Is `mv dir dir2` atomic ? Post: 302170830

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Top Forums UNIX for Dummies Questions & Answers Is `mv dir dir2` atomic ? Post 302170830 by Perderabo on Tuesday 26th of February 2008 10:42:52 PM

02-26-2008

Administrator Emeritus

I'm really only familiar with the HP-UX version, but I'm guessing that Linux version must be similar. HP's mvdir is confined to operating in a filesystem...never between filesystems. To understand it, consider the first unix implemention of it... If you have /parent/A which has a lot of files in it and you move it to /parent/B, the original version would invoke the link system call and link A to B. At this point, both A and B exist and they are just two names to the same directory. Then, as soon as possible, it would unlink A. This would leave just B. The operation took two very quick system calls. This was NOT atomic. Between the system calls, the filesystem was in an illegal state. A and B should not be links to the same directory even for a millisecond. Today, instead of two system calls, we use just one...and rename(). This is absolutely atomic. For all intents and purposes A just morphs into B. A and B must be in the same filesystem, but not necessarily have the same exact parent as I have indicated here.

Perderabo

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LEARN ABOUT SUNOS

mvdir

mvdir(1M)						  System Administration Commands						 mvdir(1M)

NAME

       mvdir - move a directory

SYNOPSIS

       /usr/sbin/mvdir dirname name

DESCRIPTION

       mvdir  moves  directories within a file system. dirname must be a directory. If name does not exist, it will be created as a directory.	If
       name does exist, and is a directory, dirname will be created as name/dirname. dirname and  name may not be on the same path; that  is,  one
       may not be subordinate to the other. For example:

	      example% mvdir x/y x/z

       is legal, but

	      example% mvdir x/y x/y/z

       is not.

OPERANDS

       dirname	       The name of the directory that is to be moved to another directory in the filesystem.

       name	       The  name  of the directory into which dirname is to be moved. If name does not exist, it will be created. It may not be on
		       the same path as dirname.

USAGE

       See largefile(5) for the description of the behavior of mvdir when encountering files greater than or equal to 2 Gbyte ( 2**31 bytes).

EXIT STATUS

       0	Successful operation.

       >0	Operation failed.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWcsu			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       mkdir(1), mv(1), attributes(5), largefile(5)

SunOS 5.10							    14 Mar 1997 							 mvdir(1M)