topas reports disk busy 0.0%
AnthonyEnglish 270000RKFN Visits (12303)
Have you ever noticed a system where the users are reporting slow response times, but topas -D (or pressing D from within the topas main screen) shows the Disk Busy is constantly 0.0%, even on disks with high I/O?
Suspicious, isn't it?
Let's try iostat -d:
System configuration: lcpu=4 drives=7 paths=14 vdisks=2
" Disk history since boot not available. "
What's going on?
The iostat command documentation explains
So, it's time to change the iostat statistics collection using the chdev command shown above. You can also change this using SMIT > System Environments > Change / Show Characteristics of Operating System and setting the field to True (use the tab key).
The lsattr command now shows:
This change will take effect immediately and it will survive the reboot (assuming you fix your performance problems and survive the reboot yourself). No need to run chdev in a startup script.
After changing this iostat setting to true in SMIT, or on the command line using chdev, the busy disk percentages are looking a bit healthier (or unhealthier).
The best performance alerting: UBA
We may have a performance problem, but we wouldn't have known if disks were the issue if we'd relied on a Disk Busy showing 0.0%. And so we thought there was no I/O issue, until we heard from that most reliable and diligent of performance monitors:
Free advice for patients
It's important to know what the real symptom is before you can address the cause. The art of being a doctor is correct diagnosis.
I'm told that when people visit doctors, they sometimes don't reveal the primary reason they're there. First they tell the doc about an ingrown toenail and then, once they think the doctor has a clue, they mention the little accident they had while practicing their Chainsaw Juggling Tightrope act.
At AIX Down Under, as a free service to our loyal readers (both of them - Hi Mum!), here's some advice for when you've got a medical complaint or a system performance problem:
The system that had the iostat statistics disabled - for performance reasons - was a pretty old one, running AIX 5.3 TL 4. It needs a bit of love and care, but the first step is to identify the real problem. Once you've got that sorted out, you're well on the way to the solution.