Re: [vnrg] Logical vs. virtual

Roland Bless <roland.bless@kit.edu> Fri, 11 February 2011 15:59 UTC

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Date: Fri, 11 Feb 2011 16:59:19 +0100
From: Roland Bless <roland.bless@kit.edu>
Organization: Institute of Telematics, Karlsruhe Institute of Technology
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Subject: Re: [vnrg] Logical vs. virtual
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Hi,

the difference between logical and virtual isn't
that easy and sometimes seems artificial.
According to an older (but nice) German book on operating system
architecture (H. Wettstein, Systemarchitektur)
the following distinction is made:

* a virtual resource appears to a user of that resource
  as if he is the (exclusive) owner of that resource.
  A user's access is always carried out on the real resource.

Thus, a virtual resource simplifies the handling of
a scarce real resource, while hiding the shortage. Virtual
memory is a typical example here - a process can use
a larger virtual memory than there is physically real
memory available. However, real memory is accessed
in case the virtual resource is used - the virtual memory
manager has to decide which parts are mapped into
that physical memory region or swapped to disc respectively.

So splitting a physical link into several virtual
links is also a valid example if the virtual
links offer the same interface and features as the
real medium, e.g., each VLAN may appear in a host
as separate Ethernet interface.

The book explicitly distinguishes virtual from logical
resources as follows:

* a logical resource is created by transforming one
  or several features of a real resource and by
  offering it in this way to the user.

This requires use of a mapping layer that transforms
the interface of the real resource(s) into a more
purposeful interface. Thus, the logical resource/device
may be simpler to handle from a user's point of view
and constitutes an improved but different resource
or device.

For example, block-oriented devices are logical
devices if the physical underlying device offers
only character-oriented access. A well-known example
is to create a single (larger) logical volume out of
several different physical disks. File-oriented access
is also a logical concept, because it abstracts from
block-oriented sequential structures of the data storage.
Consider a logical desktop appearing as single one although
composed of two different adjacent screens in contrast
to larger virtual desktop.

An example in the networking context are all layers:
a) a link layer offering frame-oriented transport on
   top of the bit-oriented physical layer
b) an end-to-end IP packet transport on top of the concatenated
   link layer connections
c) a reliable transport link on top of an unreliable link
etc...

Note that both concepts are providing abstractions.
I'm not quite sure that the distinction between logical
and virtual is helpful or useful in our context. However,
it might explain why IP is not a virtual network, but
logical one (same for TCP on top of IP) - as I also postulated
in an earlier mail on this list.

Just my two cents.

Regards,
 Roland