Re: [Libguestfs] Cached filesystem data
On Sun, Sep 14, 2014 at 09:27:46AM +0100, Doug Winter wrote:
I am trying to use guestfs to mount, read-only, a VDI filesystem that
is
also open, read-write, by a separate Oracle VirtualBox VM.
Opening the filesystem works fine, however changes made to the
filesystem in the VirtualBox VM are not reflected in guestfs.
Even unmounting and then mounting the drive again does not show the
changes made since the first mount.
However, shutdown() and then launch() of the guests does then show the
changes.
I have tried the drop_caches() call, but this doesn't help either (which
is kind of unsurprising if unmounting is insufficient).
When I looked into this before, I suspected the reason was because
qemu is caching (or maybe the host kernel). You could patch the cache
mode (in src/launch-direct.c) so it allows you to use O_DIRECT again
and see if that makes a difference.
In any case yes, unfortunately you need to shutdown and re-launch the
appliance to be sure of seeing changes.
One day in the future qemu will be able to export a consistent
snapshot of a running guest, and move between snapshots. Work is
being done on this upstream.
Is there any way of causing guestfs to see the changes made to the
filesystem without going through the slow shutdown/launch process?
See also guestfs-performance(1). If it's taking longer than ~ 5 seconds
then there are ways to fix that.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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