The EXTENDED COPY SCSI command (XCOPY) is used extensively by VMware ESX initiators, to offload to the iSCSI target, the process of copying data between a source and destination LU region.
XCOPY is also supported by other initiators, such as ddpt (http://sg.danny.cz/sg/ddpt.html), libiscsi, etc.
The XCOPY specification in SPC-4 (revision 37) states that the XCOPY source and destination device(s) should be derived from the copy source and copy destination descriptor (CSCD) IDs in the XCOPY segment descriptor. The CSCD IDs are generally (for block -> block copies), indexes into the corresponding CSCD list, i.e.
- entry 0
+ LU ID
- entry 2
+ LU ID
Segment Descriptor List
- segment 0
+ src CSCD ID = (CSCD entry 0)
+ dest CSCD ID = (CSCD entry 1)
+ src lba
+ dest lba
Currently LIO completely ignores the src and dest CSCD IDs in the Segment Descriptor List, and instead assumes that the first entry in the CSCD list corresponds to the source, and the second to the destination.
This assumption appears to stand true for XCOPY requests from ESXi initiators. However, deviation from the spec may break other initiators - it causes a number of libiscsi tests to fail.
I've pushed a reproducer to:
examples/iscsi-dd --xcopy --src iscsi://192.168.155.101:3260/<iqn>/<src_lun> --dst iscsi://192.168.155.101:3260/<iqn>/<dst_lun>
iscsi-dd has been modified to issue XCOPY requests with the CSCD List order modified such that the destination (specified via segment descriptor ID) appears before the source.
LIO XCOPY fixes merged to Linus' tree: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/log/?id=87156518da94a696f2b27ab8945d531af2f1d339&showmsg=1