Re: POC: Cleaning up orphaned files using undo logs - Mailing list pgsql-hackers

> On Fri, Dec 04, 2020 at 10:22:42AM +0100, Antonin Houska wrote:
> Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> > On Fri, Nov 13, 2020 at 6:02 PM Antonin Houska <ah@cybertec.at> wrote:
> > >
> > > Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > > > On Thu, Nov 12, 2020 at 2:45 PM Antonin Houska <ah@cybertec.at> wrote:
> > > >
> > > > If you want to track at undo record level, then won't it lead to
> > > > performance overhead and probably additional WAL overhead considering
> > > > this action needs to be WAL-logged. I think recording at page-level
> > > > might be a better idea.
> > >
> > > I'm not worried about WAL because the undo execution needs to be WAL-logged
> > > anyway - see smgr_undo() in the 0005- part of the patch set. What needs to be
> > > evaluated regarding performance is the (exclusive) locking of the page that
> > > carries the progress information.
> > >
> >
> > That is just for one kind of smgr, think how you will do it for
> > something like zheap. Their idea is to collect all the undo records
> > (unless the undo for a transaction is very large) for one zheap-page
> > and apply them together, so maintaining the status at each undo record
> > level will surely lead to a large amount of additional WAL. See below
> > how and why we have decided to do it differently.
> >
> > > I'm still not sure whether this info should
> > > be on every page or only in the chunk header. In either case, we have a
> > > problem if there are two or more chunks created by different transactions on
> > > the same page, and if more than on of these transactions need to perform
> > > undo. I tend to believe that this should happen rarely though.
> > >
> >
> > I think we need to maintain this information at the transaction level
> > and need to update it after processing a few blocks, at least that is
> > what was decided and implemented earlier. We also need to update it
> > when the log is switched or all the actions of the transaction were
> > applied. The reasoning is that for short transactions it won't matter
> > and for larger transactions, it is good to update it after a few pages
> > to avoid WAL and locking overhead. Also, it is better if we collect
> > the undo in bulk, this is proved to be beneficial for large
> > transactions.
>
> Attached is what I originally did not include in the patch series, see the
> part 0012. I have no better idea so far. The progress information is stored in
> the chunk header.
>
> To avoid too frequent locking, maybe the UpdateLastAppliedRecord() function
> can be modified so it recognizes when it's necessary to update the progress
> info. Also the user (zheap) should think when it should call the function.
> Since I've included 0012 now as a prerequisite for discarding (0013),
> currently it's only necessary to update the progress at undo log chunk
> boundary.
>
> In this version of the patch series I wanted to publish the remaining ideas I
> haven't published yet.

Thanks for the updated patch. As I've mentioned off the list I'm slowly
looking through it with the intent to concentrate on undo progress
tracking. But before I will post anything I want to mention couple of
strange issues I see, otherwise I will forget for sure. Maybe it's
already known, but running several times 'make installcheck' against a
freshly build postgres with the patch applied from time to time I
observe various errors.

This one happens on a crash recovery, seems like
UndoRecordSetXLogBufData has usr_type = USRT_INVALID and is involved in
the replay process:

    TRAP: FailedAssertion("page_offset + this_page_bytes <= uph->ud_insertion_point", File: "undopage.c", Line: 300)
    postgres: startup recovering 000000010000000000000012(ExceptionalCondition+0xa1)[0x558b38b8a350]
    postgres: startup recovering 000000010000000000000012(UndoPageSkipOverwrite+0x0)[0x558b38761b7e]
    postgres: startup recovering 000000010000000000000012(UndoReplay+0xa1d)[0x558b38766f32]
    postgres: startup recovering 000000010000000000000012(XactUndoReplay+0x77)[0x558b38769281]
    postgres: startup recovering 000000010000000000000012(smgr_redo+0x1af)[0x558b387aa7bd]

This one is somewhat similar:

    TRAP: FailedAssertion("page_offset >= SizeOfUndoPageHeaderData", File: "undopage.c", Line: 287)
    postgres: undo worker for database 36893 (ExceptionalCondition+0xa1)[0x5559c90f1350]
    postgres: undo worker for database 36893 (UndoPageOverwrite+0xa6)[0x5559c8cc8ae3]
    postgres: undo worker for database 36893 (UpdateLastAppliedRecord+0xbe)[0x5559c8ccd008]
    postgres: undo worker for database 36893 (smgr_undo+0xa6)[0x5559c8d11989]

There are also here and there messages about not found undo files:

    ERROR:  cannot open undo segment file 'base/undo/000008.0000020000': No such file or directory
    WARNING:  failed to undo transaction

I haven't found out the trigger yet, but got an impression that it
happens after create_table tests.