This is useful for upgrading an existing storage to zstd compression or others.
Chunks need to be decompressed and re-compressed during copy anyway. Only
the bit-identical option requires the same compression level
Also fix a typo: compatiable -> compatible
Zstd compression can be enabled by providing `-zstd` or `-zstd-level <level>`
to `init`, `add`, or `backup`. With `-zstd` the compression level will be
`default`, and with `-zstd-level` the level can be any of `fastest`, `default`,
`better`, or `best`.
This option is useful only when erasure coding is enabled. It will
download and re-upload chunks that contain corruption but are
generally recoverable. It can also be used to fix chunks that
are created by 3.0.1 on arm64 machines with wrong hashes.
This is to support public key encryption in the backup operation. You can use
the -key option to supply the public key to the backup command, and then the
same option to supply the private key when restoring a previous revision.
The storage must be encrypted for this to work.
