[ovirt-users] VM resource allocation and IO Threads

[Darrell Budic]

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 <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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