On Wed, 2006-10-25 at 02:01 +1000, David Chinner wrote: > On Tue, Oct 24, 2006 at 09:51:41AM -0500, Dave Kleikamp wrote: > > On Tue, 2006-10-24 at 23:59 +1000, David Chinner wrote: > > > That's the wrong way to look at it. if you want the userspace > > > process to specify a location, then you should preallocate it first > > > before doing anything else. There is no need to clutter a simple > > > data mover interface with all sorts of unnecessary error handling. > > > > You are implying the the 2-step interface, creating a new inode then > > swapping the contents, is the only way to implement this. > > No, it's not the only way to implement it, but it seems the cleanest > way to me when you have to consider crash recovery. With a temporary > inode, you can create it, hold a reference and then unlink it so > that any crash at that point will free the inode and any extents > it has on it. > > The only way I can see anything different working is having the > filesystem hold extents somewhere internally that provides us the > same recovery guarantees while we copy the data and insert the new > extents. This is obviously a filesystem specific solution and is > more complex to implement than a swap extent transaction. it > probably also needs on disk format changes to support properly.... This is definitely filesystem-dependent. I would think allocating an extent would be like any other allocation done by the filesystem, and there are already recovery mechanisms for that. > > > Once you've separated the destination allocation from the data > > > mover, the mover is basically a splice copy from source to > > > destination, an fsync and then an atomic swap blocks/extents operation. > > > Most of this code is generic, and a per-fs swap-extents vector > > > could be easily provided for the one bit that is not.... > > > > The benefit of having such a simple data mover is negated by moving the > > complexity into the allocator. > > What complexity does it introduce that the allocator doesn't already > have or needs to provide for the single call interface to work? I don't see it as any more or less complex than a single interface. > > A single interface that would move a part of a file at a time has the > > advantage that a large file which is only fragmented in a few areas does > > not need to be completely moved. > > And the two-step process can do exactly this as well - splice can > work on any offset within the file... I wasn't aware of that. That makes your proposal sound a lot better. > > > The allocation interface, OTOH, is anything but simple and is really > > > a filesystem specific interface. Seems logical to me to separate > > > the two. > > > > So what then is the benefit of having a simple generic data mover if > > every file system needs to implement it's own interface to allocate a > > copy of the data? > > I assume you meant "....allocate the space to store the copy of the data." Yeah. > The allocation interface needs to be be able to be extended > independently of the data mover interface. XFS already exposes > allocation ioctls to userspace for preallocation and we've got plans > to extnd this further to allow userspace controlled allocation for > smart defrag tools for XFS. Tying allocation to the data mover > just makes the interface less flexible and harder to do anything > smart with.... Okay. It would be nice to standardize the interface so we don't have every filesystem introducing new ioctls. > Cheers, > > Dave. -- David Kleikamp IBM Linux Technology Center - 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
