[Xen-users] Understanding memory ballooning

[Christian SchrÃder <cs@xxxxxxxxx>]

Hi list,
I try to understand memory ballooning. I have a xen server (kernel 
2.6.27.54-0.2-xen) with 32 gb total memory. A domU has been started with 
"memory=4096" and "maxmem=8192". This is the status of the balloon 
driver in the domU:
test:~ # cat /proc/xen/balloon
Current allocation:  4194304 kB
Requested target:    4194304 kB
Minimum target:       565248 kB
Maximum target:      8388608 kB
Low-mem balloon:     4194304 kB
High-mem balloon:          0 kB
Driver pages:              0 kB

I then try to increase the memory of this domU using the following 
command: "xm mem-set test 8192". However, the memory only increases to 
about 5 gb. This is what the balloon driver says:
test:~ # cat /proc/xen/balloon
Current allocation:  5281180 kB
Requested target:    8388608 kB
Minimum target:       565248 kB
Maximum target:      8388608 kB
Low-mem balloon:     3107428 kB
High-mem balloon:          0 kB
Driver pages:              0 kB

Can anybody explain why the current allocation is less than the 
requested target? "free" reports the same size:
test:~ # free -k
             total       used       free     shared    buffers     cached
Mem:       5281180    2478960    2802220          0        768     584432
-/+ buffers/cache:    1893760    3387420
Swap:      2096444          0    2096444

What can I do to make the full 8 gb usable to the domU?

Kind regards,
Christian

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