This is my contribution to the debate Zero or Thick disks – debunking the performance myth.
The last couple of years all sorts of VMware engineers worked very hard to reduce the performance difference between thin disks and thick disks. Many white-papers have been written by performance engineers to explain the improvements made on thin-disk. Therefore today the question whether to use Thin-provisioned disks or Eager zero thick is not about the difference in performance but the difference in management.
When using Thin-provisioned VMDKs you need to have a very clear defined process. What to do, when your datastore, which stores the thin provisioned disks is getting full? You need to define a consolidation ratio, you need to understand which operational process might be dangerous to your environment (think Patch-Tuesday) and what space utilization threshold you need to define before migrating thin-provisioned disks to other datastores.
Today Storage DRS can help you with many of the fore mentioned challenges. For more information please read the article: Avoiding VMDK level over-commitment while using Thin-provisioned disks and Storage DRS.
If Storage DRS is not used, Thin-provisioned disks can require a seamless collaboration between virtualization teams (provisioning and architecture) and storage administrators. When this is not possible due to organizational cultural differences, thin provisioning is rather a risk, than bliss.
Zero out process: Eager zero thick on the other hand might provide in some (corner) cases a marginal performance increase; the costs involved could outweigh the perceived benefits. First of all, Eager zero thick disks need to be zeroed out during creation, when your array doesn’t support the VAAI initiatives, this can take a hit on performance and the time to provision is extended. With terabyte sized disks becoming more common this will impact provisioning time immensely.
Waste of space: Most virtualized environments use virtual machines, typically configured with oversized OS disks and over-specced data disks, resulting in wasted space full of zero’s. Thin-provisioned disks only occupy the space used for storing data, not zero’s.
Migration: Storage vMotion goes out of its way to migrate every little bit of a virtual disk, this means it needs to copy over every zeroed out block. Combined with the oversized disks, you are creating unnecessary overhead on your hosts and storage subsystem copying and verifying the integrity of zeroed out blocks. Migrating thin disks only requires migrating the “user-data”, resulting in faster migration times, lesser overhead on hosts and storage subsystem.
In essence, Thin-provisioned disks versus Eager zero thick is all about resource/time saving versus risk avoidance. Choose wisely