Sar output

[Paula J Frazer-Campbell]

Hi to all

Output from sar -d :-

05:00:01 device %busy
05:01:01 c1t6d0 16.03

The question is what params / timescale are used by sar to calculate the %busy.

Paula

[Marcin Wicinski · Accepted solution]

Hi Paula !
sar uses procedures from pstat library. For disk utilization the procedure is pstat_getdisk. sar executes pstat_getdisk every n miliseconds ( I dont know the interval ) and checks if tested disk is working or not. According to collected answares: "busy", "idle" calculates the percentage of disk utilization. More information you can find on pstat man pages.
Later,

[John Palmer]

Hi Paula,

The equivalent metric in glance is described as:-

The percentage of the time during the interval that the disk device had IO in progress from the point of view of the Operating System. In other words, the utilization or percentage of time busy servicing requests for this device.

This is a measure of the ability of the IO path to meet the transfer demands being placed on it. Slower disk devices may show a higher utilization with lower IO rates than faster disk devices such as disk arrays. A value of greater than 50%
utilization over time may indicate that this device or its IO path is a
bottleneck, and the access pattern of the workload, database, or files may need reorganizing for better balance of disk IO load.

I'd guess that sar is reporting the same sort of information.

Regards,
John

[Paula J Frazer-Campbell]

John, Marcin

What I am trying to establish is the value calculated by a single hit or multiple hits and then averaged out?

Paula

[John Palmer]

Multiple checks over the monitoring period and the proportion that are 'busy' are returned as a percentage.

Regards,
John

[John Palmer]

Paula,

Having tusc'd sar, it appears that sar makes one call to pstat per disk in each monitoring period. The kernel must therefore maintain the %busy information by regular sampling of the disk queues.

Regards,
John

[Paula J Frazer-Campbell]

Hi John

Good idea I have tusc installed but forgot I could use it for this sort of info.

Only one point for the tusc info as you already have the 10.

Thanks paula.