Horton

Re: speed up a logical replica setup

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Alexander Lakhin <exclusion@gmail.com>, Peter Eisentraut <peter@eisentraut.org>, Euler Taveira <euler@eulerto.com>, Shlok Kyal <shlok.kyal.oss@gmail.com>, Tomas Vondra <tomas.vondra@enterprisedb.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>, Michael Paquier <michael@paquier.xyz>, Andres Freund <andres@anarazel.de>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Fabrízio de Royes Mello <fabriziomello@gmail.com>, vignesh C <vignesh21@gmail.com>
Date: 2024-07-02T12:24:07Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. pg_createsubscriber: Remove obsolete comment

  2. pg_createsubscriber: Fix an unpredictable recovery wait time.

  3. Fix unstable test in 040_pg_createsubscriber.

  4. Fix the testcase introduced in commit 81d20fbf7a.

  5. Further weaken new pg_createsubscriber test on Windows.

  6. Temporarily(?) weaken new pg_createsubscriber test on Windows.

  7. Make pg_createsubscriber warn if publisher has two-phase commit enabled.

  8. Make pg_createsubscriber more wary about quoting connection parameters.

  9. pg_createsubscriber: Remove failover replication slots on subscriber

  10. pg_createsubscriber: Remove replication slot check on primary

  11. pg_createsubscriber: Only --recovery-timeout controls the end of recovery process

  12. pg_createsubscriber: creates a new logical replica from a standby server

  13. Add some const decorations

  14. Add option force_initdb to PostgreSQL::Test::Cluster:init()

  15. Remove MSVC scripts

On Mon, Jul 1, 2024 at 8:22 PM Hayato Kuroda (Fujitsu)
<kuroda.hayato@fujitsu.com> wrote:
>
> > I have a different but possibly-related complaint: why is
> > 040_pg_createsubscriber.pl so miserably slow?  On my machine it
> > runs for a bit over 19 seconds, which seems completely out of line
> > (for comparison, 010_pg_basebackup.pl takes 6 seconds, and the
> > other test scripts in this directory take much less).  It looks
> > like most of the blame falls on this step:
> >
> > [12:47:22.292](14.534s) ok 28 - run pg_createsubscriber on node S
> >
> > AFAICS the amount of data being replicated is completely trivial,
> > so that it doesn't make any sense for this to take so long --- and
> > if it does, that suggests that this tool will be impossibly slow
> > for production use.  But I suspect there is a logic flaw causing
> > this.
>
> I analyzed the issue. My elog() debugging said that wait_for_end_recovery() was
> wasted some time. This was caused by the recovery target seeming unsatisfactory.
>
> We are setting recovery_target_lsn by the return value of pg_create_logical_replication_slot(),
> which returns the end of the RUNNING_XACT record. If we use the returned value as
> recovery_target_lsn as-is, however, we must wait for additional WAL generation
> because the parameter requires that the replicated WAL overtake a certain point.
> On my env, the function waited until the bgwriter emitted the XLOG_RUNNING_XACTS record.
>

IIUC, the problem is that the consistent_lsn value returned by
setup_publisher() is the "end +1" location of the required LSN whereas
the recovery_target_lsn used in wait_for_end_recovery() expects the
LSN value to be "start" location of required LSN.

> One simple solution is to add an additional WAL record at the end of the publisher
> setup. IIUC, an arbitrary WAL insertion can reduce the waiting time. The attached
> patch inserts a small XLOG_LOGICAL_MESSAGE record, which could reduce much execution
> time on my environment.
>

This sounds like an ugly hack to me and don't know if we can use it.
The ideal way to fix this is to get the start_lsn from the
create_logical_slot functionality or have some parameter like
recover_target_end_lsn but I don't know if this is a good time to
extend such a functionality.

-- 
With Regards,
Amit Kapila.