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Why does "eager" vs "lazy" zeroing take much longer when provisioning a VM from a template?

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http://serverfault.com – Slightly related to a previous question about thick vs thin provisioning, why does eager zeroing the to-be-cloned VM take longer than lazy zeroing? For example, in a recent test we ran, an eager-zeroed clone took ~85 minutes to complete, whereas changing to lazy zeroing made it complete in ~50 minutes. How much overhead is incurred when eager zeroing vs lazy? When would 'eager' be the right mode to use? (HowTos)