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Re: [WIP] In-place upgrade

From: "Robert Haas" <robertmhaas(at)gmail(dot)com>
To: "Zdenek Kotala" <Zdenek(dot)Kotala(at)sun(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [WIP] In-place upgrade
Date: 2008-11-03 04:22:16
Message-ID: 603c8f070811022022x582a9cbdp60798a6b87910edf@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
I tried to apply this patch to CVS HEAD and it blew up all over the
place.  It doesn't seem to be intended to apply against CVS HEAD; for
example, I don't have backend/access/heap/htup.c at all, so can't
apply changes to that file.  I was able to clone the GIT repository
with the following command...

git clone http://git.postgresql.org/git/~davidfetter/upgrade_in_place/.git

...but now I'm confused, because I don't see the changes from the diff
reflected in the resulting tree.  As you can see, I am not a git
wizard.  Any help would be appreciated.

Here are a few initial thoughts based mostly on reading the diff:

In the minor nit department, I don't really like the idea of
PageHeaderData_04, SizeOfPageHeaderData04, PageLayoutIsValid_04, etc.
I think the latest version should just be PageHeaderData and
SizeOfPageHeaderData, and previous versions should be, e.g.
PageHeaderDataV3.  It looks to me like this would cut a few hunks out
of this and maybe make it a bit easier to understand what is going on.
 At any rate, if we are going to stick with an explicit version number
in both versions, it should be marked in a consistent way, not _04
sometimes and just 04 other times.  My suggestion is e.g. "V4" but
YMMV.

The changes to nodeIndexscan.c and nodeSeqscan.c are worrisome to me.
It looks like the added code is (nearly?) identical in both places, so
probably it needs to be refactored to avoid code duplication.  I'm
also a bit skeptical about the idea of doing the tuple conversion
here.  Why here rather than ExecStoreTuple()?  If you decide to
convert the tuple, you can palloc the new one, pfree the old one if
ShouldFree is set, and reset shouldFree to true.

I am pretty skeptical of the idea that all of the HeapTuple* functions
can just be conditionalized on the page version and everything will
Just Work.  It seems like that is too low a level to be worrying about
such things.  Even if it happens to work for the changes between V3
and V4, what happens when V5 or V6 is changed in such a way that the
answer to HeapTupleIsWhatever is neither "Yes" nor "No", but rather
"Maybe" or "Seven"?  The performance hit also sounds painful.  I don't
have a better idea right now though...

I think it's going to be absolutely imperative to begin vacuuming away
old V3 pages as quickly as possible after the upgrade.  If you go with
the approach of converting the tuple in, or just before,
ExecStoreTuple, then you're going to introduce a lot of overhead when
working with V3 pages.  I think that's fine.  You should plan to do
your in-place upgrade at 1AM on Christmas morning (or whenever your
load hits rock bottom...) and immediately start converting the
database, starting with your most important and smallest tables.  In
fact, I would look whenever possible for ways to make the V4 case a
fast-path and just accept that the system is going to labor a bit when
dealing with V3 stuff.  Any overhead you introduce when dealing with
V3 pages can go away; any V4 overhead is permanent and therefore much
more difficult to accept.

That's about all I have for now... if you can give me some pointers on
working with this git repository, or provide a complete patch that
applies cleanly to CVS HEAD, I will try to look at this in more
detail.

...Robert

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