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From Ian Boston <...@tfd.co.uk>
Subject Re: content hash of a tree
Date Tue, 15 Oct 2013 07:32:44 GMT
On 15 October 2013 08:13, Thomas Mueller <mueller@adobe.com> wrote:
> Hi,
>
>>I thought this was one of the fundamental principles that
>>allow for quick diff/merge and help identify a commit within the MVCC
>>timeline.
>
> I'm not sure where you heard or read that, but a content hash is not
> needed for this; a revision number that is changed whenever there is a
> modification is enough. (For each change in this node or any direct or
> indirect child node.) For the MongoMK, the revision is a combination of
> the timestamp, the cluster node id, and a counter; very similar to the
> MongoDB object id: http://docs.mongodb.org/manual/reference/object-id/
>
>>what was the reason to abandon this idea?
>
> As for the MongoDB: performance and scalability. A node lookup by content
> hash would be bad for performance, as it would require an index on a
> randomly distributed data. See also
> http://fr.slideshare.net/daumdna/mongodb-scaling-write-performance - page
> 9 (the red line is with an index on the content hash, the green line
> without). But even without such an index: maintaining the content hash
> would be prohibitively expensive and would prevent scalable writes.

With the current state of Oak, is Oak on Cassandra possible ?
Slide 45.
Ian

>
> Regards,
> Thomas
>

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