[sheepdog] Questions on the virtual disk's cache type

[Daniel P. Berrange]

On Wed, Jan 23, 2013 at 06:09:01PM +0800, Liu Yuan wrote:
> On 01/23/2013 05:30 PM, Daniel P. Berrange wrote:
> > FYI There is a patch proposed for customization
> > 
> >   https://review.openstack.org/#/c/18042/
> > 
> 
> Seems that this patch is dropped and declined?
> 
> > 
> > I should note that it is wrong to assume that enabling cache mode will
> > improve the performance in general. Allowing caching in the host will
> > require a non-negligable amount of host RAM to have a benefit. RAM is
> > usually the most constrained resource in any virtualization environment.
> > So while the cache may help performance when only one or two Vms are
> > running on the host, it may well in fact hurt performance once the host
> > is running enough VMs to max out RAM. So allowing caching will actually
> > give you quite variable performance, while the cache=none will give you
> > consistent performance regardless of host RAM utilization (underlying
> > contention of the storage device may of course still impact things).
> 
> Yeah, allowing page cache in the host might not be a good idea to run
> multiple VMs, but cache type in QEMU has different meaning for network
> block devices. For e.g, we use 'cache type' to control client side cache
> of Sheepdog cluster, which implement a object cache in the local disk
> for performance boost and reducing network traffics. This doesn't
> consume memory at all, just occupy the disk space where runs sheep daemon.

BTW, where is this local disk cache kept in the filesystem ? For proper
security isolation between VMs, the cache location needs to be different
for each VM and needs to be included in the SELinux policy.

Daniel
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