Thick Provisioned virtual disks – Eager or Lazy?

It’s been a while since I posted anything, and that’s just mainly down to how busy I’ve been since the turn of the new year!!

I guess it’s good to be busy as it occupies the mind! =)

Although really busy, the MTI TAMs still managed to fly over to Cork to visit EMC’s factory….. it was my first time there and I must say I was very impressed…. we got to see all the processes behind testing and building the EMC storage arrays (VMAX, VNX) and also how rigorous the testing process was!! Amazed at how much investment EMC have put into their testing equipment (temperature, vibration, etc). Supposedly if it’s tested to bits, there is less chance of an DoA (Dead on Arrival) when shipped and also stops people ringing support to complain about their array not functioning! =)

 

Anyways, the reason for this post is I had to explain briefly the difference between Eager Zeroed and Lazy Zeroed for Thick Provisioning within VMware.

I believe the default format for Thick Provisioned virtual disks is ‘Lazy Zero’.

Thick Provision Lazy Zeroed
When selected this basically means that the space required for the virtual disk is allocated when the virtual disk is created but the blocks on the physical drives are not erased (zeroed) during creation, but will be zeroed out on demand at a later time on first write from the virtual machine – hence the ‘Lazy’ terminology.

So as you can imagine there’s a slight overhead when writing to a block for the first time (subsequent writes to the same block does not require it to be zeroed).

Thick Provision Eager Zeroed
When selected this basically means that the space required for the virtual disk is allocated when the virtual disk is created. However, the data remaining on the physical device is zeroed out when the virtual disk is created.

Because it has to zero out the whole space allocated to the virtual disk at creation, this means it might take longer to create the disk over say Lazy or Thin.

 

The difference in performance between the two is minimal (Read is obviously not affected)…. However, just bear in mind that if they’re using MSCS or VMware FT you will need to use Eager Zeroed Thick.

Advertisements