Monthly Archives: September 2012

VHDX file investigations on regular spinning disk

Refer my earlier blog on VHDX file best practices. This blog explains the details behind how I arrived at those recommended best practices. Based upon a TechEd talk, I ran a series of experiments with VHDX files. While the investigations led to the summary published in my previous blog, there also remain some unanswered questions. So far, my attempts to find answers to these questions have remained unfruitful. Presumably the appropriate Microsoft personnel are extremely busy. Once I publish what I found, I will publish my questions as purely that – questions, in the hope that some fellow MVPs may have some insight.

For the investigation, I created a Windows 8 based guest VM running on a Windows Server 2012 Hyper-V host. I created a dynamically expanding VHDX file and attached it as a SCSI attached volume to the Windows 8 guest. The investigation consists of performing a series of operations on the Windows 8 SCSI attached volume and observing the size of the VHDX file backing that SCSI attached volume. For the purposes of this particular blog, the VHDX file was placed on an external USB spinning hard disk, which was not thin provisioning capable.

Operation

VHDX file size in   bytes

Create an   empty dynamically expanding VHDX file that is not yet initialized and not yet   NTFS formatted

4,194, 304

 

Initialize   the disk as MBR partitioned & NTFS format the volume

205,520,896

Copy a   single file of size 949,350,400 into the VHDX file

1,178,599,124

Delete the   file, make sure it is deleted from recycle bin as well

1,178,599,124

Shut down   the VM

1,178,599,124

Run PS   cmdlet Optimize-VHD with Mode Retrim on the VHDX

1,178,599,124

 

The fact that Optimize-VHD did not shrink the VHDX file was both a little surprising and disappointing, but presumably the error is mine. After numerous presentations that claimed the TRIM/UNMAP commands flowed from the guest VM into the parent partition, I was under the impression that given a Windows 8 guest OS and a Windows Server 2012 Hyper-V host, the TRIM/UNMAP commands flowed natively. Perhaps they do, and I don’t understand it. But requiring a PS cmdlet to make them flow is not what I would call native.

The next experiment I performed was to see if the VHDX file reused space freed up

Operation

VHDX file size in   bytes

Create an   empty dynamically expanding VHDX file that is not yet initialized and not yet   NTFS formatted

4,194, 304

 

Initialize   the disk as MBR partitioned & NTFS format the volume

205,520,896

Copy a   single file of size 949,350,400 into the VHDX file

1,178,599,124

Delete the   file, make sure it is deleted from recycle bin as well

1,178,599,124

Copy the   file again

2,118,123,520

 

Note: The implication is quite obvious. Without any intervention, the VHDX fie does not neccesarily reuse the recently freed up disk space, even when the amount of new space required is exactly equal to the recently freed up space

The next experiment I consisted of using the PS cmdlet Optimize-Volume, again with the ReTrim option.

Operation

VHDX file size in   bytes

Create an   empty dynamically expanding VHDX file that is not yet initialized and not yet   NTFS formatted

4,194, 304

 

Initialize   the disk as MBR partitioned & NTFS format the volume

205,520,896

Copy a   single file of size 949,350,400 into the VHDX file

1,178,599,124

Delete the   file, make sure it is deleted from recycle bin as well

1,178,599,124

Run PS   cmdlet Optimize-Volume with Retrim option

1,178,599,124

Shut down   the VM

205,520,896

 Note: Shutting down the VM is not really an option in production systems, but it certainly is an option for people like my fellow MVPs running VMs on their laptops.

And now, let me describe the last, but most important experiment for this blog. I expect to write more blogs on this topic.

Operation

VHDX file size in   bytes

Create an   empty dynamically expanding VHDX file that is not yet initialized and not yet   NTFS formatted

4,194, 304

 

Initialize   the disk as MBR partitioned & NTFS format the volume

205,520,896

Copy a   single file of size 949,350,400 into the VHDX file

1,178,599,124

Delete the   file, make sure it is deleted from recycle bin as well

1,178,599,124

Run PS   cmdlet Optimize-Volume with Retrim option

1,178,599,124

Copy the   same single file again

1,178,599,124

 

Note: The implication again is quite obvious. Running the PS cmdlet Optimize-Volume allows the VHDX to reuse the newly freed up space.  

 

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Best practices for utilizing VHDX files in Windows Server 2012

A number of blogs and presentations have explained the advantages of VHDX files in Windows Server 2012 over the VHD files in Windows Server 2008. These advantages include:

  • VHDX files can host larger data volumes – in particular, VHDX files can host 64 TB volumes versus 2 TB for VHD files
  • VHDX files have  their metadata aligned on 1MB boundaries whereas VHD files have 512 byte sized metadata that cause alignment issues and in particular, cause significant slowdown when used with newer disks that use 4096 byte sectors instead of 512 byte sectors
  • VHDX files are more resistant to corruption

However, VHDX files have one more huge advantage that has not been as adequately explained, or at least, seems to be less widely known. This is the fact that VHDX files are much more disk space efficient in a number of ways, especially so when the system administrator follows proper best practices. This blog and some successor blogs will explain the necessary and sufficient conditions for each particular use case, and also lay out the best practices to obtain the maximum benefit.

In particular:

  • VHDX files declare themselves to be storage volumes capable of thin provisioning
  • When used properly, VHDX files based on appropriate type of storage can free up disk space and utilize the thin provisioning features of the underlying storage
  • Even when VHDX files are placed on regular storage, best practices will prevent the VHDX files from growing in size by reusing free space within the VHDX file

So what are the necessary and sufficient conditions to take advantage of these VHDX file features? They can be summarized as:

  • These tips apply to running Windows Server 2012 and Windows 8 as guest operating systems. I am hoping to find a way to have them apply to Windows Server 2008R2 and Windows 7 as well, but as of now, that is just a possibility and not a reality.
  • Separate the data volume from the system volume within the VM. Don’t just put data into the system volume
  • Seriously consider using dynamically expanding VHDX files rather than statically fully allocated VHDX files. Meta data overheads, for VHDX files have been considerably reduced in Windows Server 2012, VHDX files grow less often, and with the tips in this blog, they will grow even less often. The idea is to accept the minimal overhead of expanding VHDX files in exchange for storage space optimization. Note that if the VHDX file is placed on thin provisioned storage e.g. a thin provisioned Storage Space, using a fixed size VHDX file does not guarantee that the VHDX file will never face an issue of running out of disk space.
  • Always present the VHDX based data volume as either SCSI or virtual HBA attached storage within the VM .  
  • Periodically run a PS cmdlet to optimize the VHDX based volume. In particular, run the PS cmdlet Optmize-Volume with the ReTrim option. Optimize-Volume is described at this MSDN link, and the description is certainly incomplete. While the MSDN web page claims this PS cmdlet applies to only Windows Server 2012, it certainly also applies to Windows Server 8 as well. Yours truly has submitted the web page feedback and hopefully the page is updated soon. Further note that this PS cmdlet needs to be run with administrative privileges, again something the MSDN web page does not mention.

Details of the investigation that led to this summary of recommendations will be published in follow up blogs soon.