On Mon, Dec 6, 2010 at 7:05 PM, Chris Mason <chris.mason@xxxxxxxxxx> wrote: > Excerpts from Nirbheek Chauhan's message of 2010-12-06 07:41:16 -0500: >> 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? > > It would work yes. ÂThe operation has three cases: > > 1) file size doesn't change > 2) extend the file with new bytes in the middle > 3) make the file smaller removing bytes in the middle > > #1 is the easiest case, you can just use the clone range ioctl directly Tis doesn't seem to be interesting, looking just like traditional COW overwrite. > > For #2 and #3, all of the file pointers past the bytes you want to add > or remove need to be updated with a new file offset. ÂI'd say for an > initial implementation to use the IOC_CLONE_RANGE code, and after > everything is working we can look at optimizing it with a shift ioctl if > it makes sense. Not sure how btrfs implements versioned B-trees, but other snapshot-capable file-systems I'm aware of utilize DITTO B-tree entry that says "for tis range, consult previous version tree". One can imagine DITTO(n) extension that would tell "subtract n from look-up key and then consult previous version tree", effectively achieving range shift behavior. FWIW. Regards, Andrey > > Of the use cases you list, video editors seems the most useful. > Databases already have things pretty much under control, and delta > patching wants to go to a new file anyway. ÂVideo editing software has > long been looking for ways to do this. > > -chris > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at Âhttp://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
