[ovirt-users] VM resource allocation and IO Threads

[Martin Polednik]

As stated previously, 1 IO thread is somewhat sane choice. Multiple IO
threads make sense if you have multiple storage devices across
different NUMA nodes.

mpolednik

On Mon, Oct 30, 2017 at 4:29 PM, Darrell Budic <budic at onholyground.com> wrote:
> Best explanation I’ve found is
> https://wiki.mikejung.biz/KVM_/_Xen#virtio-blk_iothreads_.28x-data-plane.29
> If you google a bit, you’ll find some more under QEMU topics, I saw some
> discussion of threads and queues in virtio-scsi, but that seems to be a
> slightly different thing than this setting.
>
> In short, having at least 1 offers advantages for all your VM’s disks, and
> if you want to be optimal (at the possible expense of extra CPU for IO), one
> per drive attached. There is (currently) no benefit to having more than 1
> thread per drive. From what I can tell, if you have more drives than threads
> they share the threads evenly and are statically assigned to a thread. Seems
> to be effective at QEMU start, so you have to change it with the VM down or
> stop and start it again.
>
> I currently enable it on all VMs and assign 1 thread per drive on my
> systems.
>
> ________________________________
> From: Gianluca Cecchi <gianluca.cecchi at gmail.com>
> Subject: [ovirt-users] VM resource allocation and IO Threads
> Date: October 27, 2017 at 9:26:59 AM CDT
> To: users
>
> Hello,
> can anyone give any pointer to deeper information about what in subject and
> the value for "Num Of IO Threads" configuration, best practices and
> to-be-expected improvements?
>
> I read also here:
> https://access.redhat.com/documentation/en-us/red_hat_virtualization/4.1/html-single/virtual_machine_management_guide/#Editing_IO_Threads
>
> but in some points it seems not so clear to me:
>
> eg:
>
> If a virtual machine has more than one disk, you can enable or change the
> number of IO threads to improve performance.
>
> but also
>
> Red Hat recommends using the default number of IO threads, which is 1.
>
> There is also a note about deactivation and activation of disks: does it
> mean that even if I poweroff the VM and change its config I have to make
> this step after?
>
> Anyone has run benchmarks?
> Does it make sense if my VM has 3 disks to configure 6 IO threads for
> example?
> Do IO threads map to SCSI controllers inside the guest or what?
>
> Thanks in advance,
> Gianluca
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