Host-based backup of VMware vSphere VMs.
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coolgod
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Questions on Changed Block Tracking (CBT)

Post by coolgod »

what mechanism is used to determine changed block by veeam when CBT is not used ?

what's the performance and processing impact when CBT is disabled ?
Whatever the mind of man can conceive and believe, it can achieve. - Napoleon Hill
Shestakov
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Re: Questions on Changed Block Tracking (CBT)

Post by Shestakov »

Hello,
coolgod wrote:what mechanism is used to determine changed block by veeam when CBT is not used ?
When CBT is not in use it fails over to Veeam’s proprietary filtering mechanism.
Instead of tracking changed blocks of data, VBR filters out unchanged data blocks. During backup, VBR consolidates virtual disk content, scans through the VM image and calculates a checksum for every data block. Checksums are stored as metadata to backup files next to VM data. When incremental backup is run, VBR opens all backup files in the chain of previous full and incremental backups, reads metadata from these files and compares it with checksums calculated for a VM in its current state. If a match is found (which means the block already exists in the backup), the corresponding block is filtered out.
coolgod wrote:what's the performance and processing impact when CBT is disabled?
If you disable CBT same blocks will be filtered, but with less efficient mechanism, means it will take more time.
Thank you.
coolgod
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Re: Questions on Changed Block Tracking (CBT)

Post by coolgod »

thanks shestakov for clarifying this.
Whatever the mind of man can conceive and believe, it can achieve. - Napoleon Hill
NightBird
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Re: Questions on Changed Block Tracking (CBT)

Post by NightBird »

What about this VMware kb ?
http://kb.vmware.com/selfservice/micros ... Id=2090639

What do Veeam think about it ?
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Re: Questions on Changed Block Tracking (CBT)

Post by VladV »

Interesting. Why would invalidating the CBT by expanding the disk jeopardize a running VM? Curious about the explanation on why this would happen.
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Re: Questions on Changed Block Tracking (CBT)

Post by Vitaliy S. »

NightBird wrote:What do Veeam think about it ?
When virtual disk allocation is changed, then Veeam backup server uses our own proprietary change tracking engine to transfer changed data. See this topic for more info > Procedure for expanding virtual disks
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Re: Questions on Changed Block Tracking (CBT)

Post by NightBird »

So what about Gostev forum digest post ? if Veeam is so magical ;)
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Re: Questions on Changed Block Tracking (CBT)

Post by TommyB »

NightBird wrote:So what about Gostev forum digest post ? if Veeam is so magical ;)
Regarding Gostev's email and the CBT issue - I would like to know if there is any way to find out if my VM's are actually affected by this bug?
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Re: Questions on Changed Block Tracking (CBT)

Post by foggy »

Only actual restore attempt (in a virtual lab, using SureBackup, for example) will tell you whether the restore point is recoverable. Data integrity on data disks should be verified manually.

Btw, there's a separate discussion regarding that CBT issue, covering similar questions. Worth reviewing.
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Re: Questions on Changed Block Tracking (CBT)

Post by TommyB »

Thanks foggy :D
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