Regarding the last post above about eager zero, I just want to say that I have also experienced similar problems with VMware 5.x and equallogic.
Problems are not specific to backup repository but to any large write to un-zeroed blocks.
for example - deploy template, copy files within guest os, etc...
My workaround was to disable
Set it to 0 in the esxi host configuration (advanced settings). No restart needed.
VMware KB Disabling the VAAI functionality in ESXi-ESXhttp://kb.vmware.com/selfservice/micros ... Id=1033665