Re: [RFC PATCH 00/18] Multi-pack index (MIDX)
- Date: Wed, 10 Jan 2018 11:25:47 -0700
- From: Martin Fick <mfick@xxxxxxxxxxxxxx>
- Subject: Re: [RFC PATCH 00/18] Multi-pack index (MIDX)
On Sunday, January 07, 2018 01:14:41 PM Derrick Stolee
> This RFC includes a new way to index the objects in
> multiple packs using one file, called the multi-pack
> index (MIDX).
> The main goals of this RFC are:
> * Determine interest in this feature.
> * Find other use cases for the MIDX feature.
My interest in this feature would be to speed up fetches
when there is more than one large pack-file with many of the
same objects that are in other pack-files. What does your
MIDX design do when it encounters multiple copies of the
same object in different pack files? Does it index them all,
or does it keep a single copy?
In our Gerrit instance (Gerrit uses jgit), we have multiple
copies of the linux kernel repos linked together via the
alternatives file mechanism. These repos have many different
references (mostly Gerrit change references), but they share
most of the common objects from the mainline. I have found
that during a large fetch such as a clone, jgit spends a
significant amount of extra time by having the extra large
pack-files from the other repos visible to it, usually around
an extra minute per instance of these (without them, the
clone takes around 7mins). This adds up easily with a few
repos extra repos, it can almost double the time.
My investigations have shown that this is due to jgit
searching each of these pack files to decide which version of
each object to send. I don't fully understand its selection
criteria, however if I shortcut it to just pick the first
copy of an object that it finds, I regain my lost time. I
don't know if git suffers from a similar problem? If git
doesn't suffer from this then it likely just uses the first
copy of an object it finds (which may not be the best object
It would be nice if this use case could be improved with
MIDX. To do so, it seems that it would either require that
MIDX either only put "the best" version of an object (i.e.
pre-select which one to use), or include the extra
information to help make the selection process of which copy
to use (perhaps based on the operation being performed)
This also leads me to ask, what other additional information
(bitmaps?) for other operations, besides object location,
might suddenly be valuable in an index that potentially
points to multiple copies of objects? Would such
information be appropriate in MIDX, or would it be better in
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