Rich Freeman on 12 Oct 2015 10:27:09 -0700

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Re: [PLUG] Video link: "Virtualizing Bare-Metal Systems with QEMU" and meeting bookmarks

On Mon, Oct 12, 2015 at 1:17 PM, Keith C. Perry
<> wrote:
> Yep, that's might I made mention of "raw" disk images.  :)

Sure, but I'm just not sure what the connection is to bind mounts.

> Since I usually keep the system volume separate for my vm (or at least when I am building them) I can loop mount the partition then do whatever bulk data moves are needed (e.g. move source code for later compiling) and then convert back to qcow2.

Makes sense.

> Fun fact... sharing a raw disk image between a VM host and guest does seem to work (I did that a when I was building some FOSScon usb multiboot keys) but I'm not sure its "safe".  You would have to take care to make sure you are not making changes to the same files at the same time. YMMV

Yeah, you DEFINITELY do not want to mount a disk image on multiple
kernels unless the filesystem is specifically designed to handle this
(very few are, and nothing you'd find on a typical linux install).
Filesystems designed to handle being mounted by multiple kernels are
called clustered filesystems.  It isn't sufficient to just avoid
changing the same file at the same time - the kernels will be stomping
all over each other's metadata and can hose potentially anything on
the device.

The other approach to multiple mounting is a distributed filesystem.
In this arrangement any particular device is only accessed by a single
kernel, but the filesystem itself exists at a higher level than any
single kernel and uses networking/etc to distribute data across all
nodes that access it.  The most popular examples of this sort of thing
are HDFS and Ceph.  This approach tends to be more popular than
clustered filesystems, since it does not require giving high-speed
access to a single device to all the nodes that need to share it.

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