ReFS – Microsoft releases cumulative update 4077525!

Hi, last week Microsoft has finally released the ReFS stability patch KB4077525 (February 22, 2018) for Windows Server 2016 and Windows 10 that fixes ReFS lockup problems with Veeam repositories. In the “Last Word from Gostev, he wrote that in the change log, there are just a couple of modest lines about ReFS performance improvements, some of the affected users will see performance improve from zero (due to file server lockups) to nominal. It just does not translate a real magnitude of the issue, and the complexity of bugs behind…

New - Windows Server 2016


ReFS deduplication functionality with Windows Server 2016 version 1709 was incompatible with Veeam B&R v9.5 advanced ReFS integration. Specifically, the issue is that block cloning API calls fail on deduplicated backup files with data integrity streams enabled which does look like a bug on Microsoft side. Thus, to make ReFS volumes with deduplication enabled actually usable for backup repositories, Veeam Software has decided to temporarily disable fast cloning functionality on such volumes in the Veeam Backup and Replication 9.5 Update 3. Then and after some weeks, the reason it took so long to fix is that there are multiple separate issues. It was to identify and separate all these individual issues when some were outside of ReFS even, for example, caused by NTFS-specific optimization in the OS memory management. And it also makes perfect sense now why only some of users were affected. For example, small ReFS volumes should be much less impacted by these bugs.

Second birth – Improved ReFS performance:

The ReFS development team at Microsoft did an outstanding job discovering and nailing down each and every cause of the resulting behavior, and these two lines just don’t do justice to their effort. A couple of important notes before you apply the update. First, if you are running a pre-release driver from Microsoft support, it MUST be uninstalled before you install the patch (ReFS team has stressed to Gostev that this is essential). Second, if you will wonder about the referenced KB4090104 and whether you should create those values: “You notice heavy memory use in the Resilient File System (ReFS) file system type on a computer that is running Windows Server 2016. You may also notice an ReFS volume become unresponsive or freeze when you perform backups. This can specifically occur when you use a backup application that does large block-clone operations“. Anton Gostev asked Microsoft folks the same question, and the answer is no, just installing the patch is sufficient for most. Tweaking these values are for corner cases only, for example reducing memory pressure with large ReFS volumes when your file server does not have enough RAM. Basically, Anton feels pretty confident about this patch knowing how many users had success with the pre-release driver useful for ReFS Veeam backup repositories. But of course, not until Veeam Support get confirmations from all the affected customers that the patch solved their issues. So, do deploy this patch at your convenience then let everyone know your experience here.

KB Microsoft: February 22, 2018—KB4077525 (OS Build 14393.2097)

Thanks to Anton Gostev for sharing on Veeam Community forum digest and for his work with Microsoft! 🙂


Microsoft ReFS new driver version fixed all issues!

General Windows Server 2016 licensing information, including licensing criteria and terminology can be viewed here.

Here Windows Server 2016 datasheet: Download

Veeam B&R 9.5 U3 – New protection feature here!

You can learn more about Veeam Backup & Replication v10 here.


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  1. […] UPDATED (February 26, 2018): ReFS – Microsoft releases cumulative update 4077525! […]

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  3. […] Windows 10 that fixes ReFS lockup problems with Veeam repositories. I wrote about the changelog a few weeks ago. Another change is that this update will be downloaded and installed automatically from Windows […]

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