An option in the future to write data with a fast zfs level so everything is speedy and recompress blocks which have not changed in some time with a more efficient compression ratio would be really great. So you would have almost no performance penalty writing data and very high compression ratio for old data.
Fast compression levels of a given algorithm means lower compression ratio.
I don't know if ZFS supports variable compression levels (maybe per dataset), but Btrfs ZSTD support uses a mount option, e.g. mount -o compress=zstd:[1-15]
Thus it's possible to use a higher level (high compression ratio, slower speed, more CPU and RAM) for e.g. an initial archive. And later use a lower level (or even no compression) when doing updates. Writes use the compression algorithm and level set at mount time; and it's possible to change it while remaining mounted, using -o remount.
> I don't know if ZFS supports variable compression levels (maybe per dataset)
> Thus it's possible to use a higher level (high compression ratio, slower speed, more CPU and RAM) for e.g. an initial archive. And later use a lower level (or even no compression) when doing updates.
yup. works the same in ZFS. you can change the compression setting any time you like, for future writes.
I think that would require the same architectural changes needed for offline deduplication (i.e. probably not going to happen any time soon, unless I've missed some recent developements).
