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thorsfall
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VeeamZIP (Free) enables CBT. Why?

Post by thorsfall » Dec 03, 2013 3:50 am

I was previously trialling Veeam B&R 6.5 for backing up VMs in my environment, and I have since let the trial licence lapse so it's reverted the the "Free" product. That's fine for the basic testing I want to perform, i.e. ad-hoc VM backups. If I can't get this to work, then there's no way I'm forking over $$.

I am able to backup VMs from my ESXi 5.1 server just fine, but I am a little puzzled about something. I'm using the "Free" product's ZeeamZIP operation to make a backup of a single powered-off VM. Why is it enabling CBT? The "free" version cannot make an incremental backup, so all this is really doing is hurting my I/O times and filling up my limited VMFS datastore disk space. Is this meant to be happening, or is this a bug?

In the meantime, I'm forced to manually disable CBT after every ad-hoc backup. Very annoying. Has anyone else experienced this?

Trevor

foggy
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Re: VeeamZIP (Free) enables CBT. Why?

Post by foggy » Dec 03, 2013 9:54 am

Trevor, during VeeamZIP CBT is used to filter out unused blocks, which makes the full backup to complete much faster.
thorsfall wrote:so all this is really doing is hurting my I/O times and filling up my limited VMFS datastore disk space.
Could you please clarify what do you mean by this?

Gostev
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Re: VeeamZIP (Free) enables CBT. Why?

Post by Gostev » Dec 03, 2013 7:12 pm

CBT has no measurable impact on I/O performance, and it is most certainly not capable of "filling up" VMFS datastores.

thorsfall
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Re: VeeamZIP (Free) enables CBT. Why?

Post by thorsfall » Dec 04, 2013 3:29 am

Enabling CBT causes the creation of a "-ctk.vmdk" file for each VM that has been backed up. This file is now taking up space on the VMFS datastore (for no reason at all, as it is never used), and being written to as data in the real VMDK file changes. Couple this with a development environment running on a Mac Mini (yes, legal Apple OS X VMs) with its woeful 5400rpm disk, and trust me every additional I/O hurts.

VMware previously published advice that CBT defaults to the disabled state due to the (albeit small) performance impact of having it enabled, especially when snapshots come into play. This is immediately offset by the massive improvement in incremental backup performance when using Veeam or VMware backup products, making it well worth enabling, but I'm not running incremental backups.

This is all probably beside the point - my question is far simpler: Why is the free Veeam B&R product enabling CBT for a VeeamZIP operation? It doesn't even support incremental backups, so it will never use CBT as far as I can tell.

Vitaliy S.
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Re: VeeamZIP (Free) enables CBT. Why?

Post by Vitaliy S. » Dec 04, 2013 8:21 am

Hi Trevor,

To the best of my knowledge CBT is used to avoid copying of zero blocks and filter unused blocks of the VM, though you're right VeeamZIP does not perform incremental ad-hoc backups.

Thanks!

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Re: VeeamZIP (Free) enables CBT. Why?

Post by Gostev » Dec 04, 2013 9:17 pm

thorsfall wrote:my question is far simpler: Why is the free Veeam B&R product enabling CBT for a VeeamZIP operation?
Actually, foggy has already answered this question, see the first response to your original post. Without CBT, VeeamZIP would have to read every single block, including unused ones. CBT allows VeeamZIP to skip reading known unused blocks, both speeding up the backup speed, and reducing stress on your storage.

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