On Thu, Mar 31, 2016 at 01:47:50PM -0600, Andreas Dilger wrote: > On Mar 31, 2016, at 12:08 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > > > > On Thu, Mar 31, 2016 at 10:18:50PM +1100, Dave Chinner wrote: > >> On Thu, Mar 31, 2016 at 12:54:40AM -0700, Christoph Hellwig wrote: > >>> On Thu, Mar 31, 2016 at 12:18:13PM +1100, Dave Chinner wrote: > >>>> On Wed, Mar 30, 2016 at 11:27:55AM -0700, Darrick J. Wong wrote: > >>>>> Or is it ok that fallocate could block, potentially for a long time as > >>>>> we stream cows through the page cache (or however unshare works > >>>>> internally)? Those same programs might not be expecting fallocate to > >>>>> take a long time. > >>>> > >>>> Yes, it's perfectly fine for fallocate to block for long periods of > >>>> time. See what gfs2 does during preallocation of blocks - it ends up > >>>> calling sb_issue_zerout() because it doesn't have unwritten > >>>> extents, and hence can block for long periods of time.... > >>> > >>> gfs2 fallocate is an implementation that will cause all but the most > >>> trivial users real pain. Even the initial XFS implementation just > >>> marking the transactions synchronous made it unusable for all kinds > >>> of applications, and this is much worse. E.g. a NFS ALLOCATE operation > >>> to gfs2 will probab;ly hand your connection for extended periods of > >>> time. > >>> > >>> If we need to support something like what gfs2 does we should have a > >>> separate flag for it. > >> > >> Using fallocate() for preallocation was always intended to > >> be a faster, more efficient method allocating zeroed space > >> than having userspace write blocks of data. Faster, more efficient > >> does not mean instantaneous, and gfs2 using sb_issue_zerout() means > >> that if the hardware has zeroing offloads (deterministic trim, write > >> same, etc) it will use them, and that will be much faster than > >> writing zeros from userspace. > >> > >> IMO, what gfs2 is definitely within the intended usage of > >> fallocate() for accelerating the preallocation of blocks. > >> > >> Yes, it may not be optimal for things like NFS servers which haven't > >> considered that a fallocate based offload operation might take some > >> time to execute, but that's not a problem with fallocate. i.e. > >> that's a problem with the nfs server ALLOCATE implementation not > >> being prepared to return NFSERR_JUKEBOX to prevent client side hangs > >> and timeouts while the operation is run.... > > > > That's an interesting idea, but I don't think it's really legal. I take > > JUKEBOX to mean "sorry, I'm failing this operation for now, try again > > later and it might succeed", not "OK, I'm working on it, try again and > > you may find out I've done it". > > > > So if the client gets a JUKEBOX error but the server goes ahead and does > > the operation anyway, that'd be unexpected. > > Well, the tape continued to be mounted in the background and/or the file > restored from the tape into the filesystem... Right, and SGI have been shipping a DMAPI-aware Linux NFS server for many years, using the above NFSERR_JUKEBOX behaviour for operations that may block for a long time due to the need to pull stuff into the filesytsem from the slow backing store. Best explanation is in the relevant commit in the last published XFS+DMAPI branch from SGI, for example: http://oss.sgi.com/cgi-bin/gitweb.cgi?p=xfs/xfs.git;a=commit;h=28b171cf2b64167826474efbb82ad9d471a05f75 Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
