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This maybe a linguistic question. I have checked the loop device on Wikipedia. It is just for mounting files as block device. But what does "loop" mean here? Its usage here is totally bizarre to me. I am not a native English speaker. So could someone explain this jargon to me in plain English? :)


share|improve this question – Anycorn Sep 1 '10 at 2:54
I guess you should be able to translate the english page back in to your native language!. – Chubsdad Sep 1 '10 at 2:57
up vote 5 down vote accepted

It's short for "loopback".

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Why is it called "loopback" though? I can see two parallels: Loopback of the network device (i.e having some word play), or loop back in the sense that the driver asks back into the file-system again for the file to map into. What meaning is correct? – Johannes Schaub - litb Sep 1 '10 at 4:02
I think it's the latter one. Correct me if I am wrong. – smwikipedia Sep 1 '10 at 4:14
It's the latter one. Normally a request would go userspace -> VFS -> block device layer, but when the block device driver is loop, it goes userspace -> VFS -> loop device -> VFS -> block device. So it loops back to a higher level. – hobbs Sep 1 '10 at 4:19
As far as I know the term originated in the depths of the telecom industry (and probably dates back to telegraphs). See mmonem's answer. – RBerteig Sep 1 '10 at 6:44
I mark the answer for its comment by hobbs. – smwikipedia Sep 10 '10 at 5:51

The concept is also known as a disk image. I guess because driver calls to the image get passed along to the underlying driver of the physical disk. There is no actual loop involved; it is an additional level to a driver stack which is already several layers deep.

(I've written an encrypted disk image driver, and I find the "loopback" terminology incorrect and confusing.)

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I guess this term comes from the communication realm when sometimes it is needed to test the communication system by simulating a peer using a proxy circuit loop.

The concept came also to UNIX networking where loopback network interfaces do not send network traffic to the medium.

The same concept in file systems loop means that the file system driver does not really goes through the hard disk IO stack and, instead, ends using a plain disk image file for IO.

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