Documentation on the fsync() patch from OpenLink

Submitted by:  Cees de Groot <C.deGroot@inter.nl.net>

doc/README.fsync

+Fsync() patch (backend -F option)
+=================================
+
+Normally, the Postgres'95 backend makes sure that updates are actually
+committed to disk by calling the standard function fsync() in
+several places. Fsync() should guarantee that every modification to
+a certain file is actually written to disk and will not hang around
+in write caches anymore. This increases the chance that a database
+will still be usable after a system crash by a large amount.
+
+However, this operation severely slows down Postgres'95, because at all
+those points it has to wait for the OS to flush the buffers. Especially
+in one-shot operations, like creating a new database or loading lots
+of data, you'll have a clear restart point if something goes wrong. That's
+where the -F option kicks in: it simply disables the calls to fsync(). 
+
+Without fsync(), the OS is allowed to do its best in buffering, sorting
+and delaying writes, so this can be a _very_ big perfomance increase. However,
+if the system crashes, large parts of the latest transactions will still hang
+around in memory without having been committed to disk - lossage of data
+is therefore almost certain to occur.
+
+So it's a tradeoff between data integrity and speed. When initializing a
+database, I'd use it - if the machine crashes, you simply remove the files
+created and redo the operation. The same goes for bulk-loading data: on
+a crash, you remove the database and restore the backup you made before
+starting the bulk-load (you always make backups before bulk-loading,
+don't you?).
+
+Whether you want to use it in production, is up to you. If you trust your
+operating system, your utility company, and your hardware, you might enable
+it; however, keep in mind that you're running in an unsecure mode and that
+performance gains will very much depend on access patterns (because it won't
+help on reading data). I'd recommend against it.