Re: AW: [Xen-users] Use hard drive as physical mem for xen vm

[Ernst Bachmann <e.bachmann@xxxxxxxx>]

On Friday 11 November 2005 15:15, Oliver Neumann [New Identity AG] wrote:
> >> I've got a machine that should serve 2 XenUs as mysql cluster nodes with
> >> realy big memory consumption. As mysql cluster will need approx. 4GB
> >> memory on each XenU, I wondered if I could use hard disc space as memory
> >> for the XenU that appears as real physical memory within the VM. Swap is
> >> no alternative as mysql cluster nodes check the physical memory and if
> >> that is not enough, they will not work.
> >>
> >> How can I do that?
> >
> >Invest a few extra € and put 8Gigs of ram in that server?
> >Reconfigure the mysql instances to run with just 2GB?
> >Seriously, mysql wants physical ram there for a reason, if that memory is
> >swapped, it would be faster for mysql to just read the blocks from its db
> >files.
>
> No, with that amount of data in our databases we already calculated, that
> we need at least 4GB only for each cluster node, so we woul dhave to buy at
> least 8GB of ECC RAM which isn't really cheap. The 'problem' is that we
> only need to test an application on a cluster, after that we don't need
> that big amount of memory in that machine so the money is somehow wasted.
> As for the performance: That is irrelevant, the cluster does not need to be
> fast (it actually can be damn slow!).
>
> Unfortunately there seems to be no chance to save the money using Xen with
> harddisc emulated physical RAM, right?

As Mats pointed out, xen doesn't know about harddrives, so it can't do any 
swapping itself. 
Maybe you'll have more luck with a virtualization method running at a higher 
level like linux-vserver or vmware or perhaps an emulation like qemu.

/Ernst

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