How can I check if
mv is atomic on my fs (ext4)?
The OS is Red Hat Enterprise Linux Server release 6.8.
In general, how can I check this? I have looked around, and didn't find if my OS is standard POSIX.
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Interestingly enough, it seems the answer may be, "It depends".
To be clear,
mv is specified to
mvutility shall perform actions equivalent to the
The rename function specification states:
rename()function is equivalent for regular files to that defined by the ISO C standard. Its inclusion here expands that definition to include actions on directories and specifies behavior when the new parameter names a file that already exists. That specification requires that the action of the function be atomic.
But the latest the ISO C specification for
#include <stdio.h> int rename(const char *old, const char *new);
renamefunction causes the file whose name is the string pointed to by
oldto be henceforth known by the name given by the string pointed to by
new. The file named
oldis no longer accessible by that name. If a file named by the string pointed to by
newexists prior to the call to the
renamefunction, the behavior is implementation-defined.
renamefunction returns zero if the operation succeeds, nonzero if it fails, in which case if the file existed previously it is still known by its original name.
Surprisingly, note that there is no explicit requirement for atomicity. It may be required somewhere else in the latest publicly-available C Standard, but I haven't been able to find it. If anyone can find such a requirement, edits and comments are more than welcome.
See also Is rename() atomic?
Per the Linux man page:
newpathalready exists, it will be atomically replaced, so that there is no point at which another process attempting to access
newpathwill find it missing. However, there will probably be a window in which both
newpathrefer to the file being renamed.
The Linux man page claims the replacement of the file will be atomic.
Testing and verifying that atomicity might be very difficult, though, if that is how far you need to go. You're not clear as to what you mean in your use of "How can I check if mv is atomic". Do you want requirements/specification/documentation that it's atomic, or do you need to actually test it?
Note also, the above assumes the two operand file names are in the same file system. I can find no standard restriction on the
mv utility to enforce that.
mv is based on
rename system call and
rename() is atomic. You could look at the manpage
You could find answer on Is rename() atomic? on stackoverflow.
What sort of fs, did you use ?
In addition to checking the systemcalls and their atomicity, maybe
inotify-tools can serve as a test, though I am not sure if it is a guaranteed proof of atomicity.
Open 2 shells. Watch the target directory of the move in one of them:
inotifywait -m target/
Move a file into the directory in the other:
mv foobar target/
inotifywait should show only one line:
target/ MOVED_TO foobar
It seems atomic in comparison to the response to
ls target/ and
touch target/a, which produce multiline messages like:
# the response to ls target/ target/ OPEN,ISDIR target/ ACCESS,ISDIR target/ CLOSE_NOWRITE,CLOSE,ISDIR
I think, at least it shows that asynchronous multiprocess cooperation on files is safe with
inotify (practically atomic): in any case you would respond only after
inotify gave the final signal after the operation. For example, a producer-consumer setup can be implemented easily and safely with