Re: [MirageOS-devel] multiple block devices under xen

[Richard Mortier <Richard.Mortier@xxxxxxxxxxxxxxxx>]

thanks dave!  a couple of questions inline as i start to try this out...

On 18 Feb 2014, at 12:05, David Scott <scott.dj@xxxxxxxxx> wrote:

> Background
> ----------
> 
> When a disk is added to a VM (under Xen, KVM, any hypervisor) it's like 
> plugging in a PCI device into a physical box -- a disk will appear in some 
> kind of numbered "slot" on a "bus". In xen the "slot" number is the name of 
> the directory in xenstore (/local/domain/<domid>/device/vbd/<slot>) 

just because i'm still a bit fuzzy on this -- what about when i'm not running 
xenstore?  (or am i always running xenstore, i just haven't realised yet?)

> From the Linux perspective, the *convention* is that the "slot" number is the 
> device node (major-number << 8 || minor-number):

...and from below (and a bit of googling), /dev/xvdN has major number 202 and 
minor number (N-'a')*16, right?  is that (the major/minor numbering of xvd 
devices) also convention or standard?

> Xen disk support
> ----------------
> 
> ...and then create a loop device like so:
> 
> sudo losetup /dev/loop0 disk.raw
> 
> (where /dev/loop0 is any free loop device. Note the total number of loop 
> devices is determined at module load time, it's not dynamic unfortunately 
> (last time I checked anyway))

is there an easy way to find out what this is?  (whether from CLI or 
programmatically.)

> Whatever you do, don't mix I/O to the /dev/loop device with I/O to the file 
> -- it won't be coherent. 

presumably this is unlikely to happen by accident in a mirage vm?

[snip details]

ok, so in summary the current recommendation is to:

+ create a FAT file image (done by mirage) and mount it as a loopback device 
        $ sudo losetup /dev/loopN fat.img

+ write a Xen config file that hotplugs loopback devices as physical block 
devices
        disk = [ '/dev/loop0,,xvda', ... ]

+ write a mirage config file that uses the block device node number as a name
        let block = { Block.name = "51712"; filename = "fat.img"; read_only = 
false; }

...and then hopefully everything will line up, with the Block.filename value 
used when running the mirage unikernel as a UNIX process, and the Block.name 
value being interpreted as the appropriate device node when running as a Xen VM 
(assuming the Xen config file has indicated to plug the loopback device as the 
xvdN device correctly)?

> This is obviously not ideal. Perhaps in future we can:
> * make "mirage" generate an .xl config file which references the file (rather 
> than a block device) and rely on the hotplug scripts to manage the loop 
> device. This should work, but since there are more moving parts, may be flaky 
> on your particular distro.
> * make "Block.connect" take some kind of volume label rather than a device 
> id, so mirage-block-{unix,xen} can cope with disks being presented in any 
> order on the bus

both would be nice i think -- but hopefully the info above (assuming i have it 
straight now) will be enough to get going! :)


-- 
Cheers,

R.




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