[Qemu-devel] virtio ssd caching

[Qemu-devel] virtio ssd caching

[Aarian P. Aleahmad]

Hi there

I am looking for a solution to cache a VM on a SSD without the guest
knowing about it. We can do it by caching the disk on which the VM's disk
is lcoated with a SSD or we can do it through the hypervisor (i.e. Qemu,
KVM, etc.).

I was thinking about passing an ssd device to the hypervisor as a cache
disk (e.g. passing /dev/sdb to the hypervisor) or making a new virtual disk
image and storing it on a SSD and passing the SSD located vDisk to the
hypervisor as a cache.

Thus, I would like to know whether is it possible or not, and if not, if I
wish to implement such a thing, what should I do and whether is there any
reusable code to do so or not?

Thanks

Re: [Qemu-devel] virtio ssd caching

[Stefan Hajnoczi]

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On Tue, Sep 06, 2016 at 12:44:39PM +0430, Aarian P. Aleahmad wrote:
> I am looking for a solution to cache a VM on a SSD without the guest
> knowing about it. We can do it by caching the disk on which the VM's disk
> is lcoated with a SSD or we can do it through the hypervisor (i.e. Qemu,
> KVM, etc.).
> 
> I was thinking about passing an ssd device to the hypervisor as a cache
> disk (e.g. passing /dev/sdb to the hypervisor) or making a new virtual disk
> image and storing it on a SSD and passing the SSD located vDisk to the
> hypervisor as a cache.
> 
> Thus, I would like to know whether is it possible or not, and if not, if I
> wish to implement such a thing, what should I do and whether is there any
> reusable code to do so or not?

QEMU itself doesn't offset tiering/caching but you can use the host
kernel's bcache or dm-cache features to achieve this.

I haven't tried them myself so I can't give exact command-lines.
Richard Jones posted a tutorial in 2014:
https://rwmj.wordpress.com/2014/05/22/using-lvms-new-cache-feature/

Stefan

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Re: [Qemu-devel] virtio ssd caching

[Richard W.M. Jones]

On Wed, Sep 07, 2016 at 03:36:38PM -0400, Stefan Hajnoczi wrote:
> On Tue, Sep 06, 2016 at 12:44:39PM +0430, Aarian P. Aleahmad wrote:
> > I am looking for a solution to cache a VM on a SSD without the guest
> > knowing about it. We can do it by caching the disk on which the VM's disk
> > is lcoated with a SSD or we can do it through the hypervisor (i.e. Qemu,
> > KVM, etc.).
> > 
> > I was thinking about passing an ssd device to the hypervisor as a cache
> > disk (e.g. passing /dev/sdb to the hypervisor) or making a new virtual disk
> > image and storing it on a SSD and passing the SSD located vDisk to the
> > hypervisor as a cache.
> > 
> > Thus, I would like to know whether is it possible or not, and if not, if I
> > wish to implement such a thing, what should I do and whether is there any
> > reusable code to do so or not?
> 
> QEMU itself doesn't offset tiering/caching but you can use the host
> kernel's bcache or dm-cache features to achieve this.
> 
> I haven't tried them myself so I can't give exact command-lines.
> Richard Jones posted a tutorial in 2014:
> https://rwmj.wordpress.com/2014/05/22/using-lvms-new-cache-feature/

I sure hope it works better today than it did back in 2014 :-(

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-builder quickly builds VMs from scratch
http://libguestfs.org/virt-builder.1.html