Horton

Re: making relfilenodes 56 bits

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Thomas Munro <thomas.munro@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2022-06-25T01:30:26Z
Lists: pgsql-hackers 
Hi,

On 2022-06-24 10:59:25 -0400, Robert Haas wrote:
> A preliminary refactoring that was discussed in the past and was
> originally in 0001 was to move the fields included in BufferTag via
> RelFileNode/Locator directly into the struct. I think maybe it doesn't
> make sense to include that in 0001 as you have it here, but maybe that
> could be 0002 with the main patch to follow as 0003, or something like
> that. I wonder if we can get by with redefining RelFileNode like this
> in 0002:
> 
> typedef struct buftag
> {
>     Oid     spcOid;
>     Oid     dbOid;
>     RelFileNumber   fileNumber;
>     ForkNumber  forkNum;
> } BufferTag;

If we "inline" RelFileNumber, it's probably worth reorder the members so that
the most distinguishing elements come first, to make it quicker to detect hash
collisions. It shows up in profiles today...

I guess it should be blockNum, fileNumber, forkNumber, dbOid, spcOid? I think
as long as blockNum, fileNumber are first, the rest doesn't matter much.


> And then like this in 0003:
> 
> typedef struct buftag
> {
>     Oid     spcOid;
>     Oid     dbOid;
>     RelFileNumber   fileNumber:56;
>     ForkNumber  forkNum:8;
> } BufferTag;

Probably worth checking the generated code / the performance effects of using
bitfields (vs manual maskery). I've seen some awful cases, but here it's at a
byte boundary, so it might be ok.

Greetings,

Andres Freund

Commits

  1. Fix typos in comments, code and documentation

  2. Increase width of RelFileNumbers from 32 bits to 56 bits.

  3. Move RelFileNumber declarations to common/relpath.h.

  4. Include RelFileLocator fields individually in BufferTag.

  5. Convert macros to static inline functions (buf_internals.h)

  6. Remove the restriction that the relmap must be 512 bytes.

  7. Change internal RelFileNode references to RelFileNumber or RelFileLocator.

  8. Reorder subskiplsn in pg_subscription to avoid alignment issues.

  9. Fix DROP {DATABASE,TABLESPACE} on Windows.

  10. Restructure smgr API as per recent proposal. smgr no longer depends on