>> is there any validity to thinking that thick provisioned disks might have their blocks less spread out and hence, faster disk access?
Results will differ in a very wide range - just consider this simple example:
A newly created 10Gb virtual disk is partitioned for the first time: Effectively there will be writes to the first MB and to the last MB. So you get writes to 2 one-mb blocks.
Thick format: vmdk may get allocated in one piece. So there is one lookup in the vmdk-to-phsysical-location mapping table plus 2 one-mb writes. Distance between both writes is 10Gb.
Thin:vmdk will use 3 fragments - 2 allocated blocks plus one large reference to /dev/zero. Effectively there are 2 lookups required to find the 2 allocated blocks plus 2 small writes.
Now depending on the state of the VMFS-volume the distance between both writes will vary between just one MB and several TB - depending on the size of your datastore.
If you now do benchmarktest you probably get inconsistent results.
If the 2 needed one-mb-blocks are allocated next to each other you may get the best performance results for thin.
If the 2 needed writes are allocated with a distance of TBs you may also get very poor results for thin.
In this example results for thick would be just average.
I assume that this effects will spoil any serious attempt to produce a widely accepted benchmark-result.
Also the Thinprovisioning feature is unbeatable when it comes to buying the next hardware.
A company that decides to allow thin-provisioning may get away with a 10TB SAN - if the decision is to allow only thick provisioning they may have to buy a 30TB SAN.
So in this point of view thin - vs - thick is easy to decide : thin is so much cheaper.
How large would a performance disadvantage of thin need to be, to really make a difference in the sales decision ?
I guess often even a 20% performance disadvantage would not overrule the radically lower price of thin provisioning.
In my opinion this whole performance discussion : thin vs thick is moot and pure theory.
From my point of view (my job is the repair/recovery of dead VMs) it simply boils down to:
Do I regard this VM as disposable ? - and in case of a "NO" - do I have a tested and ready-to-use backup or replica for this VM ?
If the answer is unsure or No thin-provisioned vmdks should be avoided because of their significantly higher failure risk.
Unfortunately I see a lot of risky use of thin provisioned vmdks especially in small environments.
In environments large enough to expect failures as regular part of the daily routine and have a policy of "We always replace any production VM that fails - we never repair or try to recover" the higher failure rate can be tolerated.