On Sat 22-05-10 10:27:59, Dave Chinner wrote: > On Fri, May 21, 2010 at 08:58:46PM +0200, Jan Kara wrote: > > On Fri 21-05-10 09:05:24, Dave Chinner wrote: > > > On Thu, May 20, 2010 at 10:12:32PM +0200, Jan Kara wrote: > > > > b) E.g. ext4 can do even without hole punching. It can allocate extent > > > > as 'unwritten' and when something during the write fails, it just > > > > leaves the extent allocated and the 'unwritten' flag makes sure that > > > > any read will see zeros. I suppose that other filesystems that care > > > > about multipage writes are able to do similar things (e.g. btrfs can > > > > do the same as far as I remember, I'm not sure about gfs2). > > > > > > Allocating multipage writes as unwritten extents turns off delayed > > > allocation and hence we'd lose all the benefits that this gives... > > Ah, sorry. That was a short-circuit in my brain. But when we do delayed > > I don't see why we should actually do any hole punching... The write needs > > to: > > a) reserve enough blocks for the write - I don't know about other > > filesystems but for ext4 this means just incrementing a counter. > > b) copy data page by page > > c) release part of reservation (i.e. decrement counter) if we actually > > copied less than we originally thought. > > > > Am I missing something? > > Possibly. Delayed allocation is made up of two parts - space > reservation and recording the regions of delayed allocation in an > extent tree, page/bufferhead state or both. Yes. Ext4 records the info about delayed allocation only in buffer heads. > In XFS, these two steps happen in the same get_blocks call, but the > result of that is we have to truncate/punch delayed allocate extents > out just like normal extents if we are not going to use them. Hence > a reserve/allocate interface allows us to split the operation - > reserve ensures we have space for the delayed allocation, allocate > inserts the delayed extents into the inode extent tree for later > real allocation during writeback. Hence the unreserve call can > simply be accounting - it has no requirement to punch out delayed > extents that may have already been allocated, just do work on > counters. > > btrfs already has this split design - it reserves space, does the > copy, then marks the extent ranges as delalloc once the copy has > succeeded, otherwise it simply unreserves the unused space. > > Once again, I don't know if ext4 does this internal delayed > allocation extent tracking or whether it just uses page state to > track those extents, but it would probably still have to use the > allocate call to mark all the pages/bufferheads as delalloc so > that uneserve didn't have to do any extra work. Yes, exactly. I just wanted to point out that AFAICS ext4 can implement proper error recovery without a need for 'punch' operation. So after all Nick's copy page-by-page should be plausible at least for ext4. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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
