S-VOL" (secondary volume) are used for TC pair volumes as well as UR pair volumes.

Sharing volumes with TrueCopy

Creating a UR backup and a TC backup ensures that a copy in a third location is available in the event that both the primary and secondary sites fail.

Like UR, the TC remote replication function provides copies of production volumes in a second location. However, unlike UR, the TC S-VOLs are synchronous copies of the P-VOLs, and the secondary storage system is located in the same general region as the primary storage system.

You can combine UR and TC operations in a variety of ways. The following configurations are described in this appendix:

  • 3DC cascade In a 3-data-center cascade configuration (3DC cascade), three data centers are used: the TC production site, the TrueCopy/Universal Replicator intermediate site, and the distant UR secondary site. The figure in the 3DC cascade configuration provides an illustration. With this configuration, you can locate the TC synchronous site near the primary site to lower I/O response time, while the UR backup provides a disaster recovery solution at a secondary site.

    For details, see 3DC cascade configuration.

  • 3DC multi-target In a 3-data-center multi-target configuration (3DC multi-target), the production volume is shared by the TC and UR. This volume is the primary volume (P-VOL) for both at the primary site. The targets of each system are located at different secondary sites. They provide the TC solution and the UR asynchronous disaster recovery solution. The TC secondary site is at an intermediate location, the UR secondary site is at a distant location.

    For details, see 3DC multi-target configuration.

  • Delta resync Another application of the 3DC multi-target configuration is the UR delta resync configuration, which provides a quick way to regain a long-distance copy after disaster in the event that the TrueCopy/Universal Replicator P-VOL becomes unusable. In this case, the host continues I/O operations at the intermediate TC S-VOL site. With a second UR pair previously configured between the TC S-VOL and the UR S-VOL, only differential data needs to be copied to the UR S-VOL.

    For details, see Delta resync configuration .

NoteYou can combine UR and TC. If the primary storage system is a VSP 5100, VSP 5500, you can use the following storage system models:
  • VSP 5100, VSP 5500
  • VSP G1x00, VSP F1500
  • VSP G/F900
  • VSP
NoteVSP 5200 and VSP 5600 cannot be used for a 3DC configuration.

3DC cascade configuration

In a multi-target configuration, data is copied from the primary site to two secondary sites. As shown in the following illustration, the TC P-VOL is the primary production volume in a 3DC cascade configuration. The TC secondary site is located at a short distance from the primary site.

The host issues an update to the TC primary volume (P-VOL), which is copied synchronously to the S-VOL. The UR system copies the synchronous S-VOL data to the UR secondary site.

Data in the UR S-VOL is an asynchronous copy of the TC P-VOL. Depending on RPO and bandwidth, UR S-VOL data can be very close to P-VOL data. As always with UR, data consistency is ensured.

GUID-A95F3F98-3C5D-4113-ADE0-A74C7C26166B-low.png

Failure recovery occurs as follows:

  • If a failure occurs at the TC P-VOL, business continues because data in the UR S-VOL is restored to the primary storage system using the intermediate volume, if needed. Update sequence integrity is ensured.
  • If a failure occurs at both TC P-VOL and S-VOL, an implemented disaster recovery plan would re-direct host activities to the UR S-VOL.

For recovery information and procedures, see Recovery for 3DC UR/TC cascade configuration .

