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Re: increasing the default WAL segment size

David Steele <david@pgmasters.net>

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From: David Steele <david@pgmasters.net>
To: Stephen Frost <sfrost@snowman.net>, Robert Haas <robertmhaas@gmail.com>
Cc: Beena Emerson <memissemerson@gmail.com>, tushar <tushar.ahuja@enterprisedb.com>, Prabhat Sahu <prabhat.sahu@enterprisedb.com>, Ashutosh Sharma <ashu.coek88@gmail.com>, Jim Nasby <Jim.Nasby@bluetreble.com>, Kuntal Ghosh <kuntalghosh.2007@gmail.com>, Andres Freund <andres@anarazel.de>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2017-03-21T14:41:56Z
Lists: pgsql-hackers 
On 3/21/17 9:04 AM, Stephen Frost wrote:
> Robert,
>
> * Robert Haas (robertmhaas@gmail.com) wrote:
>> On Mon, Mar 20, 2017 at 7:23 PM, David Steele <david@pgmasters.net> wrote:
>>> With 16MB WAL segments the filename neatly aligns with the LSN.  For
>>> example:
>>>
>>> WAL FILE 0000000100000001000000FE = LSN 1/FE000000
>>>
>>> This no longer holds true with this patch.
>>
>> It is already possible to change the WAL segment size using the
>> configure option --with-wal-segsize, and I think the patch should be
>> consistent with whatever that existing option does.
>
> Considering how little usage that option has likely seen (I can't say
> I've ever run into usage of it so far...), I'm not really sure that it
> makes sense to treat it as final when we're talking about changing the
> default here.

+1.  A seldom-used compile-time option does not necessarily provide a 
good model for a user-facing feature.

> In short, I'm also concerned about this change to make WAL file names no
> longer match up with LSNs and also about the odd stepping that you get
> as a result of this change when it comes to WAL file names.

I can't decide which way I like best.  I like the filenames 
corresponding to LSNs as they do now, but it seems like a straight 
sequence might be easier to understand.  Either way you need to know 
that different segment sizes mean different numbers of segments per 
lsn.xlogid.

Even now the correspondence is a bit tenuous.  I've always thought:

00000001000000010000000F

Should be:

00000001000000010F000000

I'm really excited to (hopefully) have this feature in v10.  I just want 
to be sure we discuss this as it will be a big change for tool authors 
and just about anybody who looks at WAL.

Thanks,
-- 
-David
david@pgmasters.net

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.