Re: POC: Cleaning up orphaned files using undo logs

Antonin Houska <ah@cybertec.at> wrote:

> Dmitry Dolgov <9erthalion6@gmail.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.

-- 
Antonin Houska
Web: https://www.cybertec-postgresql.com