On Fri 21-05-10 09:05:24, Dave Chinner wrote: > On Thu, May 20, 2010 at 10:12:32PM +0200, Jan Kara wrote: > > Hmm, I was thinking about it and I see two options how to get out > > of problems: > > a) Filesystems which are not able to handle hole punching will allow > > multipage writes only after EOF (which can be easily undone by > > truncate in case of failure). That should actually cover lots of > > cases we are interested in (I don't expect multipage writes to holes > > to be a common case). > > multipage writes to holes is a relatively common operation in the > HPC space that XFS is designed for (e.g. calculations on huge sparse > matrices), so I'm not really fond of this idea.... Well, XFS could still handle them because it is able to do hole punching but I get your point. > > 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? 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
