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Re: increasing the default WAL segment size

David Rowley <david.rowley@2ndquadrant.com>

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From: David Rowley <david.rowley@2ndquadrant.com>
To: Michael Paquier <michael.paquier@gmail.com>
Cc: Jim Nasby <Jim.Nasby@bluetreble.com>, PostgreSQL mailing lists <pgsql-hackers@postgresql.org>, Beena Emerson <memissemerson@gmail.com>
Date: 2017-01-05T04:03:31Z
Lists: pgsql-hackers 
On 4 January 2017 at 01:16, Michael Paquier <michael.paquier@gmail.com> wrote:
> On Tue, Jan 3, 2017 at 6:23 AM, Jim Nasby <Jim.Nasby@bluetreble.com> wrote:
>> +               /* Check if wal_segment_size is in the power of 2 */
>> +               for (i = 0;; i++, pow2 = pow(2, i))
>> +                       if (pow2 >= wal_segment_size)
>> +                               break;
>> +
>> +               if (wal_segment_size != 1 && pow2 > wal_segment_size)
>> +               {
>> +                       fprintf(stderr, _("%s: WAL segment size must be in the power of 2\n"), progname);
>> +                       exit(1);
>> +               }
>
> I recall taht pow(x, 2) and x * x result usually in the same assembly
> code, but pow() can never be more optimal than a simple
> multiplication. So I'd think that it is wiser to avoid it in this code
> path. Documentation is missing for the new replication command
> SHOW_WAL_SEG. Actually, why not just having an equivalent of the SQL
> command and be able to query parameter values?

This would probably be nicer written using a bitwise trick to ensure
that no lesser significant bits are set. If it's a power of 2, then
subtracting 1 should have all the lesser significant bits as 1, so
binary ANDing to that should be 0. i.e no common bits.

Something like:

/* ensure segment size is a power of 2 */
if ((wal_segment_size & (wal_segment_size - 1)) != 0)
{
   fprintf(stderr, _("%s: WAL segment size must be in the power of
2\n"), progname);
   exit(1);
}

There's a similar trick in bitmapset.c for RIGHTMOST_ONE, so looks
like we already have assumptions about two's complement arithmetic

-- 
 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

Commits

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

  1. Make WAL segment size configurable at initdb time.

  2. Perform only one ReadControlFile() during startup.

  3. Introduce BYTES unit for GUCs.

  4. Remove useless duplicate inclusions of system header files.

  5. Refactor other replication commands to use DestRemoteSimple.

  6. Add a SHOW command to the replication command language.

  7. Add a new DestReceiver for printing tuples without catalog access.

  8. Support fls().

  9. Extend yesterday's patch making BLCKSZ and RELSEG_SIZE configurable to also

  10. Commit the reasonably uncontroversial parts of J.R. Nield's PITR patch, to

  11. XLOG (also known as WAL -:)) Bootstrap/Startup/Shutdown.

  12. Transaction log manager core code.