TrueCopy and ShadowImage considerations

Replication and Disaster Recovery Administration Guide for Hitachi NAS Platform

Version
15.1.x
Audience
anonymous
Part Number
MK-92HNAS009-29

When using TrueCopy and ShadowImage, keep the following in mind:

  • TrueCopy and ShadowImage functionality are managed using the Hitachi Vantara software interfaces, because the replication occurs between or within the Hitachi Vantara storage subsystems. Contact your Hitachi Vantara technical representative for assistance configuring these features. For more information about these applications, contact Hitachi Vantara Support Center or your Hitachi Vantara technical representative.
  • Storage servers rely on SCSI commands to determine that a system drive is simplex, primary mirror, secondary mirror, TrueCopy, or ShadowImage. The information is obtained using a proprietary extension that Hitachi Vantara has added to the standard SCSI inquiry.
  • Hitachi Vantara storage subsystems support the following volume states:

    Unmirrored (simplex)

    Mirrored primary (p-vol)

    Mirrored primary (p-vol)

    Unknown

  • Neither the storage server nor the TrueCopy application can automatically cause failover to secondary storage. An external agent (a person or application) must make the decision on the failover and execute the required commands. Once the storage failover is initiated, the storage server automatically starts accessing the new primary storage.

    An external utility controls TrueCopy can be used to issue commands to the storage server. This functionality must be provided by an external utility that checks the health of the system and makes appropriate decisions. This same utility can be used to initiate an EVS migration in conjunction with the storage failover.

    Switching mirror roles involves a brief changeover period when the system drives do not have a normal primary/secondary relationship. During that period, I/O is impossible. Therefore, the storage server unmounts the file systems on the storage pool. Changing mirror roles always involves unmounting and remounting file systems, and, if possible, users should unmount the file systems before the change and remount after the changeover is complete.

    Note that, in the case of automated storage failover, steps must be taken to ensure that the original primary volume does not come back online after a failover, because the storage server would then see two different primary volumes for the same data.

  • Hitachi Vantara requires the usage of ShadowImage for synchronous TrueCopy configurations to help recover in the case of data corruption in the file system. With this configuration, the storage server does not have to be configured to see the ShadowImage volume (in other words, the ShadowImage volumes should remain unlicensed). For recovery, data can be copied back to the TrueCopy primary mirror, leaving the ShadowImage copy intact until it is determined that the file system is back up and running.
  • Hitachi NAS Platforms and High-performance NAS Platforms support only the “Never” mirror fence level of TrueCopy.
  • Data Migrator can be used with TrueCopy, as the FS IDs in the mirrored file systems are identical.
  • Whenever a storage server (or cluster) shares a Hitachi Vantara storage subsystem with another storage server (or another cluster), the system configuration must use Hitachi Vantara host groups to prevent the storage servers from seeing system drives that they are not intended to use. Host groups are a LUN-mapping mechanism that controls which servers see the different system drives and LUNs. This type of configuration prevents potential problems caused by the storage server periodically sending commands to unlicensed Hitachi Vantara storage systems to determine their status.