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alx9r
Asked: 2013-04-02 08:42:17 +0800 CST

What are the tradeoffs between using hypervisor- vs. storage-array-based snapshots as backup sources?

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Suppose you have some virtual machines (running in eg. ESXi) that are stored on an iSCSI storage array (eg. EqualLogic PS4110). Now suppose you want to set up a backup and restore regime that looks something like this:

  • VMs are quiesced
  • snapshots are taken
  • snapshots are copied to tape
  • snapshots are deleted
  • tapes are taken offsite
  • disaster occurs
  • tapes are retrieved from offsite
  • snapshots are copied from tape
  • VMs recovered from tape are re-started

There seems to be two obvious ways to snapshot the VMs:

Specific questions:

  1. What are the tradeoffs I should consider between these two methods of achieving snapshots for the above backup regime?

  2. Are there some fundamental benefits of each of these snapshot methods that apply regardless of the purpose of the snapshot?

backup

2 Answers

  1. There are different ways to do it, below is an example how it is done using Commvault Simpana:

    1. What happens when you backup VMs using VADP only

      • Backup software instructs vsphere to snapshot VMs
      • VMs are snapshotted
      • Backup software copies data using snapshots to backup media.It may also take advantage of other VADP features like CBT ( Change Block Tracking) to help speed up the process
      • VMs snapshots are released.

    2. What happens when storage hardware snapshot is used

      • Backup software instructs vsphere to snapshot (also called quescing process) VMs. This is done to ensure VMs are in state ready for backup ( i.e. buffers are flushed etc).
      • Backup software instructs storage array to create hardware snapshot.This normally takes seconds.
      • Backup software instructs vsphere to delete VM snapshots.From this point, production VMs will not be affected and all backup processes will be done on snapshotted datastore.
      • Backup software instructs ESX server to mount hardware snapshot ( i.e datastore).When this is completed, VMs ( or rather their copies) are registered, the content indexed, if needed, and backed up to media using VADP method as described above.
      • Backup software instructs ESX server to unmount snapshotted datastore and also instructs array to delete hardware snapshot.

    As you see , you have following advantages when using hardware snapshot:

    • you only need to hold VM snapshots very short period - i.e untill all VMs you want to backup are snapshotted. Using first method however requires holding snapshots for entire backup time.
    • you can make several VM backups during the day and keep them in hardware snapshot without copying data to media.
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  2. I think that your Dell storage supports VAAI, which includes copy offloading. I don't claim to be an expert on VMWare or Dell storage, but I found a page that explains it from VMWare's perspective. I know that full copies can be offloaded to the storage because we do that here with an HDS. They say that VM Snapshots can be offloaded to the storage, but I haven't ever tried it.

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