EMC VNX – Writable Snapshots with SnapSure

SnapSure gives you the possibility to create the writable Snapshots. Writable snapshots can be mounted and exported as a read-write file systems, share the same SavVol with read-only snaphots. There is no SavVol shrink. The SavVol grows to accommodate a busy writeable snapshot file system. The space cannot be returned to the cabinet until all snapshots of a file system are deleted.

You can create, delete and restore writeable snapshots. Writeable  snapshots are branched from baseline read-only snapshot. A baseline snapshot exists for the lifetime of the writeable snaphsot. Writable snapshots and their baselines cannot be refreshed or be part of a snapshot schedule.

In case of a restore from a writeable snapshot to a PFS the writeable snapshot must be remounted as a read-only file system before the restore starts.

Architecture of Writeable Snapshots

Writeable snapshots haves their own Bitmap and Blockmap. But they share the same SavVol with all snaphots within SnapSure technology on that particular PFS (Production File System).

Writeable Snapshot

Writeable Snapshot

WCKPT = Writeable checkpoint = writeable snapshot.

  1. We have Baseline bitmap and blockmap – so we can track all the changes that happens between baseline-snapshot and preset
  2. We have WCKPT biatmap and blockmap Рso we can track all the changes that happens between baseline-snapshot  and writes to writeable snapshot
  3. One the block is changed (on WCKPT) the new value is saved on SavVol. Once the same block is changed, the data in SavVol is overwritten. That means that if Bitmap points to 1 wihtin the write request, the Blockmap locates the block in SavVol, and that block is overwritten with the new value.

 Writeable Snapshots Рlimits

  • You can have only one writeable snapshot per baseline read-only snapshot.
  • There is a maximum of 16 writeable snapshots per PFS. Writeable snapshots do not count against the 96 user snapshot limit. All together there could be a total of 112 user snapshots per PFS. However, remember that VNX Replicator uses snapshots that are within this 96 limit, so if there are 95 read-only snapshots on the PFS, VNX Replicator will fail.
  • You cannot create a snapshot from a writeable snapshot.
  • You can create a writeable snapshot from a scheduled r/o snapshot. However, if the writeable snapshot exists when the schedule executes a refresh, it will fail.
  • Writeable snapshots cannot be used with VNX Replicator.
  • Writeable checkpoints can not be created from the VNX Replicator internal snapshots.
  • There is limited support for writeable snapshots with VNX FileMover. The support is limited to creation of stub files and recall of files for write operations.
  • If the amount of data written to the snapshot is big, that could cause two different outcomes
  1. SavVol automatically extending to accommodate the writes to the snaphost. Once a SavVol is extended, it cannot be shrunk, it can only be deleted and recreated, but for that you will have to delete ALL the snaphots for that PFS.
  2. If the Savvol will not extend (either the autoextention is disabled or there is no space available), SnapSure will begin validating older snapshots to acoommodate the writes to the checkpoint. That could cause data loss.



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