> > I think it's a good idea to add file freeze semantics to the toolbox > > of useful things that could be accomplished with reflink. > > reflink is already atomic w.r.t. other writes - in what way does a > "file freeze" have any impact on a reflink operation? that is, apart > from preventing it from being done, because reflink can modify the > source inode on XFS, too.... > - create O_TMPFILE - freeze source file - read and calculate hash from source file - likely unfreeze and skip reflink+backup For the unlikely case, application could copy_file_range before unfreeze and that means that reflink of source should be allowed while file is frozen, that is, while file *data* is frozen. That mean that file freeze API needs to be able to express if both metadata and data freeze are required. > > Anyway, freeze semantics alone won't work for our backup application > > that needs to be non intrusive. Even if writes to large file are few, > > backup may take time, so blocking those few write for that long is > > not acceptable. > > So, reflink is too expensive because there are only occasional > writes, but blocking that occasional write is too expensive, too, > even though it is rare? > All right. I admit to have presented a weak example, but I am not submitting a patch to be merged. I am proposing a discussion on what I think is a gap in existing API. The feedback of "what is the measurable benefits?" is well expected, but I brought this up anyway, without concrete measurable figures to hear what others have to say. And frankly, I quite like the file freeze suggestion, so I am glad that I did. Besides, even if existing filesystems implement reflink fast "enough", this is not at all an mandated by the API. > > Blocking the writes for the setup time of a reflink > > is exactly what I was proposing and in your analogy, > > No, I proposed a way to provide a -point in time snapshot- of a > file that doesn't require reflink or any other special filesystem > support. > > > the block > > device is frozen only for a short period of time for setting up the > > snapshot and not for the duration of the backup. > > Right, it's frozen for as long as it takes to set up a -point in > time snapshot- that the backup can be taken from. You don't need > that to reflink a file. You need it if you want to do something > other than a reflink.... > Correct. As I wrote above, that could be used for conditional copy or conditional reflink on a filesystem where reflink has a measurable cost. Bottom line: I completely agree with you that "file freeze" is sufficient for the case I presented, as long as reflink is allowed while file is frozen. IOW, break the existing compound API freeze+reflink+unfreeze to individual operations to give more control over to user. Thanks, Amir.
