Top of the list of alternatives was SpiderOak, a service principally aimed at online backup (more often known as storing your data in the cloud these days) but which also provides functionality to sync backed up folders across several machines.
First problem came in the installation of SpiderOak on Slackware. Their website provides a package for Slackware 12.1. At the time, Slackware was 32 bit only, so there's no 64 bit package available. There are 64 bit packages available for a number of other distros, including Debian, Fedora, CentOS, openSUSE and Ubuntu. I decided to avoid the Slackware package since I wasn't sure it would play nicely on a more recent (13.37) version of Slackware. Instead, I set about writing a SlackBuild script to repackage one of the other distro's packages. In the end, I settled on the Ubuntu .debs.
I modified the autotools SlackBuilds.org template to unpack the .deb files instead of the more usual tarballs. Running the SlackBuild produced a package, but after I installed it I received the following error message:
Traceback (most recent call last):I found the relevant file in /usr/lib and the file utility confirmed it was not a zip file. After much head scratching, it turned out that the strip lines in the standard SlackBuild significantly alter a number of files in the SpiderOak package. After removing those lines, the package built, installed and ran as expected. edit: If you're looking for a copy of the SpiderOak SlackBuild, I've uploaded it here, with the doinst.sh, spideroak.info and slack-desc too.
File "<string>", line 5, in <module>
zipimport.ZipImportError: not a Zip file: '/usr/lib/SpiderOak/library.zip'
The next step was migrating all my data from Dropbox to SpiderOak. SpiderOak allows you to define a number of folders to back up, unlike Dropbox where only a single folder can be synced. I created a new diretory (~/SpiderOak seemed fitting) and copied the contents of my Dropbox folder over to the new SpiderOak folder. I added this folder as a folder to back up in SpiderOak and let it upload the files.
I changed all the relevant scripts which used to dump data into my Dropbox folder to do so into the new SpiderOak folder instead. After setting up similar folders on my desktop (Windows) and my Mac (laptop), I was able to create a sync whereby all three versions would be synced with one another, effectively emulating the Dropbox functionality.
After a few days of dumping data into the folder, all seemed well. A few days after that, however, I started to get worried that my 2GBs of free storage on the SpiderOak servers was filling up rapidly. After some investigation, it became apparent that the manner in which SpiderOak works is slightly, though in my case, crucially different, and it relates to the way in which overwritten or deleted files are handled.
Dropbox stores old versions of files for up to 30 days after they have been overwritten or deleted. This allowed my frequent writes to not fill up all my space. Conversely, SpiderOak do not ever discard overwritten or deleted files. This is a nice feature if you accidentally delete an important file. However, for my purposes, it merely served to swallow my free 2GBs of storage in pretty short order.
It is possible to empty the deleted items for a given profile (i.e. each machine currently being backed up to the SpiderOak servers). Irritatingly, however, it is not possible to empty the deleted items for one of the machines from a different machine; you can view the deleted files and how much space they're taking, but you can only delete them from the machine from which they were deleted. Since most of my files are created and deleted on my headless server, using VNC to open the SpiderOak GUI and emptying the files every couple of days quickly lost its appeal. I searched through the SpiderOak FAQs and documentation (such that I could find), and only found one option to do this automatically from a machine on the command line. The command is SpiderOak --empty-garbage-bin, though it is accompanied by this dire warning:
Caution: Do not use these commands unless advised by SpiderOak support. They can damage your installation if used improperly.So, for my purposes, the Dropbox approach of removing anything you've deleted or modified after 30 days is much more usable. Since I don't keep anything particularly sensitive nor critical on there, the hoohah about the privacy and encryption aren't much of a concern to me. After a month of SpiderOak, I was fed-up of the requirement for me to delete all the deleted files over VNC, and so I moved everything back to Dropbox. I suppose the lesson there is "if it ain't broke, don't fix it".