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Re: Relation forks & FSM rewrite patches

From: "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "pgsql-patches" <pgsql-patches(at)postgresql(dot)org>
Subject: Re: Relation forks & FSM rewrite patches
Date: 2008-08-01 11:26:53
Message-ID: 4892F2FD.1090704@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-patches
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(at)enterprisedb(dot)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.com

Attachment: relforks-2.patch.gz
Description: application/x-gzip (25.6 KB)
Attachment: fsm-rewrite-3.patch.gz
Description: application/x-gzip (42.6 KB)

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