Prerequisites for 3DC cascade

  • When you set up 3DC cascade using VSP 5100, VSP 5500, one VSP 5100, VSP 5500 model, VSP E series, VSP G1x00, VSP F1500, VSP, or VSP G/F900 storage system is required for each of the primary, intermediate, and secondary sites. When you set up 3DC cascade using VSP 5200, VSP 5600, one VSP 5000 series model or VSP E series storage system is required for each of the primary, intermediate, and secondary sites. Any combination of the storage system models can be used.
  • If a site has multiple storage system, 3DC cascade cannot be configured.
  • When managing the storage system with CCI only, CCI is recommended at all the sites.
  • When managing the storage system using HDvM - SN, Device Manager - Storage Navigator is required at all sites.
  • The primary site requires the following:
    • Host application
    • TC
    • TC primary volume (P-VOL), the primary production volume
    • The TC P-VOL fence level (Primary Volume Fence Level) must be Data or Secondary Volume Data.
  • The intermediate site requires the following:
    • TC
    • UR
    • TC S-VOL = UR P-VOL
    • Remote Replication Extended
    • UR master journal volume
  • The secondary site requires the following:
    • UR
    • UR S-VOL
    • UR restore journal volume
  • Differential data is used to resynchronize a suspended TC or UR pair.
  • When the TC pair is resynchronized, the UR pair in PAIR or COPY status is automatically split by the system.
  • 3DC cascade is not supported for multiple primary and secondary storage systems. The UR or TC pair in this configuration would be suspended when the status became PAIR or COPY.
  • The response time for host I/Os will be the response time for TC operation plus the creation time of journal data in the intermediate site.
  • The utilization rate on the primary site with a 3DC cascade configuration is higher than if UR and a TC systems are used independently.
  • If the TC pair is suspended before the TC pair status changes to PAIR, and then if you create the pair, the UR pair might be suspended due to an error.

    If the TC pair is suspended before the TC pair status changes to PAIR, resynchronize the TC pair so that the pair status changes to PAIR, and then create the UR pair.

Setting up the 3DC cascade configuration

Use the following procedure to set up a 3DC cascade configuration.

Procedure

  1. Install and set up TC and UR on the required systems.

  2. Create the TC pair on the primary storage system. See Hitachi TrueCopy® User Guide for instructions.

  3. When TC pair status is PAIR, create the UR pair on the secondary storage system using the TC S-VOL as the UR P-VOL.

    Mirror ID must be set between 1 and 3.
    Note The UR operation is rejected by the system if TC pair status is not already PAIR.

3DC multi-target configuration

In a multi-target configuration, data is copied from the primary site to two secondary sites.

As shown in the following illustration, the P-VOL is paired with the TC secondary volume and the UR S-VOL at separate secondary sites in a 3DC multi-target configuration. The TC secondary site is at a short distance; the UR site is located at a greater distance.

GUID-E9EAF737-5532-40F3-A0BE-177483228222-low.gif

The host issues an update to the production volume and, synchronously, to the TC S-VOL. As with a standalone UR system, the update data is asynchronously written from the primary volume (P-VOL) to the UR S-VOL.

The benefit of this configuration is that it provides a third copy of the data, helping to ensure that business can continue in the event of a failure at the other two sites.

Failure recovery occurs as follows:

  • If a failure occurs in the P-VOL, business is resumed using the TC S-VOL. When the failure is corrected at the primary site, business tasks are transferred back to the primary site.

    In addition, a quick disaster recovery solution can be put in place while the primary site is being restored, using a UR delta resync pair. In this configuration, the TC S-VOL is paired with the UR S-VOL. See Delta resync configuration for more information.

  • If a failure occurs in the TC system (P-VOL and S-VOL), business is resumed using the UR S-VOL. When the failure is corrected, business tasks are transferred back to the primary site.
  • If a failure occurs in the UR system (P-VOL and S-VOL), business is resumed using the TC S-VOL.

For recovery information and procedures, see Recovery for 3DC UR/TC multi-target configuration.

Prerequisites for 3DC multi-target

  • (VSP 5100, VSP 5500) At least one storage system (VSP 5000 series, VSP G1x00, VSP F1500, VSP E series, VSP, or VSP G/F900) is required at the primary site, TC secondary site, and UR secondary site.
  • If you set up the 3DC multi-target configuration by using VSP 5100, VSP 5500, any combination of VSP 5100, VSP 5500, VSP G1x00 or VSP F1500, VSP , VSP E series, VSP G/F900 can be used at the secondary sites.
  • (VSP 5200, VSP 5600) At least one storage system (VSP 5000 series or VSP E990) is required at the primary site, TC secondary site, and UR secondary site.
  • If you set up the 3DC multi-target configuration by using VSP 5200 or VSP 5600, any combination of VSP 5000 series or VSP E990 can be used at the secondary sites.
  • When managing the storage system with CCI only, CCI is recommended at all sites.
  • When managing the storage system using HDvM - SN, Device Manager - Storage Navigator is required at all sites.
  • The primary site requires the following:
    • TC
    • UR
    • Remote Replication Extended
    • TC P-VOL = UR P-VOL. This is the primary production volume.
    • UR master journal volume
  • (VSP 5000 series) The TC secondary site requires the following:
    • TC
    • UR
    • Remote Replication Extended
    • TC S-VOL
    • UR master journal volume
  • ( VSP Gx00 models and VSP Fx00 models, VSP E series) The TC secondary site requires the following:
    • TrueCopy
    • TC S-VOL
  • The UR secondary site requires the following:
    • UR
    • UR S-VOL
    • UR restore journal volume
  • 3DC multi-target can be used in configurations with multiple primary and secondary storage systems. However, delta resync configurations are not supported in configurations with multiple primary and secondary storage systems.

