VM with CBT disabled

I am reviewing the output of a vmware health check. one issue flagged is VM with CBT disabled, which goes onto say List all VMs with CBT status disabled. It's not a good option for backup!, can anyone elaborate in management freindly terms the risks here? if any?
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pma111Asked:
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
If you have CBT - Change Block Tracking disabled.

There is no real risk to the function of the VM, but you have disabled an important feature which enables VMs to be backed up quicker, because with CBT enabled, a Backup application can work out which blocks have changed, and only copy those changed blocks. Benefit to the business, backups are faster, smaller backup window, and less performance impact on the VM.



from Anton Gostev from Veeam on CBT:

In essence, CBT is all about CTK files, these are the files which contain change tracking information of the corresponding VMDK file.

The concept is pretty simple, and if you are familiar with AD DirSync control, or Exchange ICS (public folders change tracking) – it is essentially the same: global USN (Update Sequence Number) for each object. CTK file describes the state of each block for tracking purposes, and contain USN for each block in the corresponding VMDK. After any block is updated, it is assigned the new global USN (which is previous USN value that was used on previously processed block plus 1). This way, any application can ask VMware API “tell me if this block was changed since THIS moment”, and the API will easily tell that by simply comparing the provided sequence number with the actual USN on each block. If provided USN is smaller than actual for particular block, it means that the block was changed (and needs to be backed up, replicated or otherwise processed). So multiple processes cannot conflict with each other anyhow. Each process just memorizes the USN corresponding to the snapshot that the application created during processing, and next time it will use the memorized USN to query for changed blocks.

There should be one CTK file per VMDK file, and CTK file cannot grow out of proportion with number of blocks in VMDK (as it stores only 1 record per VMDK block). CTK file is also thousands time smaller than actual VMDK, because it stores only a few bytes of information (USN) for each corresponding 256KB VMDK block (I am 90% sure it is 256KB, used to calculate it once using CTK debug/stats data, just don’t remember for sure – unimportant info escapes my head automatically to prevent overload with useless facts ;) . For the same reasons, I/O overhead is barely noticeable with CBT: change few extra bytes to write for each 256000 bytes of data.

The CTK files are permanent, and should not be deleted after backup/replication.

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Paul SolovyovskySenior IT AdvisorCommented:
CBT is change block tracking which takes all changed blocks in a VM and puts in a journal log.  When a backup occurs it just sweeps the journal log for changed blocks instead of going into the VMDK and finding each changed block individually.  This in turn expedites the backups (incremental/differential).  Full backups will still backup all the used blocks in on the VMDK.
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