On Wed, 26 Feb 2014 12:34:26 +1100 Dave Chinner <david@xxxxxxxxxxxxx> wrote: > On Tue, Feb 25, 2014 at 03:41:28PM -0800, Andrew Morton wrote: > > On Tue, 25 Feb 2014 15:23:35 -0800 (PST) Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > > On Tue, 25 Feb 2014, Dave Chinner wrote: > > > > On Tue, Feb 25, 2014 at 02:16:01PM +1100, Stephen Rothwell wrote: > > > > > On Mon, 24 Feb 2014 11:57:10 +1100 Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > > FALLOC_FL_COLLAPSE_RANGE: I'm a little sad at the name COLLAPSE, > > > but probably seven months too late to object. It surprises me that > > > you're doing all this work to deflate a part of the file, without > > > the obvious complementary work to inflate it - presumably all those > > > advertisers whose ads you're cutting out, will come back to us soon > > > to ask for inflation, so that they have somewhere to reinsert them ;) > > > > Yes, I was wondering that. Why not simply "move these blocks from here > > to there". > > And open a completely unnecessary can of worms to do with > behavioural and implementation corner cases? But it's general. > Do you allow it to destroy data by default? Or only allow moves into > holes? Overwrite. > What do you do with range the data is moved out of? Does it just > become a hole? What happens if the range overlaps EOF - does that > change the file size? Truncate. > What if you want to move the range beyond EOF? Extend. > What if the source and destination ranges overlap? Don't screw it up. > What happens when you move the block at EOF into the middle of a > file - do you end up with zeros padding the block and the file size > having to be adjusted accordingly? Or do we have to *copy* all the > data in high blocks down to fill the hole in the block? I don't understand that. Move the block(s) and truncate to the new length. > What behaviour should we expect if the filesystem can't implement > the entire move atomically and we crash in the middle of the move? What does collapse_range do now? If it's a journaled filesystem, it shouldn't screw up. If it isn't, fsck. > I can keep going, but I'll stop here - you get the idea. None of this seems like rocket science. > In comparison, collapse range as a file data manipulation has very > specific requirements and from that we can define a simple, specific > API that allows filesystems to accelerate that operation by extent > manipulation rather than read/memcpy/write that the applications are > currently doing for this operation.... IOWs, collapse range is a > simple operation, "move arbitrary blocks from here to there" is a > nightmare both from the specification and the implementation points > of view. collapse_range seems weird, arbitrary and half-assed. "Why didn't they go all the way and do it properly". -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>