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53th chunk of `rsync.man`
6d22dcd1e9132a62b0429e468379fa98fd82ad69c5fcfa890000000100000fe1
   Beginning  in version 2.6.4, multiple --link‐dest directories may
              be provided, which will cause rsync to search the list in the or‐
              der specified for an exact match (there is a limit of 20 such di‐
              rectories).  If a match is found that differs only in attributes,
              a local copy is made and the attributes updated.  If a  match  is
              not  found, a basis file from one of the DIRs will be selected to
              try to speed up the transfer.

              This option works best when copying into an empty destination hi‐
              erarchy, as existing files may get their attributes tweaked,  and
              that  can  affect  alternate  destination  files  via hard‐links.
              Also, itemizing of changes can get  a  bit  muddled.   Note  that
              prior  to version 3.1.0, an alternate‐directory exact match would
              never be found (nor linked into the destination) when a  destina‐
              tion file already exists.

              Note  that  if you combine this option with --ignore‐times, rsync
              will not link any files together because it only links  identical
              files  together  as a substitute for transferring the file, never
              as an additional check after the file is updated.

              If DIR is a relative path, it is relative to the destination  di‐
              rectory.  See also --compare‐dest and --copy‐dest.

              Note that rsync versions prior to 2.6.1 had a bug that could pre‐
              vent  --link‐dest from working properly for a non‐super‐user when
              --owner (-o) was specified (or  implied).   You  can  work‐around
              this bug by avoiding the -o option (or using --no‐o) when sending
              to an old rsync.

       --compress, -z
              With this option, rsync compresses the file data as it is sent to
              the  destination  machine, which reduces the amount of data being
              transmitted -- something that is useful over a slow connection.

              Rsync supports multiple compression methods and will  choose  one
              for  you  unless you force the choice using the --compress‐choice
              (--zc) option.

              Run rsync --version to see the  default  compress  list  compiled
              into your version.

              When both sides of the transfer are at least 3.2.0, rsync chooses
              the  first algorithm in the client’s list of choices that is also
              in the server’s list of choices.  If no common compress choice is
              found, rsync exits with an error.  If the remote rsync is too old
              to support checksum  negotiation,  its  list  is  assumed  to  be
              "zlib".

              The  default  order  can be customized by setting the environment
              variable RSYNC_COMPRESS_LIST to a space‐separated list of accept‐
              able compression names.  If the string contains a "&"  character,
              it  is separated into the "client string & server string", other‐
              wise the same string applies to both.  If the string  (or  string
              portion)  contains no non‐whitespace characters, the default com‐
              press list is used.  Any unknown compression names are  discarded
              from  the  list,  but a list with only invalid names results in a
              failed negotiation.

              There are some older rsync versions that were configured  to  re‐
              ject  a  -z  option and require the use of -zz because their com‐
              pression library was not compatible with the  default  zlib  com‐
              pression  method.   You  can usually ignore this weirdness unless
              the rsync server complains and tells you to specify -zz.

       --compress‐choice=STR, --zc=STR
              This option can be used to override the automatic negotiation
Title: Rsync Options: --link-dest (continued), --compress, -z, and --compress-choice=STR, --zc=STR
Summary
This section continues describing the `--link-dest` option, clarifying its behavior with `--ignore-times` and noting a potential bug in older versions. It then introduces the `--compress` or `-z` option, which enables file data compression during transfer, which is useful over slow connections. Rsync supports multiple compression methods, and the section explains how rsync chooses the compression method by default and how to customize the order using the RSYNC_COMPRESS_LIST environment variable. It also mentions a historical issue with some older rsync versions requiring `-zz` instead of `-z`. Finally, it introduces the `--compress-choice` or `--zc` option for overriding the automatic negotiation.