Merging snapshots - calculating how large a VHD will grow

on 2008 r2: have 100gb dynamic vhd 10gb used inside vm (so vhd 10gb), , took snapshot, created 50gb file inside vm. deleted 50gb file inside vm, had 10gb being reported used in vm again, deleted , merged snapshot. end 60gb dynamic vhd after merge, or 10gb?

i understand best practices or stated maximum space required merge being 2x of avhd, curious how works in experience or "under hood". i'm planning merge weekend similar scenario above - logfile grew on 200gb, deleted it, , wanted know if grow vhd ~200gb, or if it's unlikely to.

thanks thoughts. 

snapshots / checkpoints use differencing disks under hood.

differencing disks block based technology mimic physical disk block style layout - there technically no relationship between size of differencing disk , resulting size of merged disk.

in vast majority of cases merge result in larger root disk - due fact more files added replaced.

and, differencing disks dynamic.  means grow on write, not shrink on delete - blocks (space) gets allocated , remains allocated on underlying file system.

having 2x avhd guidelines merging process - has enough available storage both efficient , resilient (the ability not lose data).

because during merge, happens block in differencing disk replace of parent overlays.  attempted describe long ago:

http://itproctology.blogspot.com/2009/04/never-resize-vhd-with-snapshots-or.html

is possible merge , have root disk not grow in size @ all?  yes, is.  if change in time past snapshot not additive - such temporary temp files.

but, there no science here.  there no way know sure how root disk end growing.

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brian ehlert
http://itproctology.blogspot.com
learn. apply. repeat.

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