        or zero to disable issuance of asynchronous I/O requests. Currently,
         this setting only affects bitmap heap scans.
        </p><p>
         For magnetic drives, a good starting point for this setting is the
         number of separate
         drives comprising a RAID 0 stripe or RAID 1 mirror being used for the
         database.  (For RAID 5 the parity drive should not be counted.)
         However, if the database is often busy with multiple queries issued in
         concurrent sessions,