Re: increasing the default WAL segment size

On Sun, Jan 8, 2017 at 9:52 AM, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com> wrote:
>> I agree pretty_print_kb would have been a better for this function.
>> However, I have realised that using the show hook and this function is
>> not suitable and have found a better way of handling the removal of
>> GUC_UNIT_XSEGS which no longer needs this function : using the
>> GUC_UNIT_KB, convert the value in bytes to wal segment count instead in
>> the assign hook. The next version of patch will use this.
>
>
> ... it sounds like you're going back to exposing KB to users, and that's all
> that really matters.
>
>> IMHO it'd be better to use the n & (n-1) check detailed at [3].

That would be better.

So I am looking at the proposed patch, though there have been reviews
the patch was in "Needs Review" state, and as far as I can see it is a
couple of things for frontends. Just by grepping for XLOG_SEG_SIZE I
have spotted the following problems:
- pg_standby uses it to know about the next segment available.
- pg_receivexlog still uses it in segment handling.
It may be a good idea to just remove XLOG_SEG_SIZE and fix the code
paths that fail to compile without it, frontend utilities included
because a lot of them now rely on the value coded in xlog_internal.h,
but with this patch the value is set up in the context of initdb. And
this would induce major breakages in many backup tools, pg_rman coming
first in mind... We could replace it with for example a macro that
frontends could use to check if the size of the WAL segment is in a
valid range if the tool does not have direct access to the Postgres
instance (aka the size of the WAL segment used there) as there are as
well offline tools.

-#define XLogSegSize ((uint32) XLOG_SEG_SIZE)
+
+extern uint32 XLogSegSize;
+#define XLOG_SEG_SIZE XLogSegSize
This bit is really bad for frontend declaring xlog_internal.h...

--- a/src/bin/pg_test_fsync/pg_test_fsync.c
+++ b/src/bin/pg_test_fsync/pg_test_fsync.c
@@ -62,7 +62,7 @@ static const char *progname;

static int secs_per_test = 5;
static int needs_unlink = 0;
-static char full_buf[XLOG_SEG_SIZE],
+static char full_buf[DEFAULT_XLOG_SEG_SIZE],
This would make sense as a new option of pg_test_fsync.

A performance study would be a good idea as well. Regarding the
generic SHOW command in the replication protocol, I may do it for next
CF, I have use cases for it in my pocket.
--
Michael