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Lund Performance Solutions


SOS Disc I/O Detail
The Disc I/O Detail screen is designed to allow you to gain detailed information on each of the disc devices configured on your system and to analyze the disc I/O performance indicators. Since some of the higher level data is provided on the Main screen (and Extended Disc line) you can use this screen to see cumulative counters, percentages of all I/O’s that are performed by each disc, and queue lengths, etc. Cumulative counters may be reset by selecting the RESET TOTALS key (F5).
This screen is useful for zeroing-in on detailed disc I/O data. All of your disc drives are displayed with an array of disc-related statistics.
To access the Disc I/O Detail Screen from the global screen:
  • Type S from the SOS Enter command: prompt to view the Screen Selection Menu screen.
  • From the Screen Selection Menu screen, enter D (Disc I/O Detail). The Disc I/O Detail screen will display.
  • Disc I/O Detail Screen Keys

    All of the Disc I/O Detail screen keys are identical to the keys of the CPU Detail Screen. See "CPU Detail Screen Keys" for details.

    Disc I/O Detail Screen Display Items

    The Disc I/O Detail screen can be viewed in two formats. Examples of the Disc I/O Detail screen are provided in "Graphical Format" and "Tabular Format". To toggle between a Graphic and Tabular display format press the G key or press the F4 function key (GRAPHIC DISPLAY/ TABULAR DISPLAY).

    Graphical Format

    Figure 14.1 shows an example of the Disc I/O Detail screen in graphical format. The graphic format of the Disc I/O Detail screen displays information on a drive-by-drive basis regarding overall drive utilization and I/O rates per second.


    Figure 14.1 SOS Disc I/O Detail screen (graphical format)

    Disc I/O Detail (graphical format)

    The Disc I/O Detail data items are described in the following table.
    Table 14.1 SOS Disc I/O Detail data items
    Data Item
    Description
    Dev
    This is the logical device number of the disc drive. If this is "ALL" then the values reported will represent averages for all disc drives.
    Percent Utilization
    A disc drive’s utilization measures the percentage of time a disc drive is in use.
    Performance Tip
    There is a correlation between disc utilization and disc queue lengths. In other words as utilization rises, there is more of a chance that I/O’s will begin to queue up behind an executing I/O. This can be helpful in your disc drive forum.
    I/Os per second
    This column of data represents three disc I/O statistics:
  • R Physical reads per second.
  • W Physical writes per second.
  • T Total rate of both reads and writes per second.
  • These numbers reflect the actual physical transfers between main memory and a particular disc device.
    Performance Tip
    A typical single disc drive (for example a 7937) can sustain I/O rates upward of 20-30 per second (total I/O). If rates consistently top this number it is possible that an I/O bottleneck exists. A disc bottleneck may exist even if the rates are not high, but this is an area of concern. The CPU wait for disc I/O time should be cross referenced (check the Global Screen CPU Statistics). If one or more disc drives are consistently deriving the majority of the activity while others are inactive, consider off-loading some of the busier files onto less active drives.

    Tabular Format

    Figure 14.2 shows the Disc I/O Detail tabular screen.


    Figure 14.2 SOS Disc I/O Detail screen (tabular format)

    Disc I/O Detail (tabular format)

    The following table contains a description of each component of the Disc I/O Detail tabular screen.
    Table 14.2 SOS Disc I/O Summary data items
    Data Item
    Description
    I/O %
    These numbers represent the percentage of all disc I/O performed by this disc device during the current and cumulative intervals.
    Performance Tip
    The cumulative number is especially helpful to determine how well balanced your disc drives are over a long period of time. It is nearly impossible to perfectly balance disc I/O among devices, but they can be balanced within some limit. If you notice that one or more drives are either greatly under-utilized or greatly over-utilized you should move files from one drive to another.
    Util %
    A disc drive’s utilization measures the percentage of time a disc drive is in use.
    Performance Tip
    There is a correlation between disc utilization and disc queue lengths. In other words as utilization rises, there is more of a chance that I/O’s will begin to queue up behind an executing I/O. This can be helpful in your disc drive forum.
    Rate/s:
  • Read
  • Write
  • Total
  • This section of data represents three disc I/O statistics:
  • R Physical reads per second.
  • W Physical writes per second.
  • T Total rate of both reads and writes per second.
  • These numbers reflect the actual physical transfers between main memory and a particular disc device. Bracketed numbers represent an average for each indicator.
    Counts:
  • Read
  • Write
  • Total
  • This section indicates the total number of disc reads, writes and a total of both reads and writes per second. These numbers reflect the actual physical transfers between main memory and a particular disc device (Dev - n).
    Avg. QLen
    This is the average length of the disc request queue for a particular disc device when another disc I/O request arrives at the drive. Disc I/O requests come in two flavors: cheap or expensive. While no disc I/O is a good I/O, those requiring more work from the disc devices are bad for performance. Sequentially reading an MPE file is relatively easy for a disc drive because the drive’s arm mechanism moves very little thus incurring shorter queues. If the mechanism moves wildly over the device’s platter to jump from a TurboIMAGE master set to a detail set, the cost for those particular I/O’s is substantially more and will usually incur longer queue depths.
    Performance Tip
    An average queue length of one (1.0) or greater is not acceptable. Keep in mind that it is normal to have rush hour situations during a day when the queues have high averages. This is especially true when a transaction manager reaches a check point and does a large amount of write posting. It is consistently long queue depths that are a bad sign. If you notice that a particular disc drive has consistently high queue depths you should explore the following possibilities:
  • Excessive disc arm movement because of heavily hit files on the same disc drive that depend on each other (balance files better!).
  • Database files inefficiencies (better housekeeping!).
  • Too much demand on a particular file (application design problem).
  • The disc drive is too slow of a model for your particular application (not very likely).
  • Service Time msec per I/O
    This is the amount of time in milliseconds required to service one I/O.
    Performance Tip
    If the service time is higher on one disc drive than the others, check data locality. Your data may need to be "balanced" between disc drives.

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