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Overview of Hitachi TrueCopy®

An overview of the different components that are a part of Hitachi TrueCopy® helps you to understand its function and capabilities.

Hitachi TrueCopy®

Hitachi TrueCopy® (TC) provides a continuous, nondisruptive, host-independent remote data-replication solution for disaster recovery or data migration purposes. Using the TrueCopy Remote Replication software, you can create and maintain mirror images of production volumes at a remote location. TrueCopy Remote Replication software can be deployed with Hitachi Universal Replicator software's asynchronous replication capabilities to provide advanced data replication among multiple data centers. In addition, TrueCopy Remote Replication software can be integrated with Hitachi ShadowImage® Replication software to enable robust business-continuity solutions. This lets you create a remote copy of primary site or production data that is automatically updated for executing test and development tasks, or for operations against production data.

The TrueCopy primary storage system contains the primary volume (P-VOL) of a copy pair, and the secondary storage system contains the secondary volume (S-VOL). When the primary storage system accepts a write operation for a P-VOL, the data is written on the primary volume and then sent by the primary storage system to the secondary storage system through the dedicated data paths connecting the storage systems. Subsequent write operations are not accepted by the primary volume until acknowledgement is received from the secondary storage system for the previous write operation, ensuring that the data in the secondary volume stays synchronized with the primary volume.

To reduce the overhead associated with these remote copy activities and maximize data transfer, the primary storage system uses a special write command for TrueCopy remote copy operations. This command transfers the control parameters and the FBA-format data for consecutive updated records in a track using a single write operation. The special write command eliminates the overhead required for performing FBA-to-CKD and CKD-to-FBA conversions.

TrueCopy operations can be performed using the TrueCopy Remote Replication software on Hitachi Device Manager - Storage Navigator and the Command Control Interface (CCI) command-line interface software. This document describes and provides instructions for performing TrueCopy operations using the TrueCopy software on Device Manager - Storage Navigator. For details about using CCI to perform TrueCopy operations, see the user documentation for CCI.

VSP G130 is available only in select markets. Contact your local sales representative for availability.

System components

TrueCopy operations involve the storage systems and volumes at the primary site and secondary (remote) site, the physical communications paths between these storage systems, and the TrueCopy software. A TrueCopy system configuration consists of the following components:

  • Primary and secondary storage systems The primary storage system contains the primary volumes and is connected to the hosts that access the primary volumes. The secondary storage system is connected to the primary storage system using the data paths. TrueCopy supports remote copy operations between various storage system models. This document provides instructions for performing TrueCopy operations.
  • Main control units (MCUs) and remote control units (RCUs) The MCU is the control unit in the primary storage system that controls the P-VOLs of the TrueCopy pairs. The MCU communicates with the RCU through the dedicated remote copy connections. The MCU controls the host I/O operations to the P-VOLs as well as the TrueCopy remote copy operations between the P-VOLs and S-VOLs. The MCU also manages the TrueCopy pair status and configuration information.

    The RCU is the control unit in the secondary storage system that controls the S-VOLs of the TrueCopy pairs. The RCU assists in managing the TrueCopy pair status and configuration (for example, rejects write I/Os to S-VOLs). The RCU executes the remote copy operations issued by the MCU. The RCUs should be attached to a host system to allow sense information to be reported in case of a problem with a secondary volume or remote storage system and to provide disaster recovery capabilities.

  • Hosts The hosts at the primary site are connected to the primary storage system. Hosts at the secondary site are connected to the secondary storage system for use in disaster recovery operations. If it is not possible to have hosts at the secondary site, the host at the primary site must be in communication with the secondary system for disaster recovery operations.
  • Volumes The primary volumes (P-VOLs) on the primary storage system are copied to the secondary volumes (S-VOLs) on the secondary system. The P‑VOLs contain the original data, and the S‑VOLs are the mirrored volumes that contain the backup or duplicate data. During normal TrueCopy operations, the P‑VOL remains available to all hosts at all times for read and write I/O operations and the secondary storage system rejects all host-requested write I/Os for the S‑VOLs. The S‑VOL write enable option allows write access to an S‑VOL while the pair is split, and the S‑VOL and P‑VOL differential data is used to resynchronize the pair.
  • Data paths Dedicated data paths, also called remote copy connections, are used for data transfer between the primary and secondary storage systems. You should establish at least two independent remote copy connections (one per cluster) between each MCU and RCU to provide hardware redundancy for this critical communications path.
  • Hitachi TrueCopy® software The TrueCopy software must be installed on both the primary and secondary storage systems and is used to perform TrueCopy configuration and pair operations.
  • CCI The Command Control Interface (CCI) command-line interface software can also be used for TrueCopy configuration and pair operations as well as disaster recovery operations.

The following figure shows a typical TrueCopy environment.

Storage systems

TrueCopy operations take place between a primary storage system and a secondary storage system. The primary storage system communicates with the secondary storage system over dedicated Fibre Channel or iSCSI data paths called remote copy connections.

The primary storage system is responsible for the following:

  • Managing host I/O operations to the P-VOL.
  • Managing initial copy and update copy operations between the P-VOL and the S-VOL.
  • Managing pair status and configuration information.

