Some general questions

Some general questions

[Jorma Jakowitsch]

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Hi list,

I fetched the xen-2.0.4 source for installation and let it built 2.6.10
kernels and setup some domains to understand and test xen for my needs.

I'm not too familiar with LVM so I decided to start with harddrive
partitions for my domains.

Hardware: 1024MB RAM, hda for Domain-0 and hdb for the virtual machines.

dom0 is started with 128MB running the statically compiled 2.6.10-xen0

I created two systems on hdb1 and hdb3 via debootstrap (using Woody),
configured and unmounted them later for usage with xen.
I wanted to setup a Domain1 with 512MB RAM and Domain2 with the
remaining 384MB but one of the machines didn't start due to lack of free
memory, with the following message:
Error: Error creating domain: (12, 'Cannot allocate memory')

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.


My questions:
- - 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).
- - 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?
- - 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?


Thanks in advance for any help.

Regards,
Jorma

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.
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Re: Some general questions

[Goetz Bock]

On Tue, Feb 22 '05 at 12:29, Jorma Jakowitsch wrote:
> -- Is there a way to use the whole RAM,
You can use the whole RAM, but Xen needs a little for it's own.

> -- 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?
You should not mount the disks of a running domain on domain0. You can
use LVM with snapshots to do this. 

> - - 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?
You can manualy apply the xen patch and than configure as many domU
kernels as you like. Even different versions, as long as xen supports
them.
-- 
/"\ Goetz Bock at blacknet dot de  --  secure mobile Linux everNETting
\ /       (c) 2004 Creative Commons, Attribution-ShareAlike 2.0 de
 X   [ 1. Use descriptive subjects - 2. Edit a reply for brevity -  ]
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Re: Some general questions

[Mark Williamson]

> 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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Re: Some general questions

[Jorma Jakowitsch]

<-- snipped -->

>> -- Is there a way to use the whole RAM,
> You can use the whole RAM, but Xen needs a little for it's own.

And I thought it would only use the 128M given via grub. I'll try xm info
and doublecheck that.

>> -- 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?
> You should not mount the disks of a running domain on domain0. You can
> use LVM with snapshots to do this.

I'm aware I shouldn't do this. I was just trying it on this testing
machine  and for backup-purpose I was only reading from it. But of course,
I'm searching for a live backup solution in a productive environment.

>> 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.
>
> Xen does NOT store file changes in memory only.

As previously stated, that was only for testing. I came to that conclusion
after mounting a domain1 in dom0 and creating a file from within the
domain1.
I couldn't see that file in dom0, only after shutting/reloading the domain1

>> - - 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?
> You can manualy apply the xen patch and than configure as many domU
> kernels as you like. Even different versions, as long as xen supports
> them.

Ok, I got to check what this patch is (and where I find it ;)) and see if
I'm able to built any needed custom (domU) kernels. Just in case, is there
a place where I can find more information about that?



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Some general questions

[Aya Mahfouz]

Hello Everyone,

I had a couple of questions lingering in my head for quite some time
now. I'm reading lots of articles but I'm interested in people sharing
their experiences as answers to the following questions:

Is it possible to setup a simple cloud-based compile farm? Are there
guides or projects that would help with that? My question comes after
seeing Fengguang Wu's kbuild testing farm reports.

From experience, how do you usually debug new drivers? What tools do you
use? What do you think of tools like kgdb and kdb? What do you do for
example if testing a driver freezes the computer it is running
on.

Thanks,

-- 
Kind Regards,
Aya Saif El-yazal Mahfouz