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We run VMWare ESXi 5 and are looking at backup solutions. We found Veeam backup free edition and thought we would try it.

At the start of the backup wizard, there is an option to disable "quiescing the guest (for crash consistent backup)". I've searched the Veeam site but cannot understand what this means.

If I leave this unchecked (default), does it mean my backup is not "consistant"? I thought Veeam did a snapshot of the disks and backed up the snapshot.

Can someone explain this? Is this a safe way to backup? (Guaranteed to restore 100%)?

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Note that the free edition of Veeam Backup & Replication doesn't allow for scheduled backups and it doesn't allow backing up more than one VM at a time, so this could prove onerous if you have a number of VM's that need to be backed up on a fairly regular schedule. - veeampdf.s3.amazonaws.com/datasheet/… –  joeqwerty Oct 2 '13 at 3:21

2 Answers 2

VMware snapshots can be done in two ways.

One way involves VMware telling the VMware Tools instance in the guest virtual machine to use a shadow writer to freeze I/O to the virtual disk so VMware can take a consistent or "quiesed" snapshot.

You also have the option of NOT using the VMware Tools instance to initiate a guest-side quiescing of the file system, this results in a crash-consistent backup. It would be like yanking the power cord out of a server while running - it'll most likely come up fine but there is a greater chance that data is lost or corrupt.

In some cases the application simply cannot tolerate a shadow copy operation on the file system, I have seen this before many times. Yes, you get a good backup, but you crash the application in the process. Custom apps are the worst for this.

As soon as the snapshot is taken, regardless of type, Veeam backs up the snapshot to its backup target. Once the copy of the snapshot is made, Veeam deletes the VMware snapshot (consolidating the VMDK with the delta changes that occurred during the backup operation). This is pretty normal as far as VMware backups go.

Quiescing the file system prior to taking a snapshot is the best way to go unless there is a specific requirement not to do so. The best place to look for details on this would be on VMware's web site as they have multiple KBs on working with snapshots.

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1009402 http://www.computerweekly.com/feature/VMware-backup-and-the-VMware-snapshot

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The idea is that any time you snapshot a mounted filesystem, it is going to be "unclean" due to the fact that there may be un-committed IO requests that are still held up in one of the many buffers within the kernel. When you "quiesce" the filesystem, ESXi tells the OS to try and flush out all pending writes, thereby making the filesystem closer to clean, but it will never be perfectly clean unless it's fully un-mounted.

So, snapshots taken of live filesystems should be viewed as a "restore of last resort" type of proposition due to the fact that you can never be 100% certain that the filesystem will be without corruption or missing data.

To guarantee a fool-proof backup, you'll either need to unmount filesystems before taking a snapshot (which, in most cases means shutting down a VM) or use a traditional backup tool like amanda, netbackup, etc.

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