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CIO/DIO and JFS2 read ahead

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Operating Systems AIX CIO/DIO and JFS2 read ahead
# 8  
Old 11-14-2008
pstat -a shows, like for example ps aux, only the number of current started aioservers, but not if they are busy or not. With nmon shift+a you see how many are really currently busy and this is on system with an app using AIO, going up and down. Often the maxreqs is forgotten to be configured somewhat large too so the app doesn't get errors regarding to hit the max of queued AIO requests (some "error 5" in Oracle for example).

Nevertheless, we will have to check out if the OP needs AIO at all, which he could and should find out. Maybe he uses it already, no idea. Maybe he offers us some more info to help him.
In our environments I used CIO as a kind of last ressort of tuning, when everything else with AIO and ioo tuning didn't help anymore.
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LEARN ABOUT V7

drv_usecwait

drv_usecwait(9F)					   Kernel Functions for Drivers 					  drv_usecwait(9F)

NAME

       drv_usecwait - busy-wait for specified interval

SYNOPSIS

       #include <sys/types.h>
       #include <sys/ddi.h>

       void drv_usecwait(clock_t microsecs);

INTERFACE LEVEL

       Architecture independent level 1 (DDI/DKI).

PARAMETERS

       microsecs       The number of microseconds to busy-wait.

DESCRIPTION

       drv_usecwait()  gives  drivers  a  means  of  busy-waiting  for a specified microsecond count. The amount of time spent busy-waiting may be
       greater than the microsecond count but will minimally be the number of microseconds specified.

       delay(9F) can be used by a driver to delay for a specified number of system ticks, but it has two limitations. First,  the  granularity	of
       the  wait  time	is limited to  one clock tick, which may be more time than is needed for the delay. Second,  delay(9F) may only be invoked
       from user context and hence cannot be used at interrupt time or system initialization.

       Often, drivers need to delay for only a few microseconds, waiting for a write to a device register to be picked up by the device.  In  this
       case, even in user context,  delay(9F) produces too long a wait period.

CONTEXT

       drv_usecwait() can be called from user or interrupt context.

SEE ALSO

       delay(9F), timeout(9F), untimeout(9F)

       Writing Device Drivers

NOTES

       The driver wastes processor time by making this call since drv_usecwait() does not block but simply busy-waits. The driver should only make
       calls to  drv_usecwait() as needed, and only for as much time as needed.   drv_usecwait() does not mask out interrupts.

SunOS 5.10							    12 Nov 1992 						  drv_usecwait(9F)