On 3/4/20 10:19 PM, Guerlan wrote:
I'm curious about how Qubes does this:
mounts /home/user and other user-related directories from disk B
mounts the / from disk A, but when VM shutdowns, disk is discarded
I'm curious on how it mounts disk A. I don't think it makes a copy of
disk A to a temporary disk A', because that'd move lots of gigabytes on
every VM startup.
However, it also can't mount disk A as read-only, because I can write to
it, it just gets discarded.
How does this work? And is it exclusive of Xen? Couldn't I do the same
in KVM? It's very useful
Qubes uses copy-on-write snapshots to achieve this. With a default
install, that means an LVM "thin pool" holds all of the VM volumes, and
when a VM starts a snapshot is taken of both "disk A" and "disk B" (the
*-root and *-private volumes). With a normal AppVM (base on a template)
the root and private volumes are treated differently on shutdown: Root
snapshot is discarded, and private is rotated to replace the persistent
copy (what appears in the VM as /rw and /home).
A similar snapshot routine is used if you installed Qubes with Btrfs
format instead of LVM (Btrfs is a copy-on-write filesystem).
Copy-on-write provides the ability to create new representations or
snapshots of an existing file or volume, instantly. Snapshotting is like
copying, but using a collection of pointers instead of the data itself.
Thus, when a new snapshot is changed, the system only needs to write
some new blocks in a different location and replace some pointers in the
snapshot's metadata to point to the new location. This all can save a
lot of time and disk space.
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