Setting up 3DC multi-target configuration

Use the following procedure to set up a 3DC multi-target configuration.

Procedure

  1. Install and set up TC and UR on the required systems.

  2. Create a TC pair on the primary storage system. For instructions, see the Hitachi TrueCopy® User Guide .

  3. When TC pair status is PAIR, create the UR pair on the primary storage system. The mirror ID must be set between 1 and 3.

Delta resync configuration

With the delta resync configuration, you bring the S-VOL to a consistent state more quickly after failure because only missing differential data must be copied.

You can set up two secondary sites for one primary site. If the primary site is a VSP 5100 or VSP 5500, install at least one VSP 5000 series model, VSP F1500, VSP G1x00, VSP E990, or VSP G/F900, at the intermediate site, and the secondary site respectively.

If the primary site is a VSP 5200 or VSP 5600, install at least one VSP 5000 series or VSP E series at the intermediate site, and the secondary site respectively.

NoteTo combine Universal Replicator and TrueCopy, use the previously described storage system configuration.

Copying data between two secondary sites in the event of a failure or disaster at the primary site is called delta resync. In a 3DC multi-target configuration using delta resync, only the differential data lost by a primary site failure is copied from the TC secondary site to the UR secondary site. As a result, data at a secondary site in a remote location can be restored more quickly.

You create the delta resync configuration by adding a second UR pair to the 3DC multi-target configuration, using the TC S-VOL and the UR S-VOL, as illustrated in the following figure. The delta resync operation is performed when the primary site fails.

GUID-F0E1A3A7-8BF7-446E-A75A-CAB17DA62788-low.png
Procedure
  1. Create the UR delta resync pair. See Creating the delta resync pair.
  2. Set up and map remote command devices and external ports in each site. With remote command devices, sites automatically connect with each other. See Setting up remote command devices for use with CCI.
  3. Assign mirror IDs to remote command devices. See Assigning mirrors to remote command devices for delta resync operations.
  4. In a recovery situation, run the delta resync operation. See Performing the delta resync operation.

Additional task for the 3DC multi-target configuration using delta resync

To use delta resync, you need to perform an additional task after setting up 3DC multi-target configuration.

Procedure

  1. Create the UR delta resync pair.

  2. Set up and map remote command devices and external ports in each site.

    The delta resync configuration will fail if the remote command devices for the delta resync are not set up. For setting up the remote command devices, see the Hitachi Universal Volume Manager User Guide.
  3. Assign mirror IDs to remote command devices. Delta resync configuration will also fail when mirror IDs are not assigned to remote command devices. For assigning mirror IDs to remote command devices, see the related topics. With remote command devices, the TC and UR secondary sites automatically connect with each other, and delta resync is ready. The following figure shows how remote command devices and mirror IDs respond.

    GUID-C2E6A831-95A7-4FA9-8108-67727A39697C-low.png

Creating the delta resync pair

The delta resync configuration is created after setting up the main TC and UR pairs in the multi-target configuration.

  • Data is not copied at the time you set up the delta resync pair. The P-VOL and S-VOL in this system contain data from their respective TC and UR systems
  • If a failure on the primary site occurs, the TC S-VOL is made the primary production volume using the CCI horctakeover command. Differential data stored in the journal volumes is used to synchronize the S-VOL using the resync operation.

