Make WAL segment size configurable at initdb time.

For performance reasons a larger segment size than the default 16MB
can be useful. A larger segment size has two main benefits: Firstly,
in setups using archiving, it makes it easier to write scripts that
can keep up with higher amounts of WAL, secondly, the WAL has to be
written and synced to disk less frequently.

But at the same time large segment size are disadvantageous for
smaller databases. So far the segment size had to be configured at
compile time, often making it unrealistic to choose one fitting to a
particularly load. Therefore change it to a initdb time setting.

This includes a breaking changes to the xlogreader.h API, which now
requires the current segment size to be configured.  For that and
similar reasons a number of binaries had to be taught how to recognize
the current segment size.

Author: Beena Emerson, editorialized by Andres Freund
Reviewed-By: Andres Freund, David Steele, Kuntal Ghosh, Michael
    Paquier, Peter Eisentraut, Robert Hass, Tushar Ahuja
Discussion: https://postgr.es/m/CAOG9ApEAcQ--1ieKbhFzXSQPw_YLmepaa4hNdnY5+ZULpt81Mw@mail.gmail.com

1 files changed, 6 insertions, 0 deletions

diff --git a/src/include/pg_config_manual.h b/src/include/pg_config_manual.h
index f3b35297d18..9615a389af2 100644
--- a/src/include/pg_config_manual.h
+++ b/src/include/pg_config_manual.h
@@ -14,6 +14,12 @@
  */
 
 /*
+ * This is default value for wal_segment_size to be used at intidb when run
+ * without --walsegsize option. Must be a valid segment size.
+ */
+#define DEFAULT_XLOG_SEG_SIZE	(16*1024*1024)
+
+/*
  * Maximum length for identifiers (e.g. table names, column names,
  * function names).  Names actually are limited to one less byte than this,
  * because the length must include a trailing zero byte.