On Tue, 2009-05-05 at 16:36 +0100, Jamie Lokier wrote: > Chris Mason wrote: > > The btrfs implementation is just that you have two separate files > > pointing to the same extents on disk. Each file has a reference on each > > extent, and deleting or chowning fileA doesn't change the metadata in > > fileB. > > > > The btrfs cow code makes sure that modifications in either file (even > > when mounted in -o nodatacow) are written to new extents instead of > > changing the original. If you write one block in a 1TB file, the new > > space used by the clone is only one block. (Thanks to the ceph > > developers for coding all of this up a while ago). > > Ooh, nice. > > > The main difference between reflink and the btrfs ioctl is that in the > > btrfs ioctl the destination file must already exist. The btrfs code can > > also do range replacements in the destination file, but I'd agree with > > Joel that we don't want to toss the kitchen sink into something nice and > > clean like reflink. > > Ah, now that I know about the BTRFS data-cloning ioctl... :-) > > I'm wondering why reflink() is needed at all. Can't it be done in > userspace, using the BTRFS ioctl? The hard part in userspace seems to > be copying the file attributes, but "cp -a" and other tools manage. > reflink is a subset of what the btrfs ioctl does, and that's a good thing. The way they've added support for this to ocfs2 is really cool, and the same ideas could be used in other filesystems. So, I'd rather see a system call that everyone can implement, and if btrfs hangs on to the ioctl for extra features, even better. -chris -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
