Re: Using tools such as ionice inside guests

Paolo Bonzini
2016-05-18T16:13:14Z

On 18/05/2016 17:49, Emmanuel Lacour wrote:
> Dear kvm developpers/users,
>
> I just saw that a change has been made in the kernel so the only one
> scheduler available in guests for virtio-blk is now "none".
>
> Seems introduced by [1].

Not by this one; rather this one:

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=1cf7e9c68fe84248174e998922b39e508375e7c1

> I tried to used ionice on a 3.16 guest with virtio-blk and in fact it no
> longer works because cfq is no longer available.
>
> Is there a way to use, inside a guest, a tool like ionice to set
> different IO priority in guests processes? It's very useful for loering
> down the performance impact of backups jobs for example.
>
> [1] -
> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=a98755c559e0e944a44174883b74a97019e3a367
> --

Not anymore unfortunately. The effect of this change was not
advertised enough, in my opinion. The blk-mq path should have replaced
the code used by use_bio=1, but not the code used by use_bio=0.

Thanks,

Paolo