I was just reading through the BitLooker documentation
. It states that BitLooker will "zero out dirty blocks on the VM guest OS before it copies VM data to the target location." Does this actually issue any I/O to the production storage, or is it cleverly ignoring/substituting the dirty blocks as it sends them to the repository?
The situation is that I'm about to switch this on for a well-churned multi-terabyte VM, prior to an Active Full backup. I'd like to figure out if I'm about to zero out a large chunk of data, extending the backup window while consuming ginormous amounts of bandwidth, giving my users a bad day.
To recap, I'm asking if BitLooker will:
a) Write zeroes to the VM's disks (and thereby production storage), snapshot the VM, then compress the blocks during transport.
b) Snapshot the VM, write zeroes to an ephemeral snapshot (which would also wind up on production storage), then compress the blocks during transport.
c) Snapshot the VM, skip (or do inline replacement of) the changed dirty blocks during transport.
d) Something completely different?
Thanks for any clarification that you can provide!