Before you begin

  • A UR delta resync pair can be created in a 3DC multi-target configuration only. To create the multi-target configuration, see 3DC multi-target configuration.
  • A 3DC multi-target configuration with multiple primary and secondary storage systems cannot be used for delta resync.
  • The TC S-VOL is used as the delta resync P-VOL. It must be in PAIR status.
  • The UR S-VOL is used as the delta resync S-VOL. It must be in PAIR status.
  • The mirror ID must be between 1 and 3; however, make sure it is not the same as the mirror ID assigned to the UR pair in the 3DC multi-target configuration.
  • The consistency group you use for the delta resync configuration cannot be used by another UR pair.
  • Follow volume combinations shown in .
  • System option mode 506 must be ON for delta resync.
  • If you create more than one delta resync pair, the delta resync P-VOLs in the same master journal must use the same mirror ID.
  • (VSP 5000 series) If you plan to create in-system pairs on the secondary site sharing the delta resync S-VOL, make sure to perform the delta resync create-pair operation first, and then create in-system pairs.
  • (VSP 5000 series) You can set a back-up option in the event that the delta resync operation fails. This is done on the Change Mirror Options window (see the Delta Resync Failure step in Changing options used by mirrors ).

Procedure

  1. Install and set up TC and UR on the required systems.

  2. Create a 3DC multi-target configuration.

    1. Create a TC pair from the primary storage system to the TC secondary site.

    2. When the TC pair’s status is PAIR, create the UR pair from the primary storage system to the UR secondary site.

  3. Create a second UR pair from the TC S-VOL to the UR S-VOL. Specify the Initial Copy option as Delta.

    When creating UR delta resync pair, select different mirror ID (Mirror ID 1 to 3) than the mirror ID which was used in step 2.

Setting up remote command devices for use with CCI

This procedure explains how to configure open-system command devices for use with CCI. If you are using BCM, CCI command devices are not needed. For information about how to set up command devices and a 3DC environment using BCM, see the Business Continuity Manager User Guide.

With remote command devices, communications between each pair of sites is performed automatically, allowing the delta resync operation to be run. A license for Universal Volume Manager is not required for remote command device mapping operations.

Before you begin

To set up the remote command devices, the following conditions must be met:

  • Two command devices and two remote command devices must be set up on each site: the primary site, the TC secondary site, and the UR secondary site, as shown in the figure below. See Provisioning Guide.

  • External paths must be set up on all sites for communications between command devices and remote command devices. See Hitachi Universal Volume Manager User Guide for setting up external paths and remote command devices.

To allocate mirror IDs to remote command devices, the following conditions must be met:

  • Create a 3DC multi-target configuration and UR delta resync pairs.
  • Remote command devices are set up between each site. For more information about setting up the remote command devices, see the Hitachi Universal Volume Manager User Guide.

Procedure

  1. Set up four command devices each on the primary site and at the two secondary sites.

  2. Set up and dedicate two external ports and two target ports on each site. Configure paths between external ports and target ports.

    For details about external ports, see the Hitachi Universal Volume Manager User Guide. For details about port and path settings, see the Provisioning Guide.
  3. On each site, map a command device through a target port to a device on one of the other sites. You must map to the device on the other site as a remote command device, using an external port on that system. See Hitachi Universal Volume Manager User Guide for information about mapping the remote command devices.

  4. Repeat the previous step so that two command devices on each site are mapped to a remote command device on each of the other two sites.

  5. Assign mirror IDs to the remote command devices. This is required to enable communication for delta resync operations.

Results

Thus:
  • Each site must have two command devices mapped through two target ports to the other two sites.
  • Each site must also have two remote command devices mapped-to through external ports from the other two sites.

The following illustration shows this command/remote command device configuration with port configuration.

Remote command devices in a delta resync configuration
GUID-DAE2A4DC-3404-4522-B127-7D059F510C36-low.png

Mapping remote command devices for delta resync operations

For information about command device and remote command device configurations, see Setting up remote command devices for use with CCI.

Procedure

  1. Set a path for the external port and target port of each site.

    For details about the external port, see the Hitachi Universal Volume Manager User Guide. For details about port and path settings, see the Provisioning Guide for your storage system.
  2. Set command devices for all sites.

