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well....for chds it won't play a role since decompression is only done when the belonging option is activated and then chdman is used.

For zips it would mean that the data inside the zip uses a different compression mechanism while the general zipstructure is the same, so reading shouldn't be a problem too.

When it comes to testing or unpacking it depends if ZipArchive (the used 3rd party library) is able to work with it which I haven't tested yet. When it comes to adding files to an archive, most likely a standard mode is used so that the newly added file inside the archive won't be Zstandard anymore (unless a direct copy of compressed data is used). The same applies to the new scanner/rebuilder where you might have the chance that bit7z (which reuses 7z) might already be able to decompress/compress Zstandard compressed files inside zips....have to check that. I can contact the authors of both to see if such a feature (if missing) can be added in a future release.

Add Zstandard support for zip archives and CHDs
https://github.com/mamedev/mame/pull/11827

MAME adds zstabdard support, will cmp consider supporting this compressor?

That's lot of disk space 

It's about 300GB savings over the mame chds (non-software list).

Yeah, I've seen this issue. For now feature in test but I was curious how much it could save disk space and time for full test chds. Guess we have to wait a bit before full switch to deltas

Well yes, since cmpro does a "chdman verify -i %1" command which would fail on deltas....it would require "chdman verify -i %1 -ip %2" where %2 gets replaced with the parent chd filename...but that's currently not supported (but on the to do list).
So...for delta the deep check won't work....but as I said: currently nobody is using delta chds since there are some open issues with them on chdman side, see https://github.com/mamedev/mame/issues/11675

Ok, I've converted couple of chds into delta but right now scanner (full with chd integrity check) shows those deltas as entegrity check failed 

I didn't have any, so far

Window was behind scanner window 

Since nobody I know of uses delta chd, you most likely don't see any warnings about delta chds....
So I assume you don't have any delta chds....if you select "warn about non delta" you should see a warning for nearly all of your chds. The warnings appear in the warnings window below the scanner.

I think I miss something here. I don't see any warnings about delta after full scan even if I choose option about showing delta. Do I have to full scan with crc checking of chds ?