vmware  I/O  tuning question

mokkan
mokkan used Ask the Experts™
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I'm dong stress testing on VM and when I look at the queue  ACTV is going to 32 and once it is 32, I can see QUE is increasing. I understood that max que size is 32. IF I want to get better IO for that , what can I do? Can I add another que? IF so, how can I add?
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DavidPresident
Top Expert 2010

Commented:
Add more disks; get faster disks; decrease the number of I/Os by making programs more efficient; balance I/O so that disks that don't have such a high queue depth get more I/O requests; use more intelligent RAID controllers with more cache;  if you are using a I/O intensive RAID level like RAID5, then go to RAID1 or RAID10...

Bottom line, this isn't rocket science.  There are 32 requests for I/O so you need to change things to keep the todo list shorter.

Author

Commented:
Can we increase the Que size for specific VMs,  I know that we have enough space on our storage and enough IOPS available.
DavidPresident
Top Expert 2010

Commented:
Increasing the queue size won't do squat for you.  The amount of free space has nothing to do with the fact that there are so many I/Os queued up because your disks are so busy.

A HDD can only move X amount of data (or # of transactions) in Y amount of time.   You have 32 queued up transactions to do.  So you need to lower X (buy more disks or do fewer requests), or get faster devices, which improves Y.
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Author

Commented:
Thank you very much. If  I understand correctly the queue is getting filled because storage can't handle it?
DavidPresident
Top Expert 2010

Commented:
Exactly. Hence the need to do one of the things I mentioned.  You are simply asking too much of what you have.

Equate it to not having enough RAM, and you keep piling up programs to run. Think of this queue depth as being somewhat like swap space for I/O requests.  The solution to not enough RAM is to run fewer programs, or buy more RAM.
President, IT4SOHO, LLC
Commented:
It's not quite as simple as all that... you could be slowed down in your I/O because the interface specifications (and requisite drivers) may not be optimal...

When I first switched from Xen to KVM (in a RHEL world), I was aghast at the slowness of KVM -- especially given the advertised "efficiency" KVM was supposed to have!

As it turned out, when I switched the storage from IDE to virtio, and then network from RealTek to virtio, my systems actually DID see significant performance gains... (BTW: I also migrated from raw IMG files to qcow2 formatted (pre-allocated) files...

I'm not sure of the equivalents in VMware, but I thought it worth noting that virtual machines  can have different virtual interfaces with their virtual host that can have VERY different performance characteristics...

Dan
IT4SOHO

Author

Commented:
Thanks a lot

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