Metro-based NDM is built upon SRDF/Metro active/active technology with the Metro instant activate (Metro-IA) feature. For more information about SRDF/Metro, see the SRDF/Metro Overview and Best Practices document.
Benefits of using Metro-based NDM include the following:
- Allows the complete migration from a VMAX3 or VMAX All Flash arrays to VMAX All Flash or PowerMax arrays without the need for downtime
- Leverages Metro technology to reduce the required commands users need to issue by removing the need to cut over
- Is managed by familiar and simple user interfaces: Solutions Enabler and Unisphere for VMAX
- Allows migrations to be canceled easily and failed back to the source array for any reason prior to commit
- Allows users to precopy all or a large portion of data before bringing the host live to the new array. This reduces the impact to the application during the migration period.
The previous version of NDM provided with Solutions Enabler 8.3, Unisphere 8.4, and HYPERMAX OS 5977.1125 releases allowed data to be migrated from a VMAX (5876) to a VMAX3 (5977) array without application downtime.
With the release of Solutions Enabler 9.0 and HYPERMAX OS Q2 2018, the NDM feature was enhanced to help automate the process of moving applications from a VMAX3 (5977) or VMAX All Flash (5977) array to another VMAX All Flash (5978) or PowerMax (5978) array.
With the release of Solutions Enabler 9.1, full interfamily migrations are possible (5977 to 5977, and 5978 to 5978).
The source hardware is not a limiting factor. Metro-based NDM is supported from arrays running 5977 or 5978 to 5977 or 5978 code, regardless of the underlying technology.
With SRDF/Metro, the session goes active/active only after all SCSI information and application data is synchronized from R1-R2 using SRDF adaptive copy technology. The time to be fully active/active largely depends on the time it takes for the data transfer to finish.
To improve the user experience with NDM, the software is enhanced such that the SRDF/Metro session goes active/active instantly on NDM create. This only applies to underlying Metro technology for NDM and does not apply to regular SRDF/Metro for running active/active applications. This makes both sides of the SRDF/Metro active and read/write to the host within the duration of the create command.
Metro-based NDM modes of operation
- Metro-based NDM: This is the NDM mode where the synchronization starts right after the create completes, and the create operation creates an NDM session using SRDF/Metro internally.
- Metro-based NDM with precopy: This mode of operation offers a choice to start the NDM session using adaptive copy (SRDF/ADP) which helps synchronize most of the data before moving into an active/active state. Eventually, this moves to active/active mode and synchronizes the remaining tracks. In essence, precopy allows end users to copy application data from the source array to target array while the application is still running on the source array.
Metro-based flows
The following steps and diagram describe the process flow for Metro-based NDM:
- Solutions Enabler creates an active/active SRDF group with NDM attribute.
- Solutions Enabler activates the group.
- SCSI information, device personality, and attributes are transferred to the target devices.
- Masking view to the source (R2) array created.
- Migration starts from R1 to R2 (source to target).
- User rescans host for extra paths to target array.
- Data synchronization completed.
- Commit issued and migration completed.
Figure 2. Process flow for Metro-based NDM
The following steps and diagram describe the component flow for Metro-NDM with precopy:
- Solutions Enabler creates an active/active group with NDM attribute and precopy set. The SRDF group is set to adaptive copy mode.
- Pre-copy data sync (R1-R2)
- Solutions Enabler sets READYTGT which will activate RDF group with Metro-IA.
- SCSI information, device personality, and attributes are transferred to the target devices.
- Masking view to the source array created.
- User rescans host for extra paths to the target array.
- Data synchronization completed.
- Commit issued and migration completed.
Figure 3. Component flow for Metro-based NDM with precopy
Metro-based NDM operations
- Create: This creates a Metro-based NDM session. It creates SRDF/Metro with NDM source and target attributes and puts the NDM pair in active/active mode (clear DEV-INACT on R2). This also starts data synchronization and SCSI info synchronization and moves into a migrating state. Once the data has been synchronized and the host paths to the target array have been discovered by means of a host rescan, the NDM session reaches a synchronized state. A create results in RDF mirror invalids on R1 and local mirror invalids on R2.
- Create with precopy: This creates an SRDF/Metro session with NDM attributes and puts the SRDF/Metro pair into adaptive copy disk mode. It starts data synchronization from R1 to R2. Bias is enabled on the Metro-based NDM source. This creates an SRDF/Metro session with NDM attributes and puts the SRDF/Metro pair into adaptive copy disk mode. It starts syncing data from R1 to R2. Bias is on the Metro-based NDM source.
- ReadyTGT: This continues to synchronize the remaining invalid tracks from source target and also starts synchronization of SCSI Information. It puts the NDM pair in active/active mode (clear DEV-INACT on R2) without waiting for synchronization to finish.
- Cancel: This suspends the SRDF/Metro NDM session. The R2 device moves into a DEV-INACT state and all I/O is redirected to R1.
- Sync-stop: Once in synchronized state, the user might need to test the application performance before committing. The sync-stop command would make the NDM SRC DEV-INACT, all I/O would go through Metro-NDM target R2, and the Metro-NDM session moves into a CutoverNoSync state. This operation results in moving the bias to the Metro-NDM target.
- Sync-start: This moves the Metro-NDM session from CutoverNoSync to CutoverSyncing and finally to synchronized. This command should be used once the user has finished verifying the application against the NDM target after the sync-stop command and the user is ready to either commit or cancel the NDM session. The bias remains on the Metro-NDM target commit.
Manipulation of device IDs and host paths
NDM can migrate data and cut over to the target array non-disruptively by both swapping device IDs between the source and target devices and manipulating the paths from the host to both arrays. The device ID contains the device’s unique WWN and other information about it, such as a device identifier that the user has assigned to a device through Solutions Enabler or Unisphere. All of this information is copied to the target devices.
NDM performs the data migration and device ID swap without the host being aware. The path management changes appear as either the addition of paths or the removal of paths to the existing source device. To the host and application, there is no change in the device that it is accessing and access to the device is maintained throughout the entire migration process.