    For details about setting command devices, see the Provisioning Guide for your storage system.
  3. In the site in which you are creating a pair, map command devices in all sites as remote command devices.

    For details about mapping remote command devices, see the Hitachi Universal Volume Manager User Guide.

Results

When the above procedure is performed, the status of each site changes as follows:
  • Each site has two command devices mapped to the other two sites through two target ports.
  • Each site has two remote command devices mapped from the other two sites through an external port.

In the Logical Devices window, logical devices that are command devices or remote command devices are displayed as Command Device or Remote Command Device (in the Attribute column in HDvM - SN). In CCI, command devices are displayed as CMD for VOL_ATTR of the raidcom get ldev command, and remote command devices are displayed as RCMD for VOL_ATTR of the raidcom get ldev command.

The following figure shows a configuration of command devices and remote command devices.

GUID-E452602D-9087-4AB7-AE17-6201B4B820A4-low.png

When mapping of remote command devices to each site is complete, you must assign mirror IDs to the remote command devices. This operation is required for delta resync operations.

Assigning mirrors to remote command devices for delta resync operations

You assign the mirror IDs used by the configuration’s journals to the remote command devices. This enables UR control information regarding the delta resync to be transferred and then read at the secondary sites. This is necessary for the delta resync operation.

NoteWhen SOM 1242 is set to ON, SIM=dcf6xx is issued if the remote command devices for delta resync operations are not set up.

To perform this task in CCI, use the raidcom modify journal command.

Before you begin

  • Required role: Storage Administrator (Remote Copy)
  • To assign a mirror to a remote command device, the mirror’s status must be one of the following:

    Initial, Active, Halt, Stopped, Hold, Holding, or Holding(Failure)

    To use a mirror in Initial status, it must have mirror ID 0.

  • Perform this operation for each journal.
  • In a journal, you can assign multiple mirrors to two remote command devices in one operation.
  • A maximum of 16 mirrors can be shared with a remote command device. When a remote command device is shared, you can use it to do the following:
    • Assign it to other mirrors
    • Use it with CCI

    When performing these functions with a remote command device, the number of registered journals and instances used by CCI are subtracted from the number of mirrors that can be shared.

  • A maximum of 16 remote command devices can be used for the delta resync operation per storage system.
NoteA remote command device allocated to a mirror is removed when all pairs in the 3DC delta resync configuration are deleted.

Procedure

  1. In the Explorer pane, expand the Storage Systems tree.

  2. Expand the target storage system tree, expand Replication, and click Remote Replication.

  3. Select the Mirrors tab, and then select the mirror to which you will assign the remote command device.

  4. From the Actions menu, click Remote Replication Assign Remote Command Devices.

  5. In the Available Mirrors table, select a mirror.

    • On primary site:

      - Assign mirror ID 0 to the remote command device that is mapped to the TC secondary site.

      - Assign the mirror ID used for the UR 3DC multi-target pair to the remote command device that is mapped to the secondary site.

    • On the TC secondary site:

      - Assign mirror ID 0 to the remote command device that is mapped to the primary site.

      - Assign the mirror ID used for the UR delta resync pair to the remote command device that is mapped to the UR secondary site.

    • On the UR secondary site:

      - Assign the mirror ID used for the UR 3DC multi-target configuration to the remote command device that is mapped to the primary site.

      - Assign the mirror ID used for the UR delta resync pair to the remote command device that is mapped to the TC secondary site.

    When selecting mirror IDs and remote command devices, observe the following:
    • Though two mirrors can be assigned to each remote command device, it is not required. Mirror 2 is optional.
    • The same mirror ID cannot be specified from both Mirror ID lists.
  6. Select the remote command device to be assigned from Remote Command Device.

  7. Click Add. The mirror to which the remote command device is assigned, is added to the Selected Mirrors table. If you want to remove the added mirror from the Selected Mirrors table, select the mirror and then click Remove.

  8. Click Finish.

  9. In the Confirm window, review the settings and enter a task name in the Task Name box.

  10. Click Apply to save your settings to the system.

  11. To check newly assigned remote command devices, see Remote Command Device in the Journal Volumes window.

    • If a remote command device is assigned to the mirror, the LDEV number is displayed.
    • If a remote command device is not assigned to the mirror, the field is blank.
    • If a remote command device cannot be assigned to the mirror, a hyphen is displayed.

