[tahoe-dev] Mutable or immutable?

[Shawn Willden]

Hi guys,

For GridBackup, I need to store a tree of small files that change fairly 
frequently.  These files are crucial, so I'm also a little concerned that 
there be no way to end up with neither.

One option, the one I have implemented now, uses a directory tree full of 
immutable files.  When one of them is modified, I upload the new version as 
an immutable file, then after it's safely uploaded, I unlink the old version 
and rename the new one into its place.

One concern is that this approach seems wasteful.  Since we don't have GC yet, 
the old versions may hang around for a long time consuming storage server 
space.  If I use a mutable file and modify it "in place", do the storage 
servers discard the old version?  If they do, is there a window of 
vulnerability during which BOTH versions could be lost if the new upload gets 
interrupted?

Thanks,

	Shawn.