SUMMARY: Disk IO, what is and okay avserv

From: <>
Date: Tue Feb 26 2002 - 10:30:07 EST
Thanks to everyone who responded, but to summarise, verbatim from Deborah
Santomauro's email.

Do an "iostat -xn 30" (display output in tabular format for 30 second
Look at "%b" column - this is the percentage of time that the disk is busy.
this column is over 20 for any disk, then that disk may be a potential
Next look at the "asvc_t" column - this is the average service time for
transactions in milliseconds. Typically any disk that has a "%b" greater
than 20
and an "asvc_t" exceeding 50 milliseconds, then that system is essentially
by that disk - i.e. bottleneck problem that is rather severe.

Another column to look at is "%w". This is the percentage of time there are

transactions waiting for service. Normally this number should be below "5"
each disk. Higher than that, it could be an indication that the SCSI bus
may be
overloaded. Ideal situation is when it's at "0".

With "sar" the  queue lengths and wait times are measured when there is
something in the queue. If "%busy" is small, large queues and service times

probably represent the periodic efforts by the system to ensure that
blocks are written to the disk in a timely fashion.

My original question :

I run a sar -d and get the following output which shows avserv sitting
around 50.5 all day.  Could anyone explain what if any performance hit the
box may be taking.  The sd0,h is the primary swap.  The is a rather heavily
used 5500.  Is it worthwhile addressing this or is the performance gain
minimal ?

SunOS incldk01 5.6 Generic_105181-26 sun4u    02/19/02

08:00:01   device        %busy   avque   r+w/s  blks/s  avwait  avserv

08:20:01   sd0               7     1.0      11     146    36.2    50.5
                  sd0,h             7     1.0      11     146    36.2
08:40:01   sd0               6     1.0      11     134    37.5    52.4

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Received on Tue Feb 26 10:02:03 2002

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