Microsoft is significantly improving SMB compression in Windows 11, here's how

Microsoft rolled out an optional update for Windows 11 in the form of KB5016691, and a similar update for Windows Server 2022 earlier with KB5016693. While the former will eventually become available to consumers on next month's Patch Tuesday (September 13), the cumulative update includes some new features that early adopters can take advantage of now.

Both updates mention Server Message Block (SMB) compression improvements in their changelogs, but don't really go into detail. Fortunately, Microsoft's Ned Pyle talks about all the enhancements to SMB compression in a dedicated blog post.

For those who don’t know, SMB compression in a client-server environment allows administrators, users, and applications to request file compression while content is being transferred over the network. The benefit of this approach is obviously reduced bandwidth consumption, but it comes at the cost of increased CPU usage as the hardware attempts to compress and decompress the files as they are transferred over the network.

The way SMB compression behaved before KB5016691 and KB5016693 was weird to say the least. Basically it uses a default algorithm which only tries to compress the first 500MiB of the file (note: 1MB = 1000KB but 1MiB = 1024KB), anything smaller than this threshold will not be compressed even if it is very "compressible" ".

There is an additional warning. During the reading of the first 500MiB of the file, if the algorithm detects that a file smaller than 100MiB can be compressed, it will not attempt to compress the rest of the file at all. Imagine you have a 10GiB file that is very compressible, but only 80MiB of the first 500MiB is compressible, the SMB compression algorithm will give up on compressing the file entirely and you'll end up sending almost the same 10GiB over the network. The only way to force compression is to override certain default registry settings, and unless you know exactly what you're doing, editing these settings isn't really what you should be doing.

This is a very strange behavior and probably affects a lot of use cases. However, the good news is that Microsoft is removing the restrictions from the algorithm entirely. Essentially, SMB compression will now try to compress all files you request to compress.

Obviously, this does not mean that SMB compression should be used in every use case. Pyle stressed that some formats like JPG, ZIP and DOCX are already compressed, but removing these restrictions would certainly benefit other formats like VHDX, ISO and DMP.

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