[RFC] extending splice for copy offloading

[David Disseldorp]

Hi Steve,

On Thu, 26 Sep 2013 12:22:49 -0500
Steve French <smfrench at gmail.com> wrote:

> Windows clients seem to max out at 16MB chunk size when doing copy
> offload.   I would like to increase chunk size larger than that if
> network bandwidth (returned at mount time in SMB3 on the query network
> interfaces FSCTL) is large enough, and response time is not more than
> 100 (?) milliseconds.

Sounds good, though with Samba and Btrfs the request latency will be
heavily effected by whether the FSCTL_SRV_COPYCHUNK wire request byte
offsets and lengths align to the Btrfs sector-size (4K by default
IIRC).
If they do align, then each chunk in the request can be mapped to a
BTRFS_IOC_CLONE_RANGE ioctl. Otherwise, we need to fall back to a
server-side local read/write.

We should be able to provide underlying filesystem alignment
requirements to SMB clients via the new FileFSSectorSizeInformation
ioctl, but it's not currently supported by Samba.

Cheers, David