[Qemu-devel] file system sharing

[Qemu-devel] file system sharing

[Eric S. Johansson]

can I write a file based file system and read it from the host?  I can 
guarantee I have stopped writing before I read on the host side.  I can 
even unmount before reading.

ideally, I would like to "import" my flash memory device into the guest 
OS side (USB based) but if I can create a "virtual" flash disk and when 
I'm done modifying it, physically copy the file based image to the 
physical flash, I would be happy.

---eric

Re: [Qemu-devel] file system sharing

[Mark Williamson]

> can I write a file based file system and read it from the host?  I can
> guarantee I have stopped writing before I read on the host side.  I can
> even unmount before reading.

If only one machine (host or guest) has mounted the device then it should 
always be safe to do this.  You may get away with read only mounting in one 
and writing in the other but it's not a reliable solution.  Never allow more 
than one writer to the filesystem - this does bad things to your filesystem!

If you're using a file-based disk and it's partitioned you'll need to use 
lomount http://www.dad-answers.com/qemu/utilities/QEMU-HD-Mounter/lomount/ to 
mount the right partition in the host.

> ideally, I would like to "import" my flash memory device into the guest
> OS side (USB based) but if I can create a "virtual" flash disk and when
> I'm done modifying it, physically copy the file based image to the
> physical flash, I would be happy.

I imagine just giving the guest access to the device file would work.

HTH,
Mark

Re: [Qemu-devel] file system sharing

[Mike Swanson]

You can probably also set up an ftp or smb (via Samba) server to
transfer files within the guest OS.  HTTP should also work with
Apache, but remember the protocol wasn't originally designed for
uploads, and not all clients (eg, web browsers) support it.
-- 
Mike

Re: [Qemu-devel] file system sharing

[Johannes Schindelin]

Hi,

On Sun, 31 Jul 2005, Mark Williamson wrote:

> If only one machine (host or guest) has mounted the device then it should
> always be safe to do this.  You may get away with read only mounting in one
> and writing in the other but it's not a reliable solution.

You almost certainly get not away with that if you are using Linux as
the read only part. Linux does quite a good job of caching filesystems,
but it thereby assumes to have exclusive control over the device (this
evidently only holds for non-networked FSes). Therefore, if you change
something behind the scenes, but Linux did not issue a write, it still
assumes the old data.

Ciao,
Dscho

[Qemu-devel] Re: file system sharing

[Eric S. Johansson]

Mark Williamson wrote:
> If only one machine (host or guest) has mounted the device then it should 
> always be safe to do this.  You may get away with read only mounting in one 
> and writing in the other but it's not a reliable solution.  Never allow more 
> than one writer to the filesystem - this does bad things to your filesystem!

I am well aware of these issues having worked on filesystems in the 
1980s and having designed and built one of the first RPC based networked 
filesystems in the late 80s for a CAD/CAM company.  To this day the nova 
architecture gives me the willies.

I suspect based on a comment that someone else made about caching that I 
would need some sort of event to trigger a flush either by an explicit 
flush call or a close.  Would unmounting the disk image create such an 
event?

> 
> If you're using a file-based disk and it's partitioned you'll need to use 
> lomount http://www.dad-answers.com/qemu/utilities/QEMU-HD-Mounter/lomount/ to 
> mount the right partition in the host.

so what I can do is create the partition image on the host, start up 
qemu with that multi-partition disk image, do what I need to in qemu, 
shut down qemu and then I have a modified disk image.  if I need to 
modify it from the host, then I can use lomount to make it accessible as 
the local filesystem.

Very cool.

one more question on this theme.  How do I know when the guest OS has 
finished booting?  The reason I ask is I am planning on using ssh to 
perform various operations on the guest OS once it's up.


>>ideally, I would like to "import" my flash memory device into the guest
>>OS side (USB based) but if I can create a "virtual" flash disk and when
>>I'm done modifying it, physically copy the file based image to the
>>physical flash, I would be happy.
> 
> 
> I imagine just giving the guest access to the device file would work.

