Re: New copyfile system call - discuss before LSF?

[Andy Lutomirski]

On Sat, Mar 30, 2013 at 8:52 PM, Myklebust, Trond
<Trond.Myklebust@xxxxxxxxxx> wrote:
> On Sat, 2013-03-30 at 19:53 -0700, Andreas Dilger wrote:
>> On 2013-03-30, at 16:21, Ric Wheeler <rwheeler@xxxxxxxxxx> wrote:
>>
>> > On 03/30/2013 05:57 PM, Myklebust, Trond wrote:
>> >> On Mar 30, 2013, at 5:45 PM, Pavel Machek <pavel@xxxxxx>
>> >>  wrote:
>> >>
>> >>> On Sat 2013-03-30 13:08:39, Andreas Dilger wrote:
>> >>>> On 2013-03-30, at 12:49 PM, Pavel Machek wrote:
>> >>>>> Hmm, really? AFAICT it would be simple to provide an
>> >>>>> open_deleted_file("directory") syscall. You'd open_deleted_file(),
>> >>>>> copy source file into it, then fsync(), then link it into filesystem.
>> >>>>>
>> >>>>> That should have atomicity properties reflected.
>> >>>> Actually, the open_deleted_file() syscall is quite useful for many
>> >>>> different things all by itself.  Lots of applications need to create
>> >>>> temporary files that are unlinked at application failure (without a
>> >>>> race if app crashes after creating the file, but before unlinking).
>> >>>> It also avoids exposing temporary files into the namespace if other
>> >>>> applications are accessing the directory.
>> >>> Hmm. open_deleted_file() will still need to get a directory... so it
>> >>> will still need a path. Perhaps open("/foo/bar/mnt", O_DELETED) would
>> >>> be acceptable interface?
>> >>>                                    Pavel
>> >> ...and what's the big plan to make this work on anything other than ext4 and btrfs?
>> >>
>> >> Cheers,
>> >>   Trond
>> >
>> > I know that change can be a good thing, but are we really solving a pressing problem given that application developers have dealt with open/rename as the way to get "atomic" file creation for several decades now ?
>>
>> Using open()+rename() has side effects:
>> - changes ctime/mtime on parent directory
>> - leaves temporary file in path during creation
>> - leaves temporary file in namespace during operations, and after crash
>
> So what is the actual problem that is being solved? Yes, the above may
> be disadvantages, but none of them have proven to be show-stoppers so
> far.
>
> So far, I've seen no justification for Andy's atomicity requirement
> other than "it would be nice if...". That's not enough IMO...

ISTM vpsendfile (or whatever it's called) plus a way to create deleted
files plus a way to relink deleted files gives atomic copies.  Perhaps
this is less efficient than would be ideal for OCFS2, though.

--Andy
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