On 17 May 2005, at 10:44, Peter Svensson wrote:
On Tue, 17 May 2005, tim panton wrote:
The 'if possible' thing relates to filesystem design.
Almost all of the native UNIX filesystems support mv as an atomic action
- but only within the same filesystem.
(Imagine you create the file on one physical disk then 'move' it
onto a different disk - the kernel has no option but to
copy the file).
A Posix-compliant filesystem has to perform moves as an atomic operation.
The removal of the old file name and the creation of the new file name
will from the point of view of any process be simultaneous. Also, if the
destination file name exists that name will refer either to the old file
or the new, there is no window where neither file or an incomplete file is
visible to a process.
Yep, that's what the spec says :-)
Moves are not defined across mount points. The kernel will never actually
copy the file when move is calld. That is implemented in use space by e.g.
"mv" in fileustils etc.
Yes, sorry, I wasn't clear there - what I was trying to say was that in this case it wouldn't be done as an atomic kernel action, but it came out garbled.
So create your file in a temp directory on the _same_ file system as the destination, then do the move.
If your filesystem is remote (samba or nfs) or non unix native (FAT) then it just won't work.
Nfs is a posix filesystem -> moves withing a device are guaranteed to be
atomic. I think is is defined for smb as well, though there may be
restrictions and the atomicity may not be guaranteed.
Here we disagree. I had my fingers _badly_ burnt on exactly this one some years ago. I assumed that nfs being a posix filesystem would mean that atomic actions would actually be atomic. That wasn't how it turned out.
In theory they are - but numerous bugs and 'optimizations' in various nfs implementations mean that I (personally) would never depend on it again. Especially not if there are multiple clients mounting the same filesystem read-write.
Tim.
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