Releasing a remote command device for delta resync operations assigned to a mirror

This operation is performed on the mirror when releasing the assigned remote command device.

Before you begin

  • Required role: Storage Administrator (Remote Copy).

Procedure

  1. In the Explorer pane, expand the Storage Systems tree.

  2. Expand the target storage system tree, expand Replication, and click Remote Replication.

  3. Select the Mirrors tab, and then select the mirror with the remote command device to be deleted.

  4. From the Actions menu, click Remote Replication Release Remote Command Devices.

  5. In the Release Remote Command Devices window, review your selection in the Selected Mirrors table. If you need to make a change, select Cancel.

  6. When ready, click Finish.

  7. In the Confirm window, review the settings and enter a task name in the Task Name box.

  8. Click Apply to save your settings to the system.

Relationship between deleted associated pairs and assigned remote command devices

You can change a 3DC configuration that uses the delta resync function to a configuration that does not use it by deleting pairs. At this time, along with the deleted pairs, associated pairs are also deleted automatically, and the assignment of remote command devices is released automatically.

If you delete pairs when you cannot communicate with remote command devices, the assignment of remote command devices is not released automatically. In this case, refer to information about releasing a remote command device assigned to a mirror.

If you delete pairs after a hardware failure has occurred or after a pair is suspended by error, the assignment of remote command devices you do not want to delete might be released. In this case, check if the necessary remote command devices are still assigned after the deletion. If the assignment is released, reassign the remote command devices.

NoteWhen SOM 1242 is set to ON, SIM=dcf6xx is issued if the remote command devices for delta resync operations are not set up.

Automatically deleted pairs that are automatically released

There are configurations in which pairs that are automatically deleted and remote command devices that are automatically released along with the pairs you have deleted.

When all the TC pairs linked to the UR pairs are deleted, the following pairs are also deleted automatically, and the assignment of the following remote command devices is released. The configuration changes to the UR only configuration.

  • UR delta resync pair
  • Remote command devices set for the UR mirror for delta resync
  • Remote command devices set for the mirror between the primary site and the TC secondary site

The assignment of remote command devices set for the UR mirror is not released.

When all the UR pairs between the primary site and the UR secondary site are deleted, the following pairs are also deleted automatically and the assignment of the following remote command devices is released. The configuration changes to the TC only configuration.

  • UR delta resync pair
  • Remote command devices set for the UR mirror for delta resync
  • Remote command devices set for the UR mirror
  • Remote command devices set for the mirror between the primary site and the TC secondary site

All the remote command device assignments are released.

When all the UR pairs for delta resync are deleted, the assignment of the following remote command devices is released automatically. The configuration changes to the 3DC multi-target configuration that does not use the delta resync function.

  • Remote command devices set for the UR mirror for delta resync
  • Remote command devices belonging to the TC secondary site, and set for the mirror between the primary site and the TC secondary site

The assignment of the following remote command devices is not released:

  • Remote command devices set for the UR mirror
  • Remote command devices belonging to the primary site, and set for the mirror between the primary site and the TC secondary site

Performing the delta resync operation

You perform the delta resync operation using the resynchronize mirrors operation. When the delta resync operation is performed, the differential data will be copied from the TC S-VOL to the UR S-VOL. Review the following important prerequisite information, and then follow the procedure in Resynchronizing a mirror .

  • Note the following:
    • The delta resync pair must be created.
    • Remote command devices must be set up.
    • External ports must be set.
    • Mirror IDs must be assigned to remote command devices.

    For details, see Delta resync configuration .

  • Required pair and volume statuses:
    • The TC volume used as the 3DC multi-target P-VOL: PAIR
    • The TC S-VOL used as the delta resync P-VOL: SSWS.

      Check this status after executing the CCI horctakeover command

    • The UR delta resync pair: HOLD or HOLDING. If the pair status is HLDE, change to HOLD by performing the pairresync operation.

      This applies to all pairs in the journal in which the UR delta resync pair resides.

