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Re: [RFC] : full-text instead of vdelta against empty bytestream

From: Greg Hudson <ghudson_at_MIT.EDU>
Date: 2003-11-05 02:30:58 CET

On Tue, 2003-11-04 at 17:48, C. Michael Pilato wrote:
> > That may be the reason, but I don't think it actually speeds up read
> > operations to use a plain text.
>
> Sorry, but I've read this three times, and I can't find an immediate
> reason why this statement would be correct. I can only assume that
> you mean that in the overall cost of hitting the database, perhaps the
> additional overhead of reading from a delta-versus-empty-file is lost
> in the noise?

1) As mbk points out, we'll be reading fewer blocks from the database.
Undeltification is cheap; disk I/O is expensive.

2) If we're retrieving any revision other than the head revision, we
already have to perform an undeltification operation. I think "combine
N deltas and apply the result to plaintext P" is probably about as slow
as "combine N+1 deltas and apply the result to the empty string."

3) If David succeeds in his work, we will save a deltification step for
checkouts of the head, without having to sacrifice compression. Since
deltification is relatively expensive, and checkouts of the head are
common in many contexts, that's a big win.

Actual measurements would be better than my speculations, of course.

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Received on Wed Nov 5 02:31:36 2003

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