LDKBMWND parameter: Difference between revisions

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(Added a detailed description on how LDKBMWND is calculated)
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<dl>
<dl>
<dt>Default value
<dt>Default value
<dd><var>[[NSERVS_parameter|NSERVS]]</var> plus 1 for releases up to and including V7.4. From V7.5 onwards, the default has been changed to a minimum of ( 0.5 * <var>[[NUMBUF_parameter|NUMBUF]]</var> ). NUMBUF will first be adjusted (rounded up) to be an even multiple of NLRUQ. Following that, if a value for LDKBMWND is specified in CCAIN and that value is greater than .5*NUMBUF, the value may still be adjusted:
<dd>V7.4 and earlier: <var>[[NSERVS_parameter|NSERVS]]</var> plus 1  
<li>The final value must be an even multiple of NLRUQ</li>
<p>
<li>The final value must not be greater than NUMBUF minus the total number of buffers that might possibly be open at any one time: i.e NUMBUF - (((NSERVS+NSUBTKS)*MAXOBUF)+14)*NLRUQ  </li>  
V7.5 and later: A minimum of ( 0.5 * <var>[[NUMBUF_parameter|NUMBUF]]</var> )</p>
<p>
V7.7 and later: If <var>[[NUMBUFG parameter|NUMBUFG]]</var> is greater than 0, <var>LDKBMWND</var> is ignored </p>
 
Before the value of <var>LDKBMWND</var> is adjusted or recalculated, the minimum size of <var>NUMBUF</var> must first be determined by this formula:
<p class="code">NLRUQ * ((NSERVS + NSUBTKS) * MAXOBUF + 15)</p>
<p>
If the current value of <var>NUMBUF</var> exceeds this value and is a multiple of <var>[[NLRUQ parameter|NLRUQ]]</var>, no further adjustments to <var>NUMBUF</var> are made.
</p>
Following that, if a value for <var>LDKBMWND</var> is specified in CCAIN and is greater than <code>0.5*NUMBUF</code>, the value can still be adjusted:
<ul>
<li>The final value must be an even multiple of <var>NLRUQ</var></li>
<li>The final value must <i>not</i> be greater than <var>NUMBUF</var> minus the total number of buffers that might possibly be open at any one time:  
<p class="code">NUMBUF - (((NSERVS+NSUBTKS)*MAXOBUF)+14)*NLRUQ</p> </li>
</ul>
<dt>Parameter type
<dt>Parameter type
<dd>System
<dd>System
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==Description==
==Description==
<p>
<p>
An anticipatory write window that starts writes for any modified buffers that enter the window</p>
<var>LDKBMWND</var> specifies an anticipatory write window that starts writes for any modified below-the-bar buffers that enter the window.</p>
<p>
<p>
When an anticipatory write for a modified buffer completes, the buffer is immediately reusable. </p>
When an anticipatory write for a modified buffer completes, the buffer is immediately reusable. </p>
<p>
If <var>[[NLRUQ parameter|NLRUQ]]</var> is set greater than 1, then the value of <var>LDKBMWND</var> is rounded up to a multiple of <var>NLRUQ</var>. <var>LDKBMWND</var> has a minimum size of one (1). </p>
<p>
<p>
High values of <var>LDKBMWND</var> might unnecessarily increase the number of writes done (measured by the DKWR statistic). Low values might cause excessive waiting for buffers (measured by the DKSWAIT statistic).</p>
High values of <var>LDKBMWND</var> might unnecessarily increase the number of writes done (measured by the DKWR statistic). Low values might cause excessive waiting for buffers (measured by the DKSWAIT statistic).</p>
<p>
 
Refer to [[Performance monitoring and tuning#Disk buffer monitor statistics and parameters|Disk buffer monitor statistics and parameters]] and [[Defining the runtime environment (CCAIN)#Disk buffers and Model 204 storage|Disk buffers and Model 204 storage]] for more information about disk buffers.  </p>
==See also==
<ul>
<li>[[Performance monitoring and tuning#Disk buffer monitor statistics and parameters|Disk buffer monitor statistics and parameters]] and [[Defining the runtime environment (CCAIN)#Disk buffers and Model 204 storage|Disk buffers and Model 204 storage]], for more information about anticipatory write windows and disk buffers </li>
 
<li><var>[[LDKBMWNG parameter|LDKBMWNG]]</var> </li>
</ul>


[[Category:System parameters]]
[[Category:System parameters]]
[[Category:Parameters]]
[[Category:Parameters]]

Latest revision as of 21:18, 22 September 2016

# of pages in DKBM-BTB anticipatory write window

Summary

Default value
V7.4 and earlier: NSERVS plus 1

V7.5 and later: A minimum of ( 0.5 * NUMBUF )

V7.7 and later: If NUMBUFG is greater than 0, LDKBMWND is ignored

Before the value of LDKBMWND is adjusted or recalculated, the minimum size of NUMBUF must first be determined by this formula:

NLRUQ * ((NSERVS + NSUBTKS) * MAXOBUF + 15)

If the current value of NUMBUF exceeds this value and is a multiple of NLRUQ, no further adjustments to NUMBUF are made.

Following that, if a value for LDKBMWND is specified in CCAIN and is greater than 0.5*NUMBUF, the value can still be adjusted:

  • The final value must be an even multiple of NLRUQ
  • The final value must not be greater than NUMBUF minus the total number of buffers that might possibly be open at any one time:

    NUMBUF - (((NSERVS+NSUBTKS)*MAXOBUF)+14)*NLRUQ

Parameter type
System
Where set
On User 0's parameter line by system manager
Related products
All
Introduced
Model 204 V6.1 or earlier

Description

LDKBMWND specifies an anticipatory write window that starts writes for any modified below-the-bar buffers that enter the window.

When an anticipatory write for a modified buffer completes, the buffer is immediately reusable.

High values of LDKBMWND might unnecessarily increase the number of writes done (measured by the DKWR statistic). Low values might cause excessive waiting for buffers (measured by the DKSWAIT statistic).

See also

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