Re: [Xen-devel] A Problem with Page Cache in Xen

[Tim Deegan <tim@xxxxxxx>]

Hi,

At 16:11 +0800 on 30 Jan (1327939919), hxkhust wrote:
> On a physical machine with xen virtualization platform installed ,VM
> A,VM B are VM C's virtual disks are all image files,among which VM A
> and VM B's virtual disks are QCOW2 format and VM C's disk is RAW
> format. And VM A and VM B's virtual disk image files are based on VM
> C's virtual disk image file.Now these three VMs are running on the
> same physical machine with xen installed. In the process of running VM
> A get a page data from VM C's virtual disk image file. After that ,VM
> B also need to get the same page data as VM A got just now.Under this
> situation, does VMM copy the page data from VM A's memory to VM B's
> memory or get the page data from VM C's virtual disk image file in the
> physical disk again?

It reads it from the disk again, unless you're using the (prototype)
memory-sharing code that's in blktap.

> Or if I would like to figure out this problem,what shall I do?

Instrument the disk, or read the code.

Tim.

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