pgsql: Increase default vacuum_buffer_usage_limit to 2MB.

Increase default vacuum_buffer_usage_limit to 2MB.

The BAS_VACUUM ring size has been 256kB since commit d526575f introduced
the mechanism 17 years ago. Commit 1cbbee03 recently made it
configurable but retained the traditional default. The correct default
size has been debated for years, but 256kB is certainly very small.
VACUUM soon needs to write back data it dirtied only 32 blocks ago,
which usually requires flushing the WAL. New experiments in prefetching
pages for VACUUM exacerbated the problem by crashing into dirty data
even sooner. Let's make the default 2MB. That's 1.6% of the default
toy buffer pool size, and 0.2% of 1GB, which would be a considered a
small shared_buffers setting for a real system these days. Users are
still free to set the GUC to a different value.

Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/20240403221257.md4gfki3z75cdyf6%40awork3.anarazel.de
Discussion: https://postgr.es/m/CA%2BhUKGLY4Q4ZY4f1rvnFtv6%2BPkjNf8MejdPkcju3Qii9DYqqcQ%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/98f320eb2ef05072b6fe67fcdcdc26c226e6cea4

Modified Files
--------------
doc/src/sgml/config.sgml | 2 +-
src/backend/storage/buffer/freelist.c | 2 +-
src/backend/utils/init/globals.c | 2 +-
src/backend/utils/misc/guc_tables.c | 2 +-
src/backend/utils/misc/postgresql.conf.sample | 2 +-
5 files changed, 5 insertions(+), 5 deletions(-)