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From Thomas Mueller <muel...@adobe.com>
Subject Re: [MongoMK] BlobStore garbage collection
Date Tue, 06 Nov 2012 08:24:54 GMT

>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.


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