[Borgbackup] zstd compression

[Jasper Knockaert]

Hi

I was looking at the new zstd compression option. I am now using lzma,6. 
I think zstd,19 should deliver a similar compression ratio. So the only 
question seems to be if zstd is any faster. That could be the case if it 
runs in multithread mode. However, the default for zstd seems to be to 
run as a single thread. There seems to be little evidence that 
single-threaded zstd compression is any faster than lzma for a similar 
compression ratio... (I'm avoiding 20+ modes for zstd as they use a lot 
more memory.)

Best


Jasper