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Change Block Tracking file size

Created: 26 Oct 2012 | 6 comments
Alex Zn's picture

What additional size do i need to consider if i want to use Change Block Tracking technology for VM backup?

Additional 5MB of disk space per every 10GB of vmdk size.

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samueljn's picture

Do you mean the additional size in the Datastore?

Colin Weaver's picture

Kind of a question for VMware really

Alex Zn's picture

Probably you are right, but if you ask costumer to enable CBT, the resonable question from them "What should i consider if i do so?". So you may have an answer. I post it because people search for answer, let them find it here :) 

samueljn's picture

Changed Block Tracking (CBT) is a VMware feature that helps perform incremental backups. Virtual machines running on ESX/ESXi hosts can track disk sectors that have changed.

You can't have a specific number as a reccomendation as it is more based on how often the files are getting changed/added.

You may use this link for more info:

VJware's picture

You can either choose the default which you mentioned earlier or prolly calculate it manually...

1 ctk file for each vmdk file...further on, each vmdk block would correspond to one record on the ctk...and each record is a few bytes in it stores the USN or rather 'state' of the block... now this block size is a variable depending upon the vmdk size & grows accordingly....pretty messy ;)

I would say there is no fixed additional size for one to consider if CBT is enabled... I guess, the default recommendation should be good enough.