Re: Wraparound

Thanks, I'll check it out.

On Tue, 20 Feb 2024, 13:12 Laurenz Albe, <laurenz(dot)albe(at)cybertec(dot)at> wrote:

> On Tue, 2024-02-20 at 12:57 +0530, Rajesh Kumar wrote:
> > On Tue, 20 Feb 2024, 12:53 Laurenz Albe, <laurenz(dot)albe(at)cybertec(dot)at>
> wrote:
> > > On Tue, 2024-02-20 at 10:02 +0530, Rajesh Kumar wrote:
> > > > Is autovacuum wraparound issue is when transaction id reaches
> > > > autovacuum_freeze_max_age or if transaction Reaches 2^32.
> > > >
> > > > My autovacuum_freeze_age setting is 200million.
> > > >
> > > > Db size is 150gb.
> > >
> > > 2^31 is the magical number at which very old transactions become
> > > transactions from the future, which would cause data loss unless
> > > all old rows have been frozen.
> >
> > Sorry, I could not understand. If possible, please elaborate. Otherwise,
> no issues
>
> The (32-bit) transaction numbers are stored in "xmin" and "xmax" in each
> row.
> You have to imagine these numbers as a closed ring: if you get past 2^32-1,
> you wrap around to 0 and continue.
>
> Now the 2^31 numbers before the current 32-bit transaction id are the past,
> and the 2^31 numbers after it are the future. This includes wraparound, so
> initially numbers close to 2^32 will also be past.
>
> As the current transaction id crosses 2^31, very old transactions suddenly
> become future transactions. All rows with a low "xmin" will suddenly
> appear
> to have been created in the future and become invisible. Some rows with a
> low "xmax" that have been deleted long ago can suddenly rise from the dead,
> since the transaction that deleted them now appears to be in the future.
>
> Perhaps you will find this article entertaining:
>
> https://www.cybertec-postgresql.com/en/transaction-id-wraparound-a-walk-on-the-wild-side/
>
> Yours,
> Laurenz Albe
>