Excerpts from Nirbheek Chauhan's message of 2010-12-06 14:14:59 -0500: > On Mon, Dec 6, 2010 at 9:35 PM, Chris Mason <chris.mason@xxxxxxxxxx> wrote: > > Excerpts from Nirbheek Chauhan's message of 2010-12-06 07:41:16 -0500: > [snip] > >> 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). > >> > [snip] > >> 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 > > > > 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. > > > > Alrighty, I'll try this and report back any bugs and/or suggestions. > > > 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. > > > > As an aside, my primary motivation for this was that doing an > incremental backup of things like git bare repositories and databases > using btrfs subvolume snapshots is expensive w.r.t. disk space. Even > though rsync calculates a binary delta before transferring data, it > has to write everything out (except if just appending). So in that > case, each "incremental" backup is hardly so. Oh, I see what you mean. Yes that is definitely an interesting use case. -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 http://vger.kernel.org/majordomo-info.html
