Update shell-and-data-access.md

docs.it4i/general/shell-and-data-access.md

@@ -149,6 +149,38 @@ local $ man scp
 local $ man sshfs
 ```
 
+The rsync client uses ssh to establish connection.
+
+```console
+local $ rsync my-local-file 
+```
+
+```console
+local $ rsync -r my-local-dir username@cluster-name.it4i.cz:directory
+```
+
+!!! note
+    The data transfer speed is limited by the single-core ssh encryption speed to about **150 MB/s**  
+    Use **parallel rsync** for unlimited transfers
+
+The parallel rsync transfers multiple files at the time, utilizing multiple cores to accelerate encryption.  
+First, set up ssh-agent single sign on:
+
+```
+console
+local $ eval `ssh-agent`
+local $ ssh-add
+Enter passphrase for /home/user/.ssh/id_rsa:
+```
+Then run the rsync in parallel:
+
+```console
+ls my-local-dir | xargs -I% -n 2 -P 4 rsync % username@cluster-name.it4i.cz:directory/.
+```
+
+The -n XXX argument determines how many files will be transfered per single rsync process. The -P YYY argument
+determines number of parallel rsync processes. Set to number of cores on your local machine.
+
 On Windows, use the [WinSCP client][c] to transfer data. The [win-sshfs client][d] provides a way to mount the cluster filesystems directly as an external disc.
 
 ## Connection Restrictions