Thin pool performance when allocating lots of blocks

Are thin volumes (which start as snapshots of a blank volume) efficient
for building virtual machine images?  Given the nature of this workload
(writing to lots of new, possibly-small files, then copying data from
them to a huge disk image), I expect that this will cause sharing to be
broken many, many times, and the kernel code that breaks sharing appears
to be rather heavyweight.  Furthermore, since zeroing is enabled, this
might cause substantial write amplification.  Turning zeroing off is not
an option for security reasons.

Is there a way to determine if breaking sharing is the cause of
performance problems?  If it is, are there any better solutions?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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