    • The UR volume used as the 3DC multi-target and the delta resync pair S-VOL and uses two mirror IDs, one for the 3DC multi-target pair and one for the delta resync pair:

      - 3DC multi-target S-VOL: PAIR, PSUS, or PSUE

      - Delta resync S-VOL: HOLD.

      NoteThe UR S-VOL uses two mirror IDs because of its dual use in the 3DC multi-target and delta resync configurations. The statuses shown above are required for the two mirrors as well as for the journal in which the UR delta resync pair resides.
  • If the delta resync S-VOL has been backed up on the secondary site using SI or TI, make sure to review the information for the Delta Resync Failure step in (Changing options used by mirrors ).
  • Differential data is stored in the master journal but may not be completely present after a failure at the primary site when the pair was being resynchronized (or after creating the delta resync pair configuration).
  • When a UR pair has not been suspended and not resynchronized for a long time, the data in the restore journal might exceed 70% of capacity and old journal data is automatically deleted. In this case, the P-VOL and S-VOL are not synchronized completely by just copying the data, and delta resync will fail. In case of delta resync failure, based on Delta Resync Failure settings, the entire P-VOL data will be copied to the S-VOL or pair status becomes HLDE with no processing and delta resync ends abnormally.
  • Journal data may also be destroyed in the following cases:
    • When you restore the TC or UR pair in the multi-target configuration, and then also update the delta resync pair.
    • When retry-processing occurs because of a delay of the P-VOL update.
    • When the update of a TC S-VOL is delayed.
  • The UR pair status in a delta resync configuration could become PSUS or PSUE if you split the pair, or if there is a failure when copying all data from the P-VOL to the S-VOL. If you then run the delta resync operation, all data might be copied from P-VOL to S-VOL regardless of journal option settings.
  • If the pair’s mirror is in Halting or Stopping status, the resynchronization command is rejected.
Note In a delta resync configuration, if you delete some UR pairs in a consistency group, split the pairs for each consistency group before deleting them. After that, resynchronize the pairs for each consistency group.

In a delta resync configuration, if you delete some UR pairs without splitting pairs for each consistency group, split and resynchronize them for each consistency group after the deletion.

If both UR and TC pairs are suspended, resynchronize the UR pair first.

Problems that can occur in the delta resync operation

Journal data will not exist, and therefore the delta resync operation will fail, in the following cases:

  • After creating the UR pair, the primary delta resync P-VOL is updated but not the UR P-VOL.
  • When the multi-target P-VOL is resynchronized after the TC pair was split.
  • When the UR S-VOL is resynchronized after it was split.
  • When the UR pair is resynchronized after being split and then the journal volume at the TC secondary site exceeds 70%.
  • When the delta resync P-VOL is updated, and then the journal volume at the TC secondary site exceeds 70%.
  • When the delta resync pair is created and no volumes (including volumes after failover or failback) in the primary site are updated.
  • When the status of the UR delta resync pair becomes HLDE, the journal data necessary for the delta resync operation might be discarded. In this case, all data in the delta resync P-VOL would be copied to the delta resync S-VOL.
  • Do not assign the P-VOL of an SI pair that uses a DP-VOL to the S-VOL of the UR delta resync pair.

Delta resync failure error prevention

If you specify Entire Copy for the Delta Resync Failure option (Edit Mirror Options window) in advance, you can prevent errors from occurring by copying the entire P-VOL data to the secondary volume when the necessary journal data for delta resync is not available. However, the P-VOL data will not be copied to the S-VOL in case of error even if you specify Entire Copy for the Delta Resync Failure option in the following case:

  • The UR delta resync S-VOL is assigned to the SI P-VOL or HTI P-VOL.

In the following case, entire data might be copied from the UR delta resync P-VOL to the S-VOL:

  • Before performing delta resync, the pair status is changed to PSUS by pair split or PSUE by failure while copying the entire data from the P-VOL to the S-VOL as a UR pair.

In a delta resync configuration with TCz, if you want to delete some URz pairs in a consistency group, split the pairs for each consistency group before deleting them. After that, resynchronize the pairs for each consistency group. In a delta resync configuration with TCz, if you delete some URz pairs without splitting pairs for each consistency group, you need to split and resynchronize them for each consistency group after the deletion.