AIX: I/O Tuning Parameters

I/O Tuning Parameters:

    1. numfsbufs (vmtune –b) specifies the number of file system buffer
      structures. This value is critical asVMM will put a process on the wait list
      if there are insufficient free buffer structures.


  1. Run vmtune –a (pre 5.2) vmstat –v (5.2 & >) and monitor
    fsbufwaitcnt. This is incremented each time an I/O operation has to wait for
    file system buffer structures.
  2. A general technique is to double the numfsbufs value (up to a maximum of
    512) until fsbufwaitcount no longer increases. This value, as it is dynamic,
    should be re-executed on boot prior to any mount all command.
  3. hd_pbuf_cnt (vmtune –B) determines the number of pbufs assigned to LVM.
    pbufs are pinned memory buffers used to hold pending I/O requests.
  4. Again, examine vmtune –a and review the psbufwaitcnt. If increasing,
    multiply the current hd_pbuf_cnt by 2 until psbufwaitcnt stops incrementing.
  5. Because the hd_pbuf_cnt can only be reduced via a reboot (this is
    pinned memory) – be frugal when increasing this value.

I/O Tuning :

  1. Over 35% I/O wait should be investigated.
  2. Oracle databases like async I/O, DB2 & Sybase do not care (a good
    place to start would be AIO PARMS of
  4. Recent technology disks will support higher ltg numbers
  5. lvmstat (must be enabled prior to usage) provides detailed information for
    I/O contention
  6. filemon is an excellent I/O tool (trace – ensure you turn it off)
  7. numfsbufs and hd_pbuf_cnt adjusted to reduce wait counts in vmtune or
    vmstat -v

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