Hello, I'd like to know if there has been any discussion about adding a new feature to write (add) data at an offset, but without overwriting existing data, or re-writing the existing data. Essentially, in-place addition/removal of data to a file at a place other than the end of the file.
Some possible use-cases of such a feature would be: (a) Databases (currently hack around this by allocating sparse files) (b) Delta-patching (rsync, patch, xdelta, etc) (c) Video editors (especially if combined with reflink copies) Besides I/O savings, it would also have significant space savings if the current subvolume being written to has been snapshotted (a common use-case for incremental backups). I've been told that the problem is somewhat difficult to solve properly under block-based representation of data, but I was hoping that btrfs' reflink mechanism and its space-efficient packing of small files might make it doable. A hack I can think of is to do a BTRFS_IOC_CLONE_RANGE into a new file (upto the offset), writing whatever data is required, and then doing another BTRFS_IOC_CLONE_RANGE with an offset for the rest of the original file. This can be followed by a rename() over the original file. Similarly for removing data from the middle of a file. Would this work? Would it be cleaner to implement something equivalent internally? Thanks! -- ~Nirbheek Chauhan Gentoo GNOME+Mozilla Team -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
