Private Cloud System

Thinking of building my own cloud storage solution.

For a cloud storage system, usually there are two kinds of clients:
1. Complete sync. All data are seamlessly synced between the server and client.
2. Sync on demand. Only files/data to be used are downloaded and updated for the user. This usually happens on mobile devices where the storage size is small.

For a server, it would be good if there are some features in it:
1. revision history. You can trace back the data.
2. version control. This is common for multiple users. A problem occurs when two users check in the same file with their own modification. The system should be able to identify the check-out time.

Some people have posted their solutions:
rsync: http://fak3r.com/geek/howto-build-your-own-open-source-dropbox-clone/

sparkleshare: This seems a promising open source solution. http://sparkleshare.org/
some more: http://thefinalcastle.com/tag/virtual-private-server

Some backup solutions:
http://www.aboutmyip.com/AboutMyXApp/DeltaCopy.jsp

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Update:

There are some solutions that are already in market and are nearly free:

1. http://www.tonido.com/

2. http://www.pogoplug.com/

Update 2:

1. Use plugapps for these devices

2. 破解 pogoplug:

http://www.beimeicn.com/Space/u=3M/m=blog/p=347411/s=nature_green/lang=schinese.html