I heard that the problem with duplicati with 7z is LZMA2 compression. I think LZMA is ok. I am using duplicati 2.0.3.3, and am wondering how to use 7z with LZMA. I tried changing compression module to 7z and --zip-compression-method to LZMA, but I am getting warnings that --zip-compression-method is unsupported and 7z is still dangerous. Is there a safe way to use 7zip with duplicati?
I’m eager to hear the answer as well but want to share that normal 7z usage on initial creation of my backups a year ago worked fine, but now seem to be causing issues during retention policy cleanup.
I eventually had to adjust my retention rules to never try to clean up files in the 7z timeframe. 
Good thing i used the default. I would not have wanted to find out about the 7z problems like that. I haven’t really researched much into all the different compression methods or modules dupilcati supports because usually 7z is the best or one of the best. What alternative settings do you suggest for similar compression to 7z in case there’s no way to use 7z with regular LZMA?
I ended up staying with the default zip settings so can’t be much help there.
This post is a bit old but likely still 100% accurate…
Yeah. I’ll stay with zip and use LZMA compression. Not as good though. If anyone has any ideas about using 7z
module with standard LZMA, that would be very nice.
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Generally speaking most modern compression algorithms give roughly the same compression, and with regard to the number of cores that you can use at once, it is up to you to decide how many you want to use. However, 7-zip is free and open source. The 7z format supports encryption with the AES algorithm with a 256-bit key. If the zip file exceeds that size, 7-zip will split it into multiple files automatically, such as integration_serviceLog.zip.001, integration_serviceLog.zip.002, etc. (Way back when, PK Zip used this to span zip files across multiple floppy disks.) You’ll need all the files to be present to unzip them. The 7z format provides the option to encrypt the filenames of a 7z archive.
So, just so my feeble mind understands … as of April 2019, users of Duplicati still must use zip compression because 7z, lzma, bzip, etc. implementations are all broken?
7z is broken, but zip files can be compressed via LZMA (not LZMA2) and can only be compressed with one core (but multiple instances can be run on several cores in versions after Jun. '18) and i don’t believe has solid compression. In fact, with the new multi core engine, duplicati seems to be experiencing a lot of problems. I reverted back to 2.0.3.3 (before June) and do constant database backups in case it breaks (i am now paranoid of any version beyond 2.0.3.3). Duplicati isn’t that bad if you don’t care about speed (like server backups, etc.) but its only stable with single core as far as i’m concerned 
This thread seems to have fresh information about this topic, just cross-linking: