There are a couple different ways you can design a Provisioning Services Server supported environment to boot up. Virtual Desktops can leverage PXE, Hard Drive or ISO. This week, a larger environment was set up with ISO boot disks and we ran into the following pattern detailed below.
Seemed like every couple (8 to be exact) VMs successfully registered with the DDC and then the next 2 failed. This pattern occurred down the list. After some Google-Fu, I stumbled across this VMware Forum post.
I believe in 4.x and 5.x the limit was 32 VMs that can have a file handle open. In 5.1 that increases to (I believe) 128. Also, in 4.x and 5.0, no more than 8 hosts can access a given file. In 5.1 this is increased to 32.
In our vSphere 4.1 clusters, we had 10 ESXi hosts. 2 more than the supported number of hosts that could establish access the ISO at any given time. Switching back to the vSphere console, we confirmed that all unregistered VMs were in fact isolated to 2 hosts.