[libvirt] [PATCH RFC] iohelper: Introduces a small sleep do avoid hunging other tasks

[Peter Krempa]

On Thu, May 09, 2019 at 17:59:36 -0300, Leonardo Bras wrote:
> While dumping very large VMs (over 128GB), iohelper seems to cause
> very intense IO usage on the disk, and it causes some processes
> (like journald) to hung, and depending on kernel configuration,
> to panic.
> 
> This change creates a time window, after every 10GB written, so
> this processes can write to the disk, and avoid hunging.
> 
> Signed-off-by: Leonardo Bras <leonardo at linux.ibm.com>

I don't think this approach makes sense. This looks like something on
your host is seriously wrong as anything that would write a large file
would to the same to your host.

Unfortunately libvirt's API does not have a possibility to pass in
bandwidth limit for the dump operation which would allow to limit the
bandwidth when calling the API.

You can e.g. use cgroups to limit the bandwidth of children of the
libvirtd to work around this problem or perhaps switch to a better I/O
scheduler.
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