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
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- Re: [vnrg] Logical vs. virtual Martin Stiemerling
- Re: [vnrg] Logical vs. virtual $witch
- Re: [vnrg] Logical vs. virtual Roland Bless
- Re: [vnrg] Logical vs. virtual Roland Bless