We've been talking about implementing some form of bulk data copy offloading for a while now. BTRFS and OCFS2 implement forms of copy offloading with ioctls, NFS 4.2 will include a byte-granular COPY operation, and the SCSI XCOPY command is being implemented now that Windows can issue it. In the past we've discussed promoting the ocfs2 reflink ioctl into a system call that would create a new file and implicitly copy the source data into the new file: https://lkml.org/lkml/2009/9/14/481 These draft patches take the simpler approach of only copying data between existing files. The patches 1) make a system call out of the btrfs CLONE_RANGE ioctl, 2) implement the btrfs .copy_range method with the ioctl's guts, 3) implement the nfs .copy_range by sending a COPY op, and 4) serve the COPY op in nfsd by calling the .copy_range method again. The nfs patch is an untested hack. I'm happy to beat it in to shape but I'll need some guidance. I'd like strong review feedback on the interfaces, here are some possible topics: a) Hopefully being able to specify a portion of the data to copy will avoid *huge* syscall latencies and the motivation for new async semantics. b) The BTRFS ioctl and nfs COPY let you specify a count of 0 to copy from the start offset to the end of the file. Does anyone have a strong feeling about this? I'm leaning towards not bothering with it in the syscall interface. c) I chose to return partial progess in the ssize_t return code. This limits the length of the range and the size_t count argument can be too large and return errors, much like other io syscalls. This seemed less awful than some extra argument with a pointer to a status value. d) I'm dreading mentioning a vector of ranges to copy in one syscall because I don't want to think about overlaping ranges and file systems that use range locks -- xfs for now, but more if Jan gets his way. I'd rather that we get some experience with this simpler syscall before taking on that headache. I'm sure I'm forgetting some other details. I'm going to keep hacking away at this. My next step is to get ext4 supporting .copy_range, probably with a quick hack to copy the contents of bios. Hopefully that'll give enough time to also integrate review feedback. Thoughts? - z -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
