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?

  • Is this on a shared volume of some sort? Like a LUN on a SAN? – Basil Aug 30 '12 at 15:17
  • @Basil - it's on a Lab SAN, why do you ask? – warren Aug 30 '12 at 15:21
  • Zeroing can sometimes be recognized by the SAN and offloaded to it, depending on the model. – Basil Aug 30 '12 at 16:40

Eager Zeroing writes 0s to all empty blocks. This slightly increases performance later on when those blocks need to be written to for the first time.

Lazy Zeroing does nothing to the unallocated blocks. If you have an 80GB VMDK and only 10GB worth of data, only 10GB worth of blocks are used and the rest is left as-is until needed. If this example were eager zeroed, at the time of creation all 80GB would be written to, with zeroes filling the remaining 70GB.

If you're using thin provisioning at the storage level, you want to avoid eager zeros, as that will completely defeat the purpose of thin provisioning.

When would 'eager' be the right mode to use?

Eager is the right mode to use when you're not thin provisioning LUNs and you don't mind waiting a bit longer for the VMDK to be created.

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    I always eager zero where possible to avoid the .vmdk's being fragmented, and to be confident I won't run out of space on the host if the drives in the vm's are filled. – Robin Gill Aug 30 '12 at 14:55
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    How would writing 0s to a volume increase its performance...? – Basil Aug 30 '12 at 15:15
  • @RobinGill - how can you run out of space in a thick-provisioned drive? – warren Aug 30 '12 at 15:21
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    @ warren - oops got mixed up (I'm still getting to grips with ESXi). – Robin Gill Aug 30 '12 at 15:44
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    So it's not like another file system that can simply write over the garbage data in uninitialized sectors? – Basil Aug 30 '12 at 16:39

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