Home > Storage > PowerMax and VMAX > Data Protection > Dell PowerMax 2500 and 8500: TimeFinder SnapVX Snapshots and Clones > SnapVX linked targets
A point-in-time image is applied to a volume by linking the snapshot with a target volume. Each source volume can have up to 1,024 linked volumes. All target volumes can be linked to a single snapshot, or the targets can be spread across snapshots.
Host writes to target volumes do not modify snapshot data. The snapshot point-in-time data remains unchanged even when used for immediate creation of space-efficient, host-accessible volumes for test, development, reporting, or any other purpose. Preservation of snapshot data allows the linked target to be reset to the snapshot and allows many targets to be linked to a single snapshot. The original snapshot image can be used to restore and recover data even after target data has been modified.
Best practice: The target volume should not be in use (for example, file system should be unmounted) before the link or relink command. Perform a remount after the operation completes to ensure that the host operating system is not viewing cached file system data or keeping locks on data that is being changed at the storage level. The specific host operating system may require a SCSI rescan if volumes were added to the host for the first time, and a reboot should be avoided.
Snapshots can be linked by specifying the source Storage Group, snapshot, and target Storage Group. If any volume in the Storage Group cannot be linked for some reason, no volumes in the SnapSet are linked.
Unisphere and REST API have extra functionality to either select an existing Storage Group or create a Storage Group and volumes. Solutions Enabler requires the Storage Group and volumes to be linked must be created ahead of time.
Volumes are available for use immediately after being linked. There is a background defining process that changes volume pointers to directly point to the data. The defining process creates shared allocations and improves performance when accessing target volumes. Some overhead may be incurred when accessing data that is not yet defined, but the process usually completes quickly and is transparent to the application.
Relink operations incrementally refresh a linked target. For example, relink can be used to reset the target volume to the current snapshot or to restart a test from the same point-in-time image. Relink can also move the target to another snapshot from the same Storage Group for another purpose, such as restoring the latest backups.
A relink operation is always differential. When using SRDF R1 volumes as linked targets, link operations cause a full copy to the remote array, and relink operations cause a differential copy to the remote array.
Unlink operations break the relationship between a snapshot and linked target volume. The unlink operation does not deallocate data on the target, and does not return any free capacity to the SRP. Targets volumes retain usable data after being unlinked.
Best practice: The symdev free –all command can be used to deallocate capacity of a target volume after unlinking without having to delete the volume. Use extreme caution with this command as it is a powerful tool that completely deallocates all data on the affected volumes. For safety, the volumes must be set not ready or unmapped before capacity can be freed.
Cascading refers to taking a snapshot of a linked target and linking a target to a snapshot of a linked target.
The cascading capabilities of SnapVX are robust. There is no limit on the number of cascaded hops that are allowed. The cascaded chain can be broken at any point by unlinking the target from the snapshot.
Best practice: Several traditional use cases for cascading clone or snap sessions no longer exist with SnapVX. Since snapshot data is always preserved until the snapshot is terminated, there is no need to make a “Gold Copy” snapshot. Furthermore, the defining mechanism improves performance of host I/O to target volumes and avoids target I/O from being redirected and affecting performance of source volumes.
Linked target data cannot be copied directly back to the source. Target data can be applied to the source volume with cascading snapshots taking a snapshot of the linked target, and then link that snapshot to the original source. If this copy direction is a frequent requirement, Clone may be a better option because clones can be directly and differentially restored to the source Storage Group.