The secondary storage system is responsible for the following:

  • Managing copy operations issued by the primary storage system to the S-VOL.
  • Assisting in the management of pair status and configuration (for example, rejecting write I/Os to the S-VOL).

If the primary storage system is VSP 5000 series, the secondary system can be VSP 5000 series, VSP G1x00, VSP F1500, VSP, VSP E series, VSP G130, or VSP G/F350, G/F370, G/F700, G/F900.

For VSP 5000 series,the CU can function simultaneously as a primary storage system for one or more P-VOLs and as a secondary storage system for one or more S-VOLs. This configuration requires that data paths and ports are configured for both copy directions.

If the primary storage system is VSP G/F350, G/F370, G/F700, G/F900, the secondary system can be VSP 5000 series, VSP G1x00, VSP F1500, VSP E series, VSP G/F350, G/F370, G/F700, G/F900, VSP G200, G400, G600, G800, VSP F400, F600, F800, or HUS VM.

If the primary storage system is VSP E990, the secondary system can be VSP E series, VSP G/F350, G/F370, G/F700, G/F900, VSP G200, G400, G600, G800, VSP F400, F600, F800, VSP 5000 series.

Volume pairs

Each TrueCopy volume pair consists of the P-VOL, which contains the original data, and the S-VOL, which contains the synchronous copy of the data on the P-VOL. After creating a copy pair, you can use the TrueCopy software to split, resynchronize, and reverse resynchronize pairs, and you can delete pairs as needed to return the volumes to an unpaired status.

  • When paired, the volumes are synchronized.
  • When split, new data is sent to the P-VOL but not the S-VOL.
    Note
    • It will continue writing to P-VOL but copying to S-VOL stops so that pair will not be synchronized.
    • If setting S-VOL's write option as enable, Write access from host application of secondary site to S-VOL become possible. If not, it will hold the status at the time of pair split.
    • When P-VOL and S-VOL is not synchronized, differential data will be saved as bitmap until P-VOL and S-VOL resynchronizes.
    • If you resync pairs, only the data which is not synchronized will be transferred so it will save time for copying.
  • When resynchronized, data that changed while the pair was split is copied to the S-VOL.
  • When necessary, data in the S-VOL can be copied to the P-VOL.

During normal operations, the P-VOL remains available to the host for read and write I/O operations. The secondary system rejects write I/Os for the S-VOL. The S-VOL can only be written to when the pair is split and when the write-enable option is specified for the S-VOL. In this instance, S-VOL and P-VOL track maps keep track of differential data and are used to resynchronize the pair.

Data path

TrueCopy operations are carried out between primary and secondary storage systems connected by a Fibre Channel or iSCSI interface. The data path, also referred to as the remote copy connection, connects ports on the primary storage system to the ports on the secondary storage system. Ports are assigned attributes that allow them to send and receive data.

One data path connection is required, but two or more independent connections are recommended for hardware redundancy.

Consistency groups

A consistency group is a group of pairs on which copy operations are performed simultaneously and in which the status of the pairs remains consistent. A consistency group can include pairs that reside in up to four primary and secondary systems.

You can issue a TrueCopy command to a consistency group to perform the operation on all pairs in the group at the same time. The status of the pairs changes at the same time, though this depends on the group options you have set. Some pair operations take priority under certain circumstances. For details, see Consistency group planning.

User interfaces

You can perform TrueCopy operations using one of the following user interfaces:

  • Hitachi Device Manager - Storage Navigator (HDvM - SN) is a browser-based graphical user interface (GUI) that allows you to perform TrueCopy operations from any LAN-attached computer.
    • The primary storage system must be LAN-attached to a Device Manager - Storage Navigator computer.
    • For disaster recovery purposes, the secondary storage system must be LAN-attached to a separate Device Manager - Storage Navigator computer at the secondary site so that you can perform operations on the secondary storage system in the event that the primary site is not available.
    • When you use virtual storage machine volumes, the physical LDEV ID, serial number, and the virtual information for the storage system are displayed on Device Manager - Storage Navigator. However, you cannot add remote connections by specifying the serial number of the virtual storage machine or create pairs by specifying the virtual LDEV ID. When you perform TrueCopy operations using Device Manager - Storage Navigator, specify the physical LDEV ID and serial number of the storage system.
  • Command Control Interface (CCI) is a command-line interface that allows you to perform TrueCopy operations by issuing commands to the storage system either from a client or server through the host Fibre Channel interface (in-band method) or from a LAN-attached computer (out-of-band method). CCI provides a scripting capability that enables you to automate replication operations. CCI is required for performing failover operations.

Failover software

Host failover software is used to transfer information between host servers at the primary and secondary sites and is a critical component of a disaster recovery solution.

  • When TrueCopy is used as a disaster recovery tool, host failover is required to ensure effective recovery operations.
  • When TrueCopy is used as a data migration tool, host failover is recommended.

TrueCopy does not provide host failover functions. Use the failover software most suitable for your platform and requirements (for example, Microsoft Cluster Server).

