PerfectDisk’s Zero-fill free space on thin-provisioned disks

In a previous post, I wrote about the problem of overprovisioning in data centers due to thin provisioning. To deal with this, IT professionals use the SDelete (Secure Delete) command line utility, with all its limitations.

As a followup, I will give a more detailed look at PerfectDisk’s Zero Fill feature, a fully automated alternative to SDelete that also has additional benefits over SDelete. PerfectDisk uses a standard Microsoft installation to install on any Windows guest system. It optimizes all the files on the guest and consolidates the free space into the largest possible contiguous chunk.

It then can optionally zero-fill the consolidated free space, offering several advantages over the S-delete utility. First, PerfectDisk does not consume all the free space on the disk, so applications can remain in use while it is running. Second, the zero-fill process can be scheduled to run standalone or after an optimization pass. The scheduling is just a few clicks in the Scheduling wizard; no scripting or messing with Task Scheduler is needed.

There may be some growth on the thin-provisioned disk when PerfectDisk optimizes, but if the disk is identified as thin-provisioned, PerfectDisk will use special algorithms that minimize storage growth. The same VMotion operation is required after the zero-fill operation to recover the free space. As with S-delete, be sure to do the VMotion to another datastore in order for the SHRINK to take place.

Zero-Fill Performance Benefits

Aside from shrinking the virtual disk, PerfectDisk offers several additional benefits not available with SDelete.  In any virtualized environment, there are several key performance metrics to watch. PerfectDisk can have a significant impact on:

  • total I/O operations (IOPS)
  • disk latency
  • throughput.

File fragmentation increases the number of SCSI commands that need to cross the virtualization storage stack to read a file.  For example, a file in 10 fragments generates 10 SCSI commands while the same file in one piece generates a single SCSI command.  PerfectDisk reduces the number of IOPS processed by the virtualization layer and lowers the associated resource demand for CPU and memory.

Each SCSI command requires one or more physical accesses to the disk. Since defragmentation reduces IOPS, this means the IOPS it does produce are larger. Fewer and larger IOPS mean fewer physical accesses to the disk. Disk accesses are a factor in disk latency, the time it takes an I/O to complete. Since PerfectDisk reduces the total physical accesses to the disk, the latency improves. In testing on a VMware platform, the number of I/Os taking over 30ms (VMware’s definition of slow I/O) was cut by 50%.

The combination of fewer and larger I/O means less hypervisor overhead and fewer physical disk accesses, so more work gets done per unit of time. Testing of software installations showed PerfectDisk improved the elapsed time to install by 25-33% over the fragmented disks.

If a thin-on-thin environment is using a zero-detect SAN, the hardware will be able to recover the free space PerfectDisk zero-fills after the compaction of the virtual machine.


While thin-provisioning saves labor in the data center, it has the potential to waste a great deal of disk space. Until now, the only way to remedy this situation was to use a manual utility that zero-fills the free space when the virtual machine is not in use. This, in combination with a VMotion under VMware, shrinks the disk and recovers unused space.

PerfectDisk provides the thin-provisioned disk user with an automated solution to zero-filling the free space, and allows this to happen while the virtual machine is in use. A compound benefit is that a defragmented Windows guest system reduces hypervisor overhead and disk latency while improving system throughput. The zero-fill capability is included with PerfectDisk Server, PerfectDisk Exchange and PerfectDisk vSphere and PerfectDisk Hyper-V.

Check it out: Zero Fill Free Space

Related Post:

An SDelete alternative for recovering disk space from thin-provisioned disks

Category: PerfectDiskStorageVirtualization


Leave a Reply