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xen-devel
Re: [Xen-devel] Some general questions
> After starting the domains, I thought I could mount the partition into a
> backup directory, save the data and unmount it again (so nobody would
> write external data to the machine) but xen seems to store file changes
> in memory only.
The problem isn't just in writing data externally to a virtual machine's disk
- even if you mount it read-only in dom0, you won't see all the changes.
Additionally dom0 will eventually get confused by the changes occurring on
the disk and decide it's corrupt.
It's not Xen-specific - Linux (and basically any operating system you might
use) likes to cache changes in memory in order to improve disk performance.
Most filesystems will assume that they're the only user of a filesystem.
If you want to have two domains access the same filesystem you need to either
use GFS or OCFSv2 on disk, or you need to share it using NFS.
> - Is there a way to use the whole RAM, if not, how much of the memory
> can be used for the domains? It seems I can only use approximately 384MB
> per domain (or 512 and 256).
Xen uses some memory itself. You can find out how much machine memory is free
by running the "xm info" command.
> - Possibly there are much better solutions than using disk partitions.
> What I need is a possibility to backup the domains, i.e. via cron. Is
> there a way to create backups of a running/live domain?
Not in a particularly neat way, unless you're using NFS or a cluster
filesystem. You could use LVM and do snapshots during backup.
Really, backing up a running machine is most easily done from inside that
domain, where knowledge about cached files, etc is available.
> - I'd prefer having kernel without loadable module support but need
> different support for my client kernels. Is there a way to tell during `
> `make` that I need a kernel for my vpn-system in domain1 and another
> kernel for my samba-server and so on? And can I have any type of kernel
> on my VMs, i.e. 2.6.10 for the dom0, 2.6.9 for domain1 and 2.4.10
> running domain2?
Yeah. To do it automatically in the build system you'll have to dig into the
build system magic a bit for that! I've not done this before but I imagine
you can:
* create a new file under buildconfigs/ for each kernel (copy one of the
existing ones) for each kernel
* create a correspondingly named config file under dist/install/boot/ for each
kernel
* then list all the kernels to build in the KERNELS variable in the root
Makefile.
> P.S. Almost forgot that: I also didn't find an explanation for the
> different states of a VM (r----, -b--- and so on). What states do exist
> and what do they mean? Maybe anyone could tell me where I can find that.
Running, blocked, etc. It's listed in the user manual somewhere.
Cheers,
Mark
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