Re: backup manifests - Mailing list pgsql-hackers

Hi,

On 2020-03-30 14:35:40 -0400, Robert Haas wrote:
> On Sun, Mar 29, 2020 at 10:08 PM Andres Freund <andres@anarazel.de> wrote:
> > See the attached minimal prototype for what I am thinking of.
> >
> > This would not correctly handle the case where the timeline changes
> > while taking a base backup. But I'm not sure that'd be all that serious
> > a limitation for now?
> >
> > I'd personally not want to use a base backup that included a timeline
> > switch...
>
> Interesting concept. I've never (or almost never) used the -s and -e
> options to pg_waldump, so I didn't think about using those.

Oh - it's how I use it most of the time when investigating a specific
problem. I just about always use -s, and often -e. Besides just reducing
the logging output, and avoiding spurious errors, it makes it a lot
easier to iteratively expand the logging for records that are
problematic for the case at hand.


> I think
> having a --just-parse option to pg_waldump is a good idea, though
> maybe not with that name e.g. we could call it --quiet.

Yea, I didn't like the option's name. It's just the first thing that
came to mind.


> It is less obvious to me what to do about all that as it pertains to
> the current patch.

FWIW, I personally think we can live with this not validating WAL in the
first release. But I also think it'd be within reach to do better and
allow for WAL verification.


> If we want pg_validatebackup to run pg_waldump in that mode or print
> out a hint about how to run pg_waldump in that mode, it would need to
> obtain the relevant LSNs.

We could just include those in the manifest. Seems like good information
to have in there to me, as it allows to build the complete list of files
needed for a restore.


> It's not clear to me what we would do if the backup crosses a timeline
> switch, assuming that's even a case pg_basebackup allows.

I've not tested it, but it sure looks like it's possible. Both by having
a standby replaying from a node that promotes (multiple timeline
switches possible too, I think, if the WAL source follows timelines),
and by backing up from a standby that's being promoted.


> If we don't want to do anything in pg_validatebackup automatically but
> just want to document this as a a possible technique, we could finesse
> that problem with some weasel-wording.

It'd probably not be too hard to simply emit multiple commands, one for
each timeline "segment".

I wonder if it'd not be best, independent of whether we build in this
verification, to include that metadata in the manifest file. That's for
sure better than having to build a separate tool to parse timeline
history files.

I think it wouldn't be too hard to compute that information while taking
the base backup. We know the end timeline (ThisTimeLineID), so we can
just call readTimeLineHistory(ThisTimeLineID). Which should then allow
for something pretty trivial along the lines of

timelines = readTimeLineHistory(ThisTimeLineID);
last_start = InvalidXLogRecPtr;
foreach(lc, timelines)
{
    TimeLineHistoryEntry *he = lfirst(lc);

    if (he->end < startptr)
        continue;

    //
    manifest_emit_wal_range(Min(he->begin, startptr), he->end);
    last_start = he->end;
}

if (last_start == InvalidXlogRecPtr)
   start = startptr;
else
   start = last_start;

manifest_emit_wal_range(start, entptr);


Btw, just in case somebody suggests it: I don't think it's possible to
compute the WAL checksums at this point. In stream mode WAL very well
might already have been removed.

Greetings,

Andres Freund