Re: pg_serial early wraparound

From: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>
To: Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: pg_serial early wraparound
Date: 2017-02-27 06:33:10
Message-ID: CAEepm=1F0orbiAhFA2nT3TJGT__BEVLXvguT22evpuU8FV7hMA@mail.gmail.com
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On Mon, Feb 27, 2017 at 7:28 PM, Thomas Munro
<thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> On Wed, Nov 9, 2016 at 11:07 AM, Thomas Munro
> <thomas(dot)munro(at)enterprisedb(dot)com> wrote:
>> The SLRU managed by predicate.c can wrap around and overwrite data if
>> you have more than 1 billion active XIDs. That's because when SSI was
>> implemented, slru.c was limited to four digit segment names, which
>> implied a page limit that wasn't enough for pg_serial to have space
>> for every possible XID. We should probably rip that code out, because
>> SLRUs now support five digit segment names. Something like the
>> attached. I'll post a test script to demonstrate correct wraparound
>> behaviour around in time for one of the later CFs.
>
> Here is a shell script that shows a full rotation through xid space if
> you build PostgreSQL with TEST_OLDSERXID, which you can do by
> uncommenting a line in predicate.c. On master we see the SLRU
> segments go around the clock twice for each time xid goes around.
> With the patch it goes around just once, adding an extra character to
> the segment name to double the space.

I attached the wrong version. Here is the right one.

--
Thomas Munro
http://www.enterprisedb.com

Attachment Content-Type Size
ssi-slru-wraparound-test.sh application/x-sh 1.8 KB

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