A Slashdot question today about putting to use all the unused disk space on corporate desktops got me to thinking. Now, before I start, comments there raised valid points about performance, reliability, etc.

But let’s say that we have a “cloud filesystem”. This filesystem would, at its core, have one configurable parameter: how many copies of each block of data must exist in the cloud. Now, we add servers with disk space to the cloud. As we add servers, the amount of available space on the cloud increases, subject to having enough space for replication according to our parameters.

Then, say we say we want a minimum of 3 copies of each block replicated. Each write to the filesystem will then cause a write to at least 3 different servers. Now, what if one server goes down? If the cloud filesystem is short on space, we may be down to only 2 copies of some blocks until that server comes back up. Otherwise, space permitting, it can rebuild that third copy on other servers.

Now, has this been done before? As far as I can tell, no. Wouldn’t it be sweet?

But there are some projects that are close. Most notably, GlusterFS. GlusterFS does all of the above, except the automated bits. You can have this 3 copy redundancy, but you have to manually tell it where each copy goes, manually reconfigure if a server goes offline, etc. Other options such as NBD, OpenAFS, GFS, DRBD, Lustre, GFS, etc. aren’t really well-suited for this scenario for various reasons.

So, what does everyone think? Can this work? Has it been done outside of Google?