Hi Les, On 2012‒08‒06 13:05:53-0500, Les Mikesell wrote: > On Mon, Aug 6, 2012 at 9:46 AM, Wessel Dankers > <wsl-backuppc-de...@fruit.je> wrote: > > > > The ideas overlap to a limited extent with the ideas[0] that Craig posted > > to this list. For instance, no more hardlinks, and garbage collection is > > done using flat-file databases. Some things are quite different. I'll try > > to explain my ideas here. > > Personally I think the hardlink scheme works pretty well up to about > the scale that I'd want on a single machine and you get a badly needed > atomic operation with links more or less for free.
Adding a chunk can be done atomically using a simple rename(). Removing a chunk can be done atomically using unlink(). The only danger lies in removing a chunk that is still being used (because there's a backup still in progress whose chunks aren't being counted yet by the gc procedure). The simplest way to prevent that is to grant exclusive access to the gc process. Note that hardlinks do not prevent this race either. It's a problem we need to solve anyway. Craig lists a couple of reasons to abandon the hardlink scheme in http://sourceforge.net/mailarchive/message.php?msg_id=27140176 > If you are going to do things differently, wouldn't it make sense to use > one of the naturally distributed scalable databases (bigcouch, riak, > etc.) for storage from the start since anything you do is going to > involve re-inventing the atomic operation of updating a link or replacing > it and the big win would be making this permit concurrent writes from > multiple servers? Using something like ceph/rados for storing the chunks could be interesting at some point but I'd like to keep things simple for now. cheers, -- Wessel Dankers <w...@fruit.je>
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