On Mon, Feb 26, 2018 at 09:26:01AM -0800, Darrick J. Wong wrote: > On Mon, Feb 26, 2018 at 09:26:14AM +0100, Gionatan Danti wrote: > > Hi Amir, > > > > Il 26-02-2018 08:58 Amir Goldstein ha scritto: > > > > > >Gionatan, > > > > > >First of all, the answer to your question is "just" faster copy. > > >reflinkning a file is much faster than copy, but it is not O(1). > > >I believe cp --reflink can result in cloning part of the file if the > > >system > > >crashes mid operation, so in any case, the operation is not *atomic* > > >in that sense. > > > > > >But your questions about quiescence the filesystem and your question > > >about the *atomic* nature of the clone operation are two very different > > >questions. > > > > can this result on out-of-order writes from the cloned file's point of view? > > I mean: > > - take a 10-extents file; > > - a vm/db/whatever is writing to the file; > > - a cp --reflink is executed; > > - extents are cloned one-by-one, with extents 1-4 alredy cloned, 5 is in > > progress; > > - the vm/db writes to extent n.1 - this write will *not* be present on the > > cloned file; > > - application writes to extent n.6 which will be cloned shortly; > > - the cloned file ends with the later write to extent n.6 but not the > > previous on extent n.1; > > - bad things happen! > > > > If the above is true, than cp --reflink can't be used even for > > relaxed-consistency backup/clones. > > > > >What you seem to *think* xfs reflink does, it does not actually do. > > >xfs reflink does NOT reflink the file in-memory data. > > >xfs reflink "only" reflinks the file on-disk data. > > >Right now, if you write a large file without fsync and clone it, you > > >might as well get a clone of unallocated or partly fallocated file with > > >zero or stale data. > > > > Oh, I absolutely do not expect for reflink/clone to works on in-memory data. > > I *surely* expect for dirty, not commited data to be lost: this is the very > > reason I wrote about crash-consistent backup. > > > > In short: is cloning/reflink the same as "pulling the plug" for the cloned > > file? I mean: > > - a successfull clone (so, a non-interruped/crashed one) is akin to an > > atomic process for the cloned file; > > - async writes/dirty data are lost; > > - fsynced writes are preserved; > > - writes are not reordered/commited out of order. > > > > Maybe the entire discussion is skewed by the fact that, in some cases, I am > > willing to relax my consistency model to include a crash-consistent backup > > option. Fact is, in the virtualization world there are many backup > > utilities/applications which *use* this model, and I wondered if a cp > > --reflink would give similar results without the hassle. > > > > Maybe the entire crash-vs-application consistency is out of place in a > > filesystem mailing list, where you (rightfully!!!) strive for > > perfect/maximum data consistency (and I *really* appreciate that). Hoewever, > > given the recent reflinking works on XFS, I wonder if I can put this to > > "good use" when it is considered stable. > > The way reflink is supposed to work wrt consistency is: > > 1. lock out all new io/fallocate activity on both inodes (iolock/mmaplock) > 2. wait for all directio to complete > 3. fsync both files (write all the dirty pagecache to disk) My point is that vfs_clone_file_range is not running fsync(2)i operations. It's a fdatawrite_and_wait() call, which submits dirty data to disk and waits for it, but does *not flush volatile storage caches*. IOWs, it's not a data integrity operation. Hence while the reflink now has "data on disk" and can clone the extents, Neither the data nor the extents being cloned are stable and won't be until an fsync operation is performed on either the reflink source or destination file.... > 4. lock both inodes (ilock) > 5. clone each extent atomically > 6. unlock ilock > 7. unlock iolock/mmaplock > > So at least in theory the cloned file will match whatever the host saw > on disk and page cache at the time the reflink call was initiated. > I say 'in theory' because there could be bugs. Still no cache flushes. Hence even after the clone has run, you can still lose the data (and extents!) from the host file.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
