ReFS Accelerated VHDX Operations

One of the interesting new features in Windows Server 2016 (WS2016) is ReFS Accelerated VHDX Operations (which also work with VHD). This feature is not ODX (VAAI for you VMware-bods), but it offers the same sort of benefits for VHD/X operations. In other words: faster creation and copying of VHDX files, particularly fixed VHDX files.

Reminder: while Microsoft continually tells us that dynamic VHD/Xs are just as fast as fixed VHDX files, we know from experience that the fixed alternative gives better application performance. Even some of Microsoft’s product groups refuse to support dynamic VHD/X files. But the benefit of Dynamic disks is that they start out as a small file that is extended as time requires, but fixed VHDX files take up space immediately. The big problem with fixed VHD/X files is that they take an age to create or extend because they must be zeroed out.

Those of you with a nice SAN have seen how ODX can speed up VHD/X operations, but the Microsoft world is moving (somewhat) to SMB 3.0 storage where there is no SAN for hardware offloading.

This is why Microsoft has added Accelerated VHDX Operations to ReFS. If you format your CSVs with ReFS then ReFS will speed up the creation and extension of the files for you. How much? Well this is why I built a test rig!

The back-end storage is a pair of physical servers that are SAS (6 Gb) connected to a shared DataON DNS-1640 JBOD with tiered storage (SSD and HDD); I built a WS2016 TPv3 Scale-Out File Server with 2 tiered virtual disks (64 KB interleave) using this gear. Each virtual disk is a CSV in the SOFS cluster. CSV1 is formatted with ReFS and CSV2 is formatted with NTFS, 64 KB allocation unit size on both. Each CSV has a file share, named after the CSV.

I had another WS2016 TPv3 physical server configured as a Hyper-V host. I used Switch Embedded Teaming to aggregate a pair of iWARP NICs (RDMA/SMB Direct, each offering 10 GbE connectivity to the SOFS) and created a pair of virtual NICs in the host for SMB Multichannel.

I ran a script on the host to create fixed VHDX files against each share on the SOFS, measuring the time it requires for each disk. The disks created are of the following sizes:

  • 1 GB
  • 10 GB
  • 100 GB
  • 500 GB

Using the share on the NTFS-formatted CSV, I had the following results:


A 500 GB VHDX file, nothing that unusual for most of us, took 40 minutes to create. Imagine you work for an IT service provider (which could be a hosting company or an IT department) and the customer (which can be your employer) says that they need a VM with a 500 GB disk to deal with an opportunity or a growing database. Are you going to say “let me get back to you in an hour”? Hmm … an hour might sound good to some but for the customer it’s pretty rubbish.

Let’s change it up. The next results are from using the share on the ReFS volume:


Whoah! Creating a 500 GB fixed VHDX now takes 13 seconds instead of 40 minutes. The CSVs are almost identical; the only difference is that one is formatted with ReFS (fast VHD/X operations) and the other is NTFS (unenhanced). Didier Van Hoye has also done some testing using direct CSV volumes (no SMB 3.0), comparing Compellent ODX and ReFS. What the heck is going on here?

The zero-ing out process that is done while creating a fixed VHDX has been converted into a metadata operation – this is how some SANs optimize the same process using ODX. So instead of writing out to the disk file, ReFS is updating metadata which effectively says “nothing to see here” to anything (such as Hyper-V) that reads those parts of the VHD/X.

Accelerated VHDX Operations also works in other subtle ways. Merging a checkpoint is now done without moving data around on the disk – another metadata operation. This means that merges should be quicker and use fewer IOPS. This is nice because:

  • Production Checkpoints (on by default) will lead to more checkpoint usage in DevOps
  • Backup uses checkpoints and this will make backups less disruptive

Does this feature totally replace ODX? No, I don’t think it does. Didier’s testing proves that ReFS’s metadata operation is even faster than the incredible performance of ODX on a Compellent. But, the SAN offers more. ReFS is limited to operations inside a single volume. Say you want to move storage from one LUN to another? Or maybe you want to provision a new VM from a VMM library? ODX can help in those scenarios, but ReFS cannot. I cannot say yet if the two technologies will be compatible (and stable together) at the time of GA (I suspect that they will, but SAN OEMs will have the biggest impact here!) and offer the best of both worlds.

This stuff is cool and it works without configuration out of the box!

5 thoughts on “ReFS Accelerated VHDX Operations”

  1. There is a tool which can instantly create VHDX Files of any size ultraquick. However zeroing out of the sectors is NOT done! Not a big issue, because you can still zero out later with sdelete from sysinternals, if this is really necessary.
    Furthermore it can extend a VHDX, upgrade a VHD to VHDX and convert a dynamic VHDX to a static VHDX.
    The Webpage for this tool is in german:

    1. There’s a lot of those tools out there. They usually try to hide the fact that they don’t zero out the disks, which is exactly why the tools are fast in the first place. These tools have no place in a true multi-tenant environment because of the security risks that they introduce.

  2. Hi

    Would you recommend ReFS over NTFS for a none clustered Hyper-V host running server 2012 r2?

    It has been recommended to me a lot recently but most references are made to Windows 2016.

    Any help/advice appreciated

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