From: Heikki Linnakangas <heikki@enterprisedb.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-patches <pgsql-patches@postgresql.org>
Subject: Re: Relation forks & FSM rewrite patches
Date: Fri, 01 Aug 2008 14:26:53 +0300
Message-ID: <4892F2FD.1090704@enterprisedb.com> (raw)
In-Reply-To: <22640.1215887277@sss.pgh.pa.us>
References: <48651722.8020600@enterprisedb.com>
<22640.1215887277@sss.pgh.pa.us>
Attached is an new version of the relation forks and FSM patches,
updated per Tom's comments (details below). I think the relation forks
patch is ready for commit, except that the docs on physical storage
needs to be updated. Barring any objections, I will commit the relation
forks patch in a few days, and submit a patch for the docs.
The FSM patch has been updated so that it applied over the new relation
forks patch.
Tom Lane wrote:
> "Heikki Linnakangas" <heikki@enterprisedb.com> writes:
>> The one part that I'm not totally satisfied in the relation forks patch
>> is the smgrcreate() function. The question problem is: which piece of
>> code decides which forks to create for a relation, and when to create
>> them? I settled on the concept that all forks that a relation will need
>> are created at once, in one smgrcreate() call. There's no function to
>> create additional forks later on.
>
> I think that's an extremely bad idea. You need look no further than
> Zdenek's hopes of in-place-upgrade to see a need for adding a fork
> to an existing relation; but even without that I can see possibly
> wanting to not create a fork right away. I think smgrcreate() should
> just create one fork as it's told (ie, same API you gave mdcreate).
Yeah, that's better.
> Other nits:
>
> relpath() is now in danger of buffer overrun: you need to increase
> the palloc() sizes.
Oops, fixed.
> Seems like a #define for the max number of digits
> in a ForkNumber wouldn't be out of place (though I sure hope it never
> gets past 1 ...).
Done. It makes it more obvious how the buffer length is calculated than
using plain numbers.
> Also, I strongly suggest *not* appending _0 to the name of the main fork's
> file. This'd break on-disk compatibility and people's expectations,
> for no particular reason.
Done.
> Don't like the name NUM_FORKS; it seems to suggest that's the actual
> number of forks in existence. MAX_NUM_FORKS would be better.
I don't like MAX_NUM_FORKS. I renamed it to MAX_FORKNUM (and changed its
semantics accordingly).
> I think that setting InvalidForkNumber to -1 is unportable: there is
> nothing compelling the compiler to represent enum ForkNumber as a signed
> type, so the places where you assume a ForkNumber variable can hold
> InvalidForkNumber all look like trouble. One solution is to include
> InvalidForkNumber in the enum:
>
> enum ForkNumber {
> InvalidForkNumber = -1,
> MAIN_FORKNUM = 0
> };
>
> This would be a bit annoying if someone wrote a switch statement listing
> different possible fork numbers, as the compiler would complain if
> there's no case for InvalidForkNumber; but I can't see a reason for code
> like that to be common.
I chose this approach. There's no switch constructs like that at the
moment, and I don't see any immediate need for one. In fact, at the
moment InvalidForkNumber is only used in bgwriter database fsync
requests, where the fork number field doesn't mean anything.
> BTW, don't put a comma at end of enum ForkNumber, I think some compilers
> will barf on that.
Done.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.comAttachments:
fsm-rewrite-3.patch.gz (application/x-gzip, 42.6K) download
relforks-2.patch.gz (application/x-gzip, 25.6K) download
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To: pgsql-patches@postgresql.org
Cc: heikki@enterprisedb.com, tgl@sss.pgh.pa.us
Subject: Re: Relation forks & FSM rewrite patches
In-Reply-To: <4892F2FD.1090704@enterprisedb.com>
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