Libguestfs - Feb 2015 - host Linux - guest Win7 fast file sharing [was: getting guestfs_rsync_out to work]

Update:

Using mount (read-only) and unmount commands in a script that loops
continuously, we are able to see near
instantaneous file modifications (less than 0.5 sec) performed on a Win7 live
guest.  We have to use the sync.exe
utility (technet.microsoft.com/en-us/sysinternals/bb897438.aspx), otherwise
changes are not visible for 10-20 sec.

We tested sync.exe with our libguestfs test program (using various methods,
including rsync_out, and w/wo sync on
Linux side), but we're still not able to see live guest file changes until
we relaunch the image.  Any advice on how
to get around the relaunch requirement (which takes 3-5 sec due to loading the
appliance Linux kernel), would be
greatly appreciated.  There are many reasons why using libguestfs is a better
solution for us in the long run.
Thanks.

-Jeff

---------------- Original Message ---------------
Subject: Re: [Libguestfs] getting guestfs_rsync_out to work
From:    "Jeff Brower" <jbrower@signalogic.com>
Date:    Mon, February 9, 2015 3:39 pm
To:      "Richard W.M. Jones" <rjones@redhat.com>
Cc:      libguestfs@redhat.com
-------------------------------------------------

Richard-
> On Mon, Feb 09, 2015 at 08:27:22AM -0600, Jeff Brower wrote:
>> Yes we know that it doesn't know.  We'd be ok to re-launch the
image to pick up changes except for the time it
takes (if we could get into 1 to 10 msec range it would be
ok).
>
> There's no feasible way to start a Linux kernel in under about 1-2
seconds.  It just takes that long for the kernel
to initialize itself. Even if you use the User-mode Linux backend (which just
runs vmlinux as a userspace process)
you'll experience approximately the same overhead.

Ok got it.  Thanks again for your reply.  We tried guestfs_drop_caches() as
follows:

  while(1) {

    guestfs_mount(g, "/dev/sda2", "/");

    printf("%s\n", guestfs_cat(g, "/HostShared/temp.txt"));

    guestfs_umount(g, "/");

    guestfs_drop_caches(g, 3);

    usleep(1000*1000);
 }

and still we cannot see changes made to temp.txt (from inside the Win7 guest VM)
while this loop continues to run on
the host Linux.  We still have to re-launch the image to see changes.

In general, it would be great if the "kernel load" and "disk
refresh / sync" functions in guestfs_launch() could be
separated.  If there was a way to do read-only file sharing, a very limited
subset of what virtFs/9p can do, that
would be great for Win guests.

-Jeff

On Tue, Feb 10, 2015 at 02:09:33PM -0600, Jeff Brower
wrote:
> Using mount (read-only) and unmount commands in a script that loops
> continuously, we are able to see near instantaneous file
> modifications (less than 0.5 sec) performed on a Win7 live guest.
> We have to use the sync.exe utility
> (technet.microsoft.com/en-us/sysinternals/bb897438.aspx), otherwise
> changes are not visible for 10-20 sec.
>
> We tested sync.exe with our libguestfs test program (using various
> methods, including rsync_out, and w/wo sync on Linux side), but
> we're still not able to see live guest file changes until we
> relaunch the image.  Any advice on how to get around the relaunch
> requirement (which takes 3-5 sec due to loading the appliance Linux
> kernel), would be greatly appreciated.  There are many reasons why
> using libguestfs is a better solution for us in the long run.
It's not too surprising that sync doesn't work on the libguestfs
appliance side.

BTW you probably should also change the caching mode in qemu to `none'
or maybe `directsync' (default is now `writeback').

Unfortunately about 18 months ago we changed the caching modes and
removed the possibility of using `none':

https://rwmj.wordpress.com/2013/09/02/new-in-libguestfs-allow-cache-mode-to-be-selected/

So you'd have to patch libguestfs to add it back -- it's a fairly
straightforward patch.

I still think what you're trying to do is going to go wrong in
unexpected ways.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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