Meaning of "near-atomic" transfer using temporary files?

Question

I was reading 'SCP' command man page in linux, in the end it said

"No attempt is made at "near-atomic" transfer using temporary files".

Vaguely I can guess what it is but can anyone clearly tell me the technical definition of this sentence.

Thanks, paddy

Answer 1

An atomic copy would be as Craig states, to use a temporary file and then mv the temporary file to the intended destination. The mv IS an atomic providing source and destination are on the same partition. Only file operations with the tmp file open already will be able to read the contents. rename() is not atomic on files that move between partitions as the data has to be copied.

This assumes you're scp'ing to a UNIX system of course :)

Answer 2

Atomic would mean that nothing else could read or write the file until scp had finished with it. "Near-atomic" refers to the common practise of copying a file into a temporary location (on the target machine/disk) and then moving it into the final location. The move operation is much faster than a copy ("near-atomic" by comparison) but it's not necessarily atomic in the true sense of the word. Another process could still read the file in an inconsistent state during a non-atomic move.