I'm not entirely sure how to do that.  would I just do something like:

-hdc /dev/sda1

as part of the command line?
> 
> HTH,
> Mark

Re: [Qemu-devel] Re: file system sharing

[Paul Brook]

> one more question on this theme.  How do I know when the guest OS has
> finished booting?  The reason I ask is I am planning on using ssh to
> perform various operations on the guest OS once it's up.

Just like a real machine, you don't. You need to configure the guest to tell 
the host when it's ready, or just keep trying until it succeeds.

Paul

[Qemu-devel] lomount : Re: file system sharing

[Eric S. Johansson]

Mark Williamson wrote:
> If you're using a file-based disk and it's partitioned you'll need to use 
> lomount http://www.dad-answers.com/qemu/utilities/QEMU-HD-Mounter/lomount/ to 
> mount the right partition in the host.

this works great for some of the partitions but not the last one.  For 
some reason the file based image of a disk thinks the cylinder number is 
* and if I try to set it to the right value, fdisk gives me error 25 and 
something about wrong ioctk.  I reboot to flush the cache in the 
kerneland it's still not set to the right number of cylinders.

The disk image was formatted inside of qemu as part of the IPCop 
installation process.  It boots up just fine, I can create files change 
files etc. no problem.

It's just one I have stopped qemu and are trying to modify files within 
the disk image, I can't mount the partition I need to change.

---eric

Re: [Qemu-devel] Re: file system sharing

[Mark Williamson]

> Mark Williamson wrote:
> > If only one machine (host or guest) has mounted the device then it should
> > always be safe to do this.  You may get away with read only mounting in
> > one and writing in the other but it's not a reliable solution.  Never
> > allow more than one writer to the filesystem - this does bad things to
> > your filesystem!
>
> I am well aware of these issues having worked on filesystems in the
> 1980s and having designed and built one of the first RPC based networked
> filesystems in the late 80s for a CAD/CAM company.  To this day the nova
> architecture gives me the willies.

Ah yes.  Somehow whenever I work on a substantial project, I'm left with a new 
phobia ;-)  Just thought I'd mention it - I've hosed my own filesystems in 
similar ways (admittedly I should have thought what I was doing).

> I suspect based on a comment that someone else made about caching that I
> would need some sort of event to trigger a flush either by an explicit
> flush call or a close.  Would unmounting the disk image create such an
> event?

As far as QEmu's block layer is concerned?  Good question - I don't know 
enough about its internals to comment.  Can anyone verify if QEmu does 
internal caching for block IOs? (or anything else that'd cause coherency 
problems here, e.g. async IOs seem a likely candidate).

> > If you're using a file-based disk and it's partitioned you'll need to use
> > lomount
> > http://www.dad-answers.com/qemu/utilities/QEMU-HD-Mounter/lomount/ to
> > mount the right partition in the host.
>
> so what I can do is create the partition image on the host, start up
> qemu with that multi-partition disk image, do what I need to in qemu,
> shut down qemu and then I have a modified disk image.  if I need to
> modify it from the host, then I can use lomount to make it accessible as
> the local filesystem.

Yep.  You *could* take a disk file and format it all with one filesystem, 
which would make it directly mountable under the guest Linux.  However, the 
Linux loop driver doesn't like /partitioned/ "file disks" - lomount sorts 
this out in userspace by just mounting one partition at a time.  My thought 
was that if you were using (say) an image of a USB key, it'd probably have a 
partition table on it.

> > I imagine just giving the guest access to the device file would work.
>
> I'm not entirely sure how to do that.  would I just do something like:
>
> -hdc /dev/sda1
>
> as part of the command line?

That might work.  Alternatively, exporting the whole device -hdc /dev/sda 
might be worth a try.  Can't say I've ever tried it (so far!)

The alternative would be to use some network-based solution.  I'm sure you've 
thought of that but I seem to remember a user-friendly way of letting a QEmu 
guest access an SMB share, which would make it rather more convenient to set 
up.

Cheers,
Mark

Re: [Qemu-devel] Re: file system sharing

[Brad Watson]

I've used both -hda /dev/sda and -hdc /dev/sda for
booting from and just accessing a flashdrive
respectively on linux.  Probably -hdc \\.\E: would
work for accessing the flashdrive from qemu running on
MS.... 

Regards,

Brad Watson



		
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