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[Xen-users] Xen and distributing memory

I'm trying to understand how Xen's memory balloon driver works.  IIRC
VMware's is "dynamic" so if my host has, say, 2 GB memory to spare then
I can still have 3 guests with 1GB of memory assigned each and so long
as all 3 aren't actually *using* all the memory then VMware will
dynamically shuffle the hosts memory around between guests (actually I
think VMware even preserves host swap space for guest RAM so
theoretically they could all could be using 1GB).

>From my understanding with Xen, this is also possible for the most part
but it has to be done manually with 'xm mem-set'.  Is this true?

The reason I am asking is because I have a few domUs that pretty much
run idle except for certain parts of the day.  For example, my backup
server is on a domU.  When it's idle it's only using about 33MB of it's
assigned ram, but once a day when backups are run it uses up to 233MB.
When it's idle I'd like it so the unused RAM is available to other
guests.  Do I have to manually do a 'xm mem-set' and guesstimate how
much memory is going to be used for each guest at certain times of the
day or is there a way that Xen can do this dynamically (a la VMware)?

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