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[Qemu-discuss] Qemu IO throttling - Long IO stalls

From: Jordan Pittier
Subject: [Qemu-discuss] Qemu IO throttling - Long IO stalls
Date: Wed, 9 Jan 2013 21:20:55 +0100

Hi guys,
I am playing with Qemu IO throttling. I've set a limit on read bandwidth (750 KBytes/s) on a disk of one of my VM. Then inside this VM I run a disk stress test (mainly dd'ing large files) to see whether the limit is effective.

At the begining of the stress test, the VM is able to perform a huge amount of IO, as reported by "iostat 1" (launched both inside the VM and the physical server). But quickly the VM is not able to perform any IO, and IO stall for several seconds, even minutes. It's like the VM has "consumed" way more than its share and is now refused to perform any read operation. I can't even login to the VM with SSH.

So my questions are : is this the expected behavior ? Is qemu io throttle able to cope with IO surge ? How can I have a "smoother" behavior ?

Thanks a lot !
QEMU emulator version 1.2.0 (qemu-kvm-1.2.0+noroms-0ubuntu2, Debian)
3.5.0-21-generic #32-Ubuntu SMP

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