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Re: increasing the default WAL segment size

Tomas Vondra <tomas.vondra@2ndquadrant.com>

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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: pgsql-hackers@postgresql.org
Date: 2016-12-22T18:08:43Z
Lists: pgsql-hackers 
On 12/20/2016 02:19 PM, Andres Freund wrote:
> On 2016-12-20 08:10:29 -0500, Robert Haas wrote:
>> We could use the GUC assign hook to compute a mask and a shift, so
>> that this could be written as (CurrPos & mask_variable) == 0.  That
>> would avoid the division instruction, though not the memory access.
>
> I suspect that'd be fine.
>
>
>> I hope this is all in the noise, though.
>
> Could very well be.
>
>
>> I know this is code is hot but I think it'll be hard to construct a
>> test case where the bottleneck is anything other than the speed at
>> which the disk can absorb bytes.
>
> I don't think that's really true. Heikki's WAL changes made a *BIG*
> difference. And pretty small changes in xlog.c can make noticeable
> throughput differences both in single and multi-threaded
> workloads. E.g. witnessed by the fact that the crc computation used to
> be a major bottleneck (and the crc32c instruction still shows up
> noticeably in profiles).  SSDs have become fast enough that it's
> increasingly hard to saturate them.
>

It's not just SSDs. RAID controllers with write cache (which is 
typically just DRR3 memory anyway) have about the same effect even with 
spinning rust.

So yes, this might make a measurable difference.

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, 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.