Controlling system wide parameters
There are two parameters that control the system wide I/O pacing:
- maxpout: High-water mark that specifies the maximum number of pending I/Os to a file
- minpout: Low-water mark that specifies the point at which programs that have reached maxpout can resume writing to the file
The high- and low-water marks can be set by:
- smit -> System Environments -> Change / Show Characteristics of Operating System (smitty chgsys) and then entering the number of pages for the high- and low-water marks
- chdev -l sys0 -a maxpout=NewValue
chdev -l sys0 -a minpout=NewValue
Controlling per file system options
In Version 5.3, the I/O pacing can be tuned on a per file system bases. This tuning is done when using the mount command.
For example, mount -o minpout=40,maxpout=60 /fs
Another way to do this is to use SMIT or edit the /etc/filesystems.
Default and recommended values
The default value for the high- and low-water marks is 0, which disables I/O pacing.
Changes to the maxpout and minpout values take effect immediately and remain in place until they are explicitly changed.
It is a good idea to make the value of maxpout (and also the difference between maxpout and minpout) large enough so that they are greater than 4*numclust. This way sequential write-behind won’t be suspended due to I/O pacing.
The recommended value for maxpout should be (a multiple of 4) + 1 so that it works well with the VMM write-behind feature. The reason this works well is for the following
interaction:
1. The write-behind feature sends the previous four pages to disk when a logical
write occurs to the first byte of the fifth page (JFS with default numclust=1).
2. If the pacing high-water mark (maxpout) were a multiple of 4 (say, 8), a process would hit the high-water mark when it requested a write that extended into the
ninth page. It would be then put to sleep before the write-behind algorithm had a chance to detect that the fourth dirty page is complete and the four pages were ready to be written.
3. The process would then sleep with four full pages of output until its outstanding writes fell below the pacing low-water mark (minpout).
4. If on the other hand, the high-water mark had been set to 9, write-behind would get to schedule the four pages for output before the process was suspended.
While enabling VMM I/O pacing may improve response time for certain workloads, the workloads generating the large amounts of I/O will be slowed down because the
processes are put to sleep periodically instead of continuously streaming the I/Os.
Disk-I/O pacing can improve interactive response time in some situations where foreground or background programs that write large volumes of data are interfering with
foreground requests. If not used properly, however, it can reduce throughput excessively.