Initial copy and update copy operations

When you create a copy pair, the initial copy operation is performed to copy all of the data in the P-VOL to the S-VOL. Once the pair is synchronized, the primary storage system performs update copy operations when it receives write I/Os for the P-VOL. The update copy operation ensures that host updates are performed on the P-VOL and the S-VOL at the same time.

The following figure illustrates the initial copy and update copy operations.

Initial copy operations

When you create a copy pair, the contents of the P-VOL are copied to the S-VOL track by track (not including diagnostic and unassigned alternate tracks). The initial copy operation synchronizes the P-VOL and S-VOL independently of host I/O processes.

If you know that the data in the P-VOL and S-VOL are already identical, or if the P-VOL does not yet contain any data, you can select the None initial copy option. When you select this option, the pair becomes synchronized (PAIR status) immediately. You can also specify the following options to control the impact of the initial copy operations on the storage system performance:

  • Copy Pace. Specifies the number of tracks that are copied simultaneously before the P-VOL accepts another host I/O request. If more tracks are copied, the initial copy operation is completed more quickly. If fewer tracks are copied, storage system performance (for example, host I/O response time) is maintained at a higher level. You can set this option when you create pairs using Device Manager - Storage Navigator or CCI. The following table lists the values for the copy pace and the transfer size per remote I/O (RIO).
    Copy pace Transfer size
    OPEN-V
    1 256 KB (256 KB x 1)
    2 512 KB (256 KB x 2)
    3 768 KB (256 KB x 3)
    4 1024 KB (256 KB x 4)
    Other than OPEN-V (VSP 5000 series)
    1 58 KB (58 KB x 1)
    2 116 KB (58 KB x 2)
    3 174 KB (58 KB x 3)
    4 232 KB (58 KB x 4)
    5 290 KB (58 KB x 5)
    6 348 KB (58 KB x 6)
    7 406 KB (58 KB x 7)
    8 464 KB (58 KB x 8)
    9 522 KB (58 KB x 9)
    10 580 KB (58 KB x 10)
    11 638 KB (58 KB x 11)
    12 696 KB (58 KB x 12)
    13 754 KB (58 KB x 13)
    14 812 KB (58 KB x 14)
    15 870 KB (58 KB x 15)
  • Maximum Initial Copy Activities. Specifies the maximum number of concurrent initial copy operations.

    When using CCI, you can specify the maximum number of initial copies by using the raidcom modify remote_replica_opt -copy_activity command.

  • Initial Copy Priority. Specifies the order in which the initial copy operations are performed. This option applies when you create more pairs at the same time than the maximum initial copy activity setting. When using CCI, you can specify the order of initial copy operations for LUs and LDEVs by changing the order they are specified in the configuration definition file.
  • Round Trip Time. You can specify the time limit for data to travel from the P-VOL to the S-VOL. This value is used by the storage system to control initial copy pace when update copying is in progress. When you use CCI, you can specify the value using the raidcom modify rcu command.

Update copy operations

When the primary storage system accepts a host write operation for a P-VOL, the primary storage system performs an update copy operation to write the data on both the P-VOL and the S-VOL. The primary storage system does not accept another write operation for the P-VOL until it receives confirmation from the secondary storage system that the write operation was completed successfully on the S-VOL. This ensures that the data on the P-VOL and S-VOL remains synchronized.

Update copy operations have a higher priority than initial copy operations. However, if an initial copy operation is in progress when the host issues a write operation to a P-VOL, the update copy operation must wait until the completion of the number of tracks specified in the copy pace setting. For example, if the copy pace setting is 15 tracks, the update copy must wait until all 15 tracks (1 cylinder) in the initial copy operation are copied. At that time the update copy operation is performed, and then the initial copy operation is resumed.

Pair status

The pair status is managed by the primary storage system which manages the P-VOLs.

  • The primary storage system is able to change the pair status of the P-VOL and the S-VOL.
  • The secondary storage system can change the pair status of the S-VOLs, but cannot change the pair status of the P-VOLs. The primary storage system detects the change of the pair status of S-VOL, and then change the status of P-VOL.
  • The pair status changes as follows:
    • If the volume is not assigned to a TrueCopy pair, the volume status is SMPL.
    • When the initial copy begins to create a pair, the primary storage system changes the status of both volumes to COPY (volumes to be copying).
    • When the initial copy completes, the primary storage system changes the status of both volumes to PAIR (volumes become a pair).
    • When user splits the pair from the primary storage system or the secondary storage system, the status of the P-VOL and the S-VOL are changed to PSUS (pair suspended-split: split by a command).
    • The primary storage system cannot keep the synchronization of the P-VOL and the S-VOL by some reasons, for example errors, the primary storage system changes the status of the P-VOL and the S-VOL to PSUE (pair suspended-error: split by an error).
    • When the user removes the pair from the primary storage system, the primary storage system changes the status of the P-VOL and the S-VOL to SMPL.
    • When the user removes the pair from the secondary storage system, the secondary storage system changes the status of the S-VOL to SMPL, and then the primary storage system detects the removal of the pair at the secondary system (if the path is normal), the primary storage system changes the status of the P-VOL to PSUS.