Responses to Vivek Sharma's multiple postings on Oracle-L

From: Gaja Krishna Vaidyanatha <oraperfman_at_yahoo.com>
Date: Thu, 28 Jun 2001 12:42:57 -0700
Message-ID: <F001.0033C916.20010628125115@fatcity.com>

Dear Vivek,

I totally respect your "requests for clarification" and I will be more than happy to give them to you. But going forward, I urge you NOT to send "specific questions" on bullet points in the powerpoint slides of my presentations to the list (and that too multiple times), instead send it to me directly at - gajav_at_yahoo.com.

Your questions "directed to me" but posted on the list may not be appropriate for the following reasons:

People may not have access to the PPT you are referring to, and thus may not have any "point of reference".
Not everyone (yourself included) have the background material that is covered during the presentation, as part the talk (explanations that are done during a presentation). There may be a lot of content that is missed. Bullets in my PPTs are
"outlines" for me to talk and expand further.
#2 can potentially lead people to arrive at
"erroneous conclusions" as some bullet points may be
"taken out of context". As a result, this may raise
more questions.
Lastly, you may be cluttering the Inbox of a few thousand people for "questions directed to me NOT to the list" and people may not appreciate that.

Here are responses to your queries:

sar -d Interpretation. "High disk queue numbers + high service times -> I/O contention". What values of disk queue numbers, high service times are Considered High ?

Ans. For most storage architectures available today, an average service time for I/O requests that exceeds 20ms is considered a "bottleneck". Service times of 20ms or above are usually accompanied by "wait queue and run queue" numbers that are non-zero. What is
"high" for one environment, may not be relevant for
another. The 20ms threshold may not be applicable to some of the more esoteric storage devices (such as solid state disks). These are "guidelines" NOT
"draconian rules".

2) sar -d Interpretation. "%busy can be high if service times are within (4 *CPU time-slice)". What does CPU time-slice mean & how can it be Calculated ?

Ans. The "4*CPU time-slice" guideline is a method we used in the past to diagnose I/O bottlenecks. I mention this to "lay some foundation and history" during my presentation.

For example, if your system's CPU timeslice was set at 10ms, any service time above 40ms was considered to be
"high". CPU time slice is the amount of CPU time a
process gets when requesting the CPU, until it consumes the slice, or undergoes a context switch to perform I/O. The CPU timeslice is a OS kernel configurable parameter. Please check your OS docs for specific details. Again, a guideline NOT a rule.

3) How to configure sort_write_buffers and sort_write_buffer_size?

Ans. These parameters are configured (prior to Oracle8i) to enable Oracle to write the temporary segments directly to disk, by bypassing the database buffer cache. This is only relevant when configuring SORT_DIRECT_WRITES to TRUE. The 2 related parameters namely SORT_WRITE_BUFFERS and SORT_WRITE_BUFFER_SIZE can be set to 4 and 65536 (but the max. values for these may be OS dependent). From Oracle 8.1 onwards, the only configurable sort parameters are SORT_AREA_SIZE, SORT_AREA_RETAINED_SIZE and SORT_MULTIBLOCK_READ_COUNT, as all other sort-related parameters have been de-supported.

Hope that helps,

Gaja

Gaja Krishna Vaidyanatha
Director, Storage Management Products,
Quest Software, Inc.
Co-author - Oracle Performance Tuning 101 http://www.osborne.com/database_erp/0072131454/0072131454.shtml

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Received on Thu Jun 28 2001 - 14:42:57 CDT