Re: Transaction ID wraparound: problem and proposed solution

Tom Lane wrote:
> 
> Philip Warner <pjw@rhyme.com.au> writes:
> >> * disk space --- letting pg_log grow without bound isn't a pleasant
> >> prospect either.
> 
> > Maybe this can be achieved by wrapping XID for the log file only.
> 
> How's that going to improve matters?  pg_log is ground truth for XIDs;
> if you can't distinguish two XIDs in pg_log, there's no point in
> distinguishing them elsewhere.

One simple way - start a new pg_log file at each wraparound and encode 
the high 4 bytes in the filename (or in first four bytes of file)

> > Maybe I'm really missing the amount of XID manipulation, but I'd be
> > surprised if 16-byte XIDs would slow things down much.
> 
> It's not so much XIDs themselves, as that I think we'd need to widen
> typedef Datum too, and that affects manipulations of *all* data types.

Do you mean that each _field_ will take more space, not each _record_ ?

> In any case, the prospect of a multi-gigabyte, ever-growing pg_log file,
> with no way to recover the space short of dump/initdb/reload, is
> awfully unappetizing for a high-traffic installation...

The pg_log should be rotated anyway either with long xids or long-long
xids.

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Hannu