• dedicated
• DiscIOPerformanceTuning

Dedicated server disc I/O performance tuning

Follow the I/O path from the OS, down to the volume level, down to the individual disc, for a world of adventure...

• Optimise the web application!
  • Reduce the amount of logging done
  • Use indexes to access data blocks rather than bulk scans (i.e fewer read requests)
  • Cluster hot data blocks together (i.e fewer disc head seeks)
• Add RAM:
  • Avoids disc reads by caching more web pages;
  • Increases the percentage used for dirty buffers so it helps conglomerate writes as well;
  • Reduces the amount of virtual memory swapping performed.

• See Server file system performance tuning;

• Play with the OS I/O scheduler

• Use a battery backed write cache for:
  • Applications that perform an intensive number of writes to a single location. This will reduce the number of writes that physically hit the disc.
  • Transaction processing on an ACID database. This will hide the write latency.
• Split the work load over different physical volumes. For example, have separate volumes for small database, log files, user data, etc.
  • Or put the FS journal on another disc
• Try increasing the number of spindles:

  • RAID 0 will have the best read/write performance (BUT NO DATA SAFETY);

  • RAID 10 is the next best;
  • RAID 1 characteristics:
    • Write performance is slightly less than a single disc;
    • Linear read performance only good with good OS readahead;
    • Read performance otherwise can scale as requests split over drives;
  • RAID 5 characteristics:
    • Decent read performance;
    • Write performance is only good when writing entire strides (i.e. bulk data writes);
    • Database performance will suck;
    • RAID 5 when degraded will totally suck
  • RAID 6 great when reliability is everything.
• Ensure that you have sufficient bus bandwidth to the discs:
  • PCI-E or PCI-X
  • Point to Point (SAS, SATA) will be better than a shared bus (SCSI LVD, IDE)

    • This whitepaper contains slightly strange information about this.

  • Two IDE devices on the same cable will suck
• Increase the individual disc speed
  • Higher the RPM the better

• If data safety does not matter:

  • Turn on the disc write-back caches