A Problem with Cache in Xen

A Problem with Cache in Xen

[hxkhust]

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Hi,
 
I am a second-year graduate student.Recently,i have done some research on xen and encounter a problem.Thus i send this e-mail to you for help.
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 B and VM C's virtual disk image files are based on VM A'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 B get a page data from VM A's virtual disk image file. After that ,VM C also need to get the same page data as VM B got just now.Under this situation, does VMM copy the page data from VM B's memory to VM C's memory or get the page data from VM A's virtual disk image file in the physical disk again? 
 
 
HXK
Huazhong University of Science and Technology
Wuhan, 430074, China






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Re: A Problem with Cache in Xen

[Tim Deegan]

Hi, 
At 08:55 +0800 on 13 Jan (1326444953), 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 B and VM C's virtual disk image files are based on VM
>  A'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 B get a page data from VM A's virtual
>  disk image file. After that ,VM C also need to get the same page data
>  as VM B got just now.Under this situation, does VMM copy the page
>  data from VM B's memory to VM C's memory or get the page data from VM
>  A's virtual disk image file in the physical disk again?

Since VM C's disk is not QCOW, reads will always come directly from VM
C's image -- never from VM A's image (since the tools wouldn't know how
to do that) and never by copying from VM B.

Tim.

Re: A Problem with Cache in Xen

[hxkhust]

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I am terribly sorry for the mistake that i made in my last latter.I express my question incorrectly.
The following is the correct one:
 
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? 

At 2012-01-19 19:07:38,"Tim Deegan" <tim@xen.org> wrote:
>Hi, 
>At 08:55 +0800 on 13 Jan (1326444953), 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 B and VM C's virtual disk image files are based on VM
>>  A'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 B get a page data from VM A's virtual
>>  disk image file. After that ,VM C also need to get the same page data
>>  as VM B got just now.Under this situation, does VMM copy the page
>>  data from VM B's memory to VM C's memory or get the page data from VM
>>  A's virtual disk image file in the physical disk again?
>
>Since VM C's disk is not QCOW, reads will always come directly from VM
>C's image -- never from VM A's image (since the tools wouldn't know how
>to do that) and never by copying from VM B.
>
>Tim.

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