Will new ReFS be backward compatible with current ReFS filesystems?

[JakeSteeleFM]

Will existing archived data on Windows Server vNext ReFS be compatible with the native deduplication and compression features of the new ReFS?

[#2]

They should fix the NORMAL deduplication data corruption first, which I still can reproduce from Server 2022 up the the newest 26040 vNext build. Exact same scenario with Server 2019: No data corruption. https://techcommunity.microsoft.com/t5/windows-server-insiders/server-vnext-25987-26010-and-server-2... And now you add ReFS deduplication on top? It is amazing how many new bugs creeped in into newer OS since about 2021. And I mean actual OS bugs, like this dedup corruption, or that you cannot Access local shadowcopies on Windows 11 since May 2022 (unless you stick to Windows 11 21h1). Note that I skip the UI bugs here. This is how Microsoft is shooting its reputation. The OS needs a "full fix run" from Dave Cutler again. Like he did for Windows XP SP0, aka "worst code ever seen".

[#3]

Yes. There's just no path for ReFS in WS 2012. Dedup will be replaced by online dedup so one has to disable current dedup to use the new one.

[#4]

to learn more about the ReFS upgrade paths and features have a look at this unofficial but utmost updated page: https://gist.github.com/XenoPanther/15d8fad49fbd51c6bd946f2974084ef8

Keep in mind in-place upgrade will upgrade all "online" volumes with ReFS and there is no way back to the previous version.

So if you are testing Windows 2025 preview and upgrade to this release you may not read this data with Windows Server 2022 anymore by design, unless you backup and restore and reformat with Windows Server 2022.

Currently there is a quirk with Windows 11 (Insider) not being able to access Windows Server 2025 preview formatted ReFS drives.