I wanted to follow up on my previous post; Is a Virtual Machine Bringing Your Storage Down? with some test in my home lab. Nothing ‘real life’ but enough to get familiar with these new features.
Control the IOPS is something crucial in a shared storage resources environment. VMware vSphere 4.1 has several techniques to do that and the premium feature is known as SIOC or Storage I/O Control. If you have the proper license just go for it and turn it on! Don’t forget to follow the requirements and recommendations.
Now if you haven’t bought vSphere 4.1 Enterprise Plus licenses, you still have other built-in storage features that will be used by your vSphere 4.1 hosts to manage, with great fairness, available storage resources. To name a few: Disk.schednumreqoutstanding, QFullSampleSize and QFullThreshold.
This week in my cave man, I’ve played a bit with a couple of those vSphere 4.1 new features, name it sched.scsix:x.throughputCap and sched.scsix:x.bandwidthCap. I have recorded and published a small video you can watch below.
In summary here are my observations:
- Apparently you cannot use both parameters at the same time. It is either throughputCap or bandwidthCap per disk. If that makes sense to any one please comment 🙂
- If you do use both parameters for the same disk, they are just ignored by the host.
- sched.scsi0:0.bandwidthCap must be set in Bps. The VMware KB isn’t that clear about this. Why can’t I set IOps here?
- If for instance your VM has two disks and you set sched.scsi0:0.bandwidthCap=100IOps for the first disk and sched.scsi0:1.bandwidthCap=50IOps for the second disk, the values are added up and actual limit for any of the disks is 150IOps. Is this a bug or by design?
How this two virtual machine parameters play with Disk.schednumreqoutstanding, QFullSampleSize, QFullThreshold and SIOC? I don’t catch the whole picture yet on this subject to write something relevant here, thus if any one knows more please enlighten us 😉