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First two patches are trivial cleanups that I''ve spotted while reading send.c, can be applied independently. The third patch contains the important bits, safety checks that the subvolumes involved in send do not accidentally lose the RO status. I haven''t seen this documented anywhere that this is mandatory, implied from how I assume send works. Please let me know if this is

This patch adds the flag, BTRFS_SEND_FLAG_NO_FILE_DATA to the btrfs send ioctl code. When this flag is set, the btrfs send code will never write file data into the stream (thus also avoiding expensive reads of that data in the first place). BTRFS_SEND_C_UPDATE_EXTENT commands will be sent (instead of BTRFS_SEND_C_WRITE) with an offset, length pair indicating the extent in question. This patch