IBM DS6000 Series Unix performance monitoring tools, Iostat, 301, Example A-1 AIX iostat output

the data, even if this pool spans several ranks. If possible, the extents for one logical volume are taken from the same rank.

To get higher throughput values than a single array can deliver, it is necessary to stripe the data across several arrays. This can only be achieved through striping on the host level.

To achieve maximum granularity and control for data placement, you will have to create an extent pool for every single rank.

However, some operating systems support only a limited number of attached disks, or make it difficult for the administrator to combine several physical disks into one big volume. In the DS6000 logical volumes cannot span several extent pools. To be able to create very large logical volumes, you must consider having extent pools that include more than one rank.

UNIX performance monitoring tools

Some tools are worth discussing because they are available for almost all UNIX variants and system administrators are accustomed to using them. You may have to administer a server and these are the only tools you have available to use. These tools offer a quick way to tell whether a system is I/O bound:

￿iostat

￿sar (System Activity Report)

￿vmstat (Virtual Memory Statistics)

IOSTAT

The base tool for evaluating I/O performance of disk devices for UNIX operating systems is iostat. Although available on most UNIX platforms, iostat varies in its implementation from system to system.

The iostat command is useful to determine whether a system’s I/O load is balanced or whether a single volume is becoming a performance bottleneck. The tool reports I/O statistics for TTY devices, disks, and CD-ROMs. It monitors I/O device throughput and utilization by observing the time the disks are active in relation to their average transfer rates.

Tip: I/O activity monitors, such as iostat, have no way of knowing whether the disk they are seeing is a single physical disk or a logical disk striped upon multiple physical disks in a RAID array. Therefore, some performance figures reported for a device, for example, %busy, could appear high.

Example A-1shows a sample iostat output, taken on an AIX host. It shows disk device statistics since the last reboot.

Example: A-1 AIX iostat output

#iostat
Disks:	% tm_act	Kbps	tps	Kb_read	Kb_wrtn
hdisk0	0.0	0.3	0.0	29753	48076
hdisk1	0.1	0.1	0.0	11971	26460
hdisk2	0.2	0.8	0.1	91200	108355
cd0	0.0	0.0	0.0	0	0

Appendix A. Operating systems specifics 301