Partially synchronising a structured folder (on a Synology machine) to a smaller storage

21 November 2014

One of the most enjoyable things writing code has to offer is automating tasks to the point where you can completely forget about them. There is nothing like walking down the street and getting an email on your phone from some long forgotten process, reminding you that something happened and thinking “I can’t remember how that works but I built that”. Recently I’ve been dusting off and cleaning up some of my personal scripts to publish up on my GitHub account and in the process I stumbled upon a problem I solved a long time ago which on initial glance seemed really simple but ended up being rather treacherous.

The situation is as follows: imagine a Synology server storing a video library where files are stored with a certain directory structure. This structure is important because Kodi (previously XBMC) requires specific naming conventions in order to succesfully index TV shows in its library.

Now imagine that you move out to go to college and scramble together a small machine to use as a HTPC. The cpu is fine, the gpu is fine, but the storage capacity is not nearly as much as you’ve grown accustomed to. You can’t mount the server at home as remote storage because your Austrian roommate will forever nag you about the traffic messing up his Dota game and the connection isn’t fast enough to stream anyway. So what do you do?

I figured that the most logical solution was to just rsync all the files in the directory to the local machine until the volume was full, starting with file that was modified last. Seems easy right? How about something with

find . -type f -printf '%[email protected] %p\n'

Just pipe that into sort and xargs to rsync the files individually to the HTPC until the volume is full. Unfortunately its not that simple. Synology machines are equipped with Busybox binaries instead of proper GNU utilities. This means that there is no -printf flag on the find or the xargs command, the stat command is severely limited and the default shell is Almquist shell. This limits the options and convenience quite a bit. Some fancy glob shelling trick won’t be possible too. A savage option would be to pipe something from ls, but even that was off the table due to the not functioning ‘-e List full date and time’ flag of the BusyBox implementation. A compromise could be made where the ‘-mtime’ find flag could be used to simply get all the files not older than a certain amount of time but I didn’t like the idea of having the hdd just sitting there idely until new files came by.

The next step could be to look into options like utilizing the Debian chroot or to find some other way to get better binaries on the system. None of that seems like the ‘clean’ way to do it but luckily there is another route to go.

My solution consists of two scripts. One Python script that runs on the Synology server to list all the files in the source directory and to create symlinks to the latest files in a temporary directory until a certain amount of bytes of data is reached. The second script is a small shell script that runs on the remote machine.

This way every time the script is run on the small server, a directory is made on the large server, the dir is filled up with symlinks to files sorted by age in a copy of the original directory structure from the specified path until this new temporary directory has reached the allocated amount of bytes specified in the script. Then rsync on the smaller server will start pulling everything from that directory down to the local volume.

To set it up drop the script somewhere on the remote server like for example

wget -P /volume1/RAID5/other/code/scripts/system

On the local server download the script to some directory

wget -P ~/.dotfiles-public/code/scripts/system/

And run the script once in a while using a cron job like this based on your configuration

*       7,12,17,22      *       *       *       SHELL=/bin/bash PATH=/bin:/sbin:/usr/bin:/usr/sbin bash /home/vdloo/.dotfiles-public/code/scripts/system/ -h -u vdloo -p 22 -a 1500000000000 -l /mnt/lvmdisk/series > /dev/null 2>&1