A. There are several ways for virtual hard disks to be stored with ESX:

  • Zeroed thick: VMDK files have all space allocated at time of creation, but the blocks that make up the VMKD aren't erased until the VM tries to access the block.
  • Thick: Same as zeroed thick, but blocks aren't erased when accessed for the first time. This means that if a thick VMDK is created over the same location as a previous VMDK and the application just tries to read certain blocks of data, the old data will be read, posing a security risk.
  • Eager zeroed thick: Same as zeroed thick, except all the blocks that make up the VMKD are cleared when you create the VMDK.
  • Thin: Data blocks are only wiped and allocated as the VMDK uses them.

If you perform a storage vMotion with vCenter 2.5, the target disk will be a thick disk, even if the source was a thin disk. While there are ways to keep the disks thin, they're very complex procedures. The ideal solution is to upgrade vCenter to version 4, which supports keeping VMDKs in their original format. The hosts can still be ESX 3.5 and be managed by vCenter 4.

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