Re: increasing the default WAL segment size

From: David Steele <david(at)pgmasters(dot)net>
To: Beena Emerson <memissemerson(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: tushar <tushar(dot)ahuja(at)enterprisedb(dot)com>, Prabhat Sahu <prabhat(dot)sahu(at)enterprisedb(dot)com>, Ashutosh Sharma <ashu(dot)coek88(at)gmail(dot)com>, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>, Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: increasing the default WAL segment size
Date: 2017-03-20 23:23:46
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Hi Beena,

On 3/20/17 2:07 PM, Beena Emerson wrote:
> Added check for the version, the SHOW command will be run only in v10
> and above. Previous versions do not need this.

I've just had the chance to have a look at this patch. This is not a
complete review, just a test of something I've been curious about.

With 16MB WAL segments the filename neatly aligns with the LSN. For

WAL FILE 0000000100000001000000FE = LSN 1/FE000000

This no longer holds true with this patch. I created a cluster with 1GB
segments and the sequence looked like:


Whereas I had expected something like:


I scanned the thread but couldn't find any mention of this so I'm
curious to know if it was considered? Was the prior correspondence
merely serendipitous?

I'm honestly not sure which way I think is better, but I know either way
it represents a pretty big behavioral change for any tools looking at
pg_wal or using the various helper functions.

It's a probably a good thing to do at the same time as the rename, just
want to make sure we are all aware of the changes.


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