I have a pool that was created in 2011, using lzjb compression, and it wasn't until a couple of years later that an upgrade allowed me to set the compression to lz4. I estimate that at least 20% of the content (by space) on the array was created prior to 2013, which means it's still compressed using lzjb.
I can think of a couple of options to fix this and regain (some) space:
Back up and restore to a new pool. Not really practical, as I do not have sufficient redundant storage to hold the temporary copy. The restore would also require the pool to be offline for several hours.
Write a script to re-copy any file with a timestamp older than 2013. Potentially risky, especially if it chokes on spaces or other special characters and ends up mangling the original name.
So... is there some way to get ZFS to recompress any legacy blocks using the current compression algorithm? Kind of like a scrub, but healing the compression.
A related question: is there some way to see the usage of each type of compression algorithm? zdb just shows overall compression stats, rather than breaking them down into individual algorithms.