btrfs will compress every file changed since it was mounted if you use:
mount -o compress-force=lzo /dev/btrfsdev /mnt/btrfsmnt
If you want to see to it that ALL files get compressed this way, I've got a little script I wrote to do it...
du -ht +$((1024*1024)) "$HOME" |\
sed -rn 's/^[^/]*(.*)/btrfs fi defrag -fvclzo "\1"/p' |\
sudo sh -n
The above only works on my $HOME directory - but you can use it on anything or everything as you like. It's also got the -n operand fed to sh as is so you can see for yourself what sh is currently not doing before removing it to tell it to do it.
Anyway, first it queries du for files in human-readable format (probably redundant here since we strip that in the next step anyway) that are larger than 1MB or $((1024*1024)).
It |pipes its info to sed which strips off everything before the leading /, "quotes" the filename, builds the btrfs filesystem defragment -verbose -flush-to-disk -compress-lzo \filename command and hands it over a |pipe to sudo sh to execute.
Again, it won't do anything so long as sh --no-execute is in effect, though.
I think some very recent btrfs userspace tool builds do defragment entire directories recursively, but if so it's a pretty new thing, so I've always had to do stuff like this.
Currently, btrfs does not support n-way mirrors.
Btrfs does have a special replace subcommand:
btrfs replace start /dev/left /dev/new_device /mnt/foo
Reading between the lines of the btrfs-replace man page, this command should be able to use both existing legs - e.g. for situations where both legs have read errors - but both error sets are disjoint.
The btrfs replace command is executed in the background - you can check its status via the status subcommand, e.g.:
btrfs replace status /mnt/foo
45.4% done, 0 write errs, 0 uncorr. read errs
Alternatively, one can also add a device to raid-1 filesytem and then delete an existing leg:
btrfs dev add /dev/mapper/new_device /mnt/foo
btrfs dev delete /dev/mapper/right /mnt/foo
The add should return fast, since it justs adds the device (issue a btrfs fi show to confirm).
The following delete should trigger a balancing between the remaining devices such that each extend is available on each remaining device. Thus, the command is potentially very long running. This method also works to deal with the situation described in the question.
In comparison with btrfs replace the add/delete cycle spams the syslog with low-level info messages. Also, it takes much longer to finish (e.g. 2-3 times longer, in my test system with 3 TB SATA drives, 80 % FS usage).
Finally, after the actual replacement, if the newer devices are larger than the original devices, you will need to issue a btrfs fi resize on each device to utilize the entire disk space available. For the replace example at the top, this looks like something like:
btrfs fi resize <devid>:max /mnt/foo
where devid stands for the device id which btrfs fi show returns.
Best Answer
Kernel 5.1 added ZSTD level support. I tested it with rc1 today using a mount option compress=zstd:12 in /etc/fstab. The default level is 3. To be clear: The change affects only files that are written after this mount command. Some benchmark results: https://lkml.org/lkml/2019/1/28/1930