Allow the wal_buffers setting to be auto-tuned to a reasonable value.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 0f73aae13def660371c34c8feda6e684e6366bdb
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2011-01-23T01:31:24Z
Releases: 9.1.0
Allow the wal_buffers setting to be auto-tuned to a reasonable value.

If wal_buffers is initially set to -1 (which is now the default), it's
replaced by 1/32nd of shared_buffers, with a minimum of 8 (the old default)
and a maximum of the XLOG segment size.  The allowed range for manual
settings is still from 4 up to whatever will fit in shared memory.

Greg Smith, with implementation correction by me.

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