On Wed, Oct 14, 2015 at 11:46:08AM -0700, Darrick J. Wong wrote: > On Tue, Oct 13, 2015 at 12:29:59AM -0700, Christoph Hellwig wrote: > > On Mon, Oct 12, 2015 at 04:41:06PM -0700, Darrick J. Wong wrote: > > > One of the patches in last week's XFS reflink patchbomb adds FALLOC_FL_UNSHARE > > > flag; at the moment it _only_ forces copy-on-write of shared blocks, and it > > > leaves holes alone. > > > > Yes, I've seen the implementation. > > > > > Obviously we haven't yet figured out what are peoples' preferences in terms of > > > "fill the holes and unshare the shared" vs. "only unshare the shared" vs. "only > > > fill the holes". It isn't that hard to add a FALLOC_FL_UNSHARE_FILL_HOLES flag > > > that fills the holes while unsharing is going on. > > > > > > Personally I suspect that the most interest is in filling holes and unsharing, > > > because they don't want to pay for allocation at a critical stage for anywhere > > > in the file. But I could be wrong, so allowing both goals to be expressed via > > > mode allows flexibility. > > > > Exactly. And a normal falloc should do just that - fill holes and > > ensure that we don't need to COW already allocated locks. So I don't > > think we need a new fallocate interface for that. > > The documentation for fallocate ought to be updated to include that as part of > guaranteeing that subsequent writes to the range won't fail due to ENOSPC, > shared blocks will be unshared. > > Incidentally, btrfs leaves shared blocks alone. OTOH, given that it's totally > COW it probably doesn't make sense to unshare blocks anyway... but maybe I > also don't want to dive into btrfs f-allocation behavior at this time. :) > > Ok, so I'll rework the XFS funshare code into something that hangs off the > regular fallocate call, and get rid of the explicit 'funshare' bits. Makes sense given we have the FALLOC_FL_ZERO_RANGE operation which returns a zero to and preallocates all the holes in the range. I would expect this operation on shared blocks to unshare blocks, too... > > The question is if we > > want a copy interface that gives you the same semantics as if you also > > called an fallocate on the destination range. For that case we'd > > usually want to avoid doing the clone and instead do a in-kernel or > > hardware assisted copy and then fill the holes with unwritten extents. If hole filling was required, then I'd do the operation the other way around - prealloc the entire range, then do hardware assisted copy of each separate data range in the source file with unwritten extent conversion on offload completion... > Probably; I can easily imagine people wanting to fill the holes and also > not wanting them filled. *nod*. 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
