documentation: add stream_compress documentation, update Changelog

ChangeLog

@@ -1,5 +1,12 @@
 btrbk-current
 
+  * KNOWN BUG: for raw targets, the send stream is not decompressed if
+    stream_compress is set and raw_target_compress is not set.
+  * MIGRATION
+    - update ssh_filter_btrbk.sh on remote hosts if "stream_compress"
+      is enabled. Also add "--compress" option to ssh_filter_btrbk.sh
+      invocation if "stream_compress" is enabled.
+  * Add "stream_compress" configuration option.
   * Bugfix: Replace "realpath" with "readlink" in ssh_filter_btrbk.sh
 
 btrbk-0.23.3

btrbk.conf.example

@@ -56,6 +56,11 @@ snapshot_dir               _btrbk_snap
 #ssh_compression            no
 #ssh_cipher_spec            default
 
+# Enable compression for remote btrfs send/receive operations:
+#stream_compress            no
+#stream_compress_level      default
+#stream_compress_threads    default
+
 # Enable lock file support: Ensures that only one instance of btrbk
 # can be run at a time.
 #lockfile                   /var/lock/btrbk.lock

doc/btrbk.conf.5

@@ -225,6 +225,29 @@ cipher_spec" option in ssh(1) for more information. Defaults to
 \[lq]default\[rq] (the ciphers specified in \fIssh_config\fR).
 .RE
 .PP
+\fBstream_compress\fR  <compress_command>|no
+.RS 4
+Compress the btrfs send stream before transferring it from/to remote
+locations. Defaults to \[lq]no\[rq]. If enabled, make sure that
+<compress_command> is available on the source and target
+hosts. Supported \fI<compress_command>\fR: gzip, pigz, bzip2, pbzip2,
+xz, lzo, lz4.
+.RE
+.PP
+\fBstream_compress_level\fR  default|<number>
+.RS 4
+Compression level for the specified <compress_command>. Refer to the
+related man-page for details (usually [1..9], where 1 means fastest
+compression). Defaults to \[lq]default\[rq] (the default compression
+level of <compress_command>).
+.RE
+.PP
+\fBstream_compress_threads\fR  default|<number>
+.RS 4
+Number of threads to use for <compress_command>. Only supported for
+"pigz", "pbzip2" and recent versions of "xz".
+.RE
+.PP
 \fBrate_limit\fR  <rate>|no
 .RS 4
 Limit the transfer to a maximum of \fI<rate>\fR bytes per second. A