[Users] Heavy Disk IO from a single VM can block the other VMs on the same host

[Kirill Korotaev]

That's most likely due to a single file system used for containers - journal becomes a bottleneck.
fsync forces journal flushes and other workloads begin to wait for journal... In reality workload looks like this are typical for
heavy loaded databases or mail systems only.

How to improve:
- increase journal size
- split file systems, i.e. run each container from it's own file system

Thanks,
Kirill


On Nov 29, 2011, at 20:13 , Hubert Krause wrote:

> Hello,
> 
> my environment is a Debian squeeze host with a few debian squeeze
> guests. The private and root filesystems of the guest are locatet on
> the same raid device (raid5) in an luksCrypt Container in an LVM
> container on an ext4 partition with nodelalloc as mountoption. If I run
> the tool stress:
> 
> stress --io 5 --hdd 5 --timeout 60s (which means fork 5 threads doing
> read/write access and 5 threads doing constantly fsync) the
> responsivness of the other VMs is very bad. That means, Isolation for
> IO operations is not given. I've tried to reduce the impact of the
> VM with 'vzctl set VID --ioprio=0'. There was only a
> minor effect, my application on the other VM where still not
> responsive.
> 
> Any Idea how to prevent a single VM to disturb the other VMs regarding
> diskIO?
> 
> Greetings
> 
> Hubert
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