Impact on VMware and Hyper-V Snapshots on Backup Technology

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What is a Snapshot?
“Snapshot” is a most used term in the storage industry to explain the capturing a volume state at a certain point in time. Most snapshots are firstly capacity-neutral. The blocks of a volume are naturally set to read only and updates to those blocks are written to a separate disk area. How these changes are tracked impacts the performance of the volume and the quantity of snapshots that can be taken and managed.


VMware Snapshots
When VMware issues a snapshot or when it’s told to by a software application or storage array it primarily creates a redo log for every VM’s snapshot. Changes to the VM’s primary disk volume are recorded within the redo log till the snapshot has expired. VMware can permit multiple snapshots per VM however because the variety will increase and also the length of your time that the snapshot is in impact, performance will degrade as a result of every represents another redo log that VMware should keep track of.

The length of your time that the snapshot is existing can jointly impacts performance. Once the snapshot has expired the recorded changes are rolled into the VM’s primary volume. To try this VM has to be “stunned” that is essentially a awfully short, usually milliseconds, pause of the VM. However, there square measure times, once rolling a awfully massive redo log into the first volume, that multiple and longer overwhelming stuns ought to occur. This could cause application degradation and even application lockup. As a result backup applications ought to end their use of snapshot knowledge quickly so fewer changes ought to be rolled back in.

Hyper-V Snapshots
Hyper-V handles the snapshot process differently than does VMware and it’s something that backup applications that count on snapshots need to be aware of. Hyper-V is more of a classic snapshot technology. The virtual volume is set to read-only and as changes to blocks come in, the old block to be updated is copied to a snapshot volume and the new block is written into the active virtual volume. This eliminates the need for a rollback and the potential issues that stunning a VM can cause. When a snapshot has expired Hyper-V has to simply drop the association to the snapshot and start using the primary volume as normal.

However Hyper-V snapshots do have their limitations. in all probability the largest issue for backup vendors is that it will solely have one active and accessible snapshot per VM. Also, once a snapshot expires it’s not deleted from disk till the virtual machine is rebooted. However the backup software package handles this distinction from the VMware methodology is important. If they figure the snapshot technique and one is already active for the VM, meaning they’re either aiming to ought to delete it or use what’s there. the matter with deleting and taking a replacement up-to-date snapshot is there could also be different tasks active that are looking forward to this snapshot, sort of a replication job or maybe a coverage perform. The matter with investing a snapshot that’s already in place is that the backup application isn’t obtaining the most recent version of the information which is able to impact the accuracy of the recovery.

There is another however; Hyper-V will support Volume Shadow Copy Service (VSS) that permits storage arrays or backup software to trigger volume level snapshots then maintains those snapshots. This brings important edges. First the number of snapshot that may be unbroken are currently restricted to the capabilities of the storage system itself. An increasing variety of storage systems will currently maintain close to limitless snapshots with very little to no performance loss. Second, it provides the backup trafficker the aptitude to use a typical interface to speak with a range of storage systems while not having to singly produce support for every one

Bridging the Differences
Backup applications have always had to deal with differences in what various platforms would be able to support. They have to balance how to provide similar functionality between platforms without manually recreating missing capabilities unless absolutely needed.

  Hyper-V, Storage and Virtualization, VMware
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