Horton

Re: Make relfile tombstone files conditional on WAL level

Thomas Munro <thomas.munro@gmail.com>

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From: Thomas Munro <thomas.munro@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Heikki Linnakangas <hlinnaka@iki.fi>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2021-09-29T03:29:16Z
Lists: pgsql-hackers 
On Wed, Aug 4, 2021 at 3:22 AM Robert Haas <robertmhaas@gmail.com> wrote:
> It's not really clear to me what problem is at hand. The problems that
> the tombstone system created for the async I/O stuff weren't really
> explained properly, IMHO. And I don't think the current system is all
> that ugly. it's not the most beautiful thing in the world but we have
> lots of way worse hacks. And, it's easy to understand, requires very
> little code, and has few moving parts that can fail. As hacks go it's
> a quality hack, I would say.

It's not really an AIO problem.  It's just that while testing the AIO
stuff across a lot of operating systems, we had tests failing on
Windows because the extra worker processes you get if you use
io_method=worker were holding cached descriptors and causing stuff
like DROP TABLESPACE to fail.  AFAIK every problem we discovered in
that vein is a current live bug in all versions of PostgreSQL for
Windows (it just takes other backends or the bgwriter to hold an fd at
the wrong moment).  The solution I'm proposing to that general class
of problem is https://commitfest.postgresql.org/34/2962/ .

In the course of thinking about that, it seemed natural to look into
the possibility of getting rid of the tombstones, so that at least
Unix systems don't find themselves having to suffer through a
CHECKPOINT just to drop a tablespace that happens to contain a
tombstone.

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