Re: POC: Cleaning up orphaned files using undo logs

From: Dmitry Dolgov <9erthalion6(at)gmail(dot)com>
To: Antonin Houska <ah(at)cybertec(dot)at>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: POC: Cleaning up orphaned files using undo logs
Date: 2021-09-17 16:20:45
Message-ID: 20210917162045.75no2bqiy6fparpi@localhost
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> On Tue, Sep 14, 2021 at 10:51:42AM +0200, Antonin Houska wrote:
> Antonin Houska <ah(at)cybertec(dot)at> wrote:
> > Dmitry Dolgov <9erthalion6(at)gmail(dot)com> wrote:
> > > * By throwing at the patchset `make installcheck` I'm getting from time to time
> > > and error on the restart:
> > >
> > > TRAP: FailedAssertion("BufferIsValid(buffers[nbuffers].buffer)",
> > > File: "undorecordset.c", Line: 1098, PID: 6055)
> > >
> > > From what I see XLogReadBufferForRedoExtended finds an invalid buffer and
> > > returns BLK_NOTFOUND. The commentary says:
> > >
> > > If the block was not found, then it must be discarded later in
> > > the WAL.
> > >
> > > and continues with skip = false, but fails to get a page from an invalid
> > > buffer few lines later. It seems that the skip flag is supposed to be used
> > > this situation, should it also guard the BufferGetPage part?
> >
> > I could see this sometime too, but can't reproduce it now. It's also not clear
> > to me how XLogReadBufferForRedoExtended() can return BLK_NOTFOUND, as the
> > whole undo log segment is created at once, even if only part of it is needed -
> > see allocate_empty_undo_segment().
> I could eventually reproduce the problem. The root cause was that WAL records
> were created even for temporary / unlogged undo, and thus only empty pages
> could be found during replay. I've fixed that and also setup regular test for
> the BLK_NOTFOUND value. That required a few more fixes to UndoReplay().
> Attached is a new version.

Yep, makes sense, thanks. I have few more questions:

* The use case with orphaned files is working somewhat differently after
the rebase on the latest master, do you observe it as well? The
difference is ApplyPendingUndo -> SyncPostCheckpoint doesn't clean up
an orphaned relation file immediately (only later on checkpoint)
because of empty pendingUnlinks. I haven't investigated more yet, but
seems like after this commit:

commit 7ff23c6d277d1d90478a51f0dd81414d343f3850
Author: Thomas Munro <tmunro(at)postgresql(dot)org>
Date: Mon Aug 2 17:32:20 2021 +1200

Run checkpointer and bgwriter in crash recovery.

Start up the checkpointer and bgwriter during crash recovery (except in
--single mode), as we do for replication. This wasn't done back in
commit cdd46c76 out of caution. Now it seems like a better idea to make
the environment as similar as possible in both cases. There may also be
some performance advantages.

something has to be updated (pendingOps are empty right now, so no
unlink request is remembered).

* What happened with the idea of abandoning discard worker for the sake
of simplicity? From what I see limiting everything to foreground undo
could reduce the core of the patch series to the first four patches
(forgetting about test and docs, but I guess it would be enough at
least for the design review), which is already less overwhelming.

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