this might be a weird question from the leftfield, but are we actually sure that the existing data store concept is worth the trouble? afaiu it saves us from storing the same binary twice, but leads into the DSGC topic. would it be possible to make it optional to store/address binaries by hash (and thus not need DSGC for these configurations)?
In any case we should definitely avoid to require repo traversal for DSGC. This would operationally limit the repo sizes Oak can support. -- Michael Marth | Engineering Manager +41 61 226 55 22 | mma...@adobe.com<mailto:mma...@adobe.com> Barfüsserplatz 6, CH-4001 Basel, Switzerland On Nov 6, 2012, at 9:24 AM, Thomas Mueller wrote: Hi, 1- What's considered an "old" node or commit? Technically, anything other than the head revision is old but can we remove them right away or do we need to retain a number of revisions? If the latter, then how far back do we need to retain? we discussed this a while back, no good solution back then[1] Yes. Somebody has to decide which revisions are no longer needed. Luckily it doesn't need to be us :-) We might set a default value (10 minutes or so), and then give the user the ability to change that, depending on whether he cares more about disk space or the ability to read old data / roll back to an old state. To free up disk space, BlobStore garbage collection is actually more important, because usually 90% of the disk space is used by the BlobStore. So it would be nice if items (files) in the BlobStore are deleted as soon as possible after deleting old revisions. In Jackrabbit 2.x we have seen that node and data store garbage collection that has to traverse the whole repository is problematic if the repository is large. So garbage collection can be a scalability issue: if we have to traverse all revisions of all nodes in order to delete unused data, we basically tie garbage collection speed with repository size, unless if we find a way to run it in parallel. But running mark & sweep garbage collection completely in parallel is not easy (is it even possible? if yes I would have guessed modern JVMs should have it since a long time). So I think if we don't need to traverse the repository to delete old nodes, but just traverse the journal, this would be much less of a problem. Regards, Thomas
