After upgrading a few clients on our Sherlock cluster from Lustre 2.13 to 2.14, we are seeing more and more "no handlers for opcode 0x16" errors on the servers (running 2.12.6).
From lustre_idl.h, I can see that ost opcode 0x16 is:
OST_FALLOCATE = 22,
So it's probably related to new fallocate support for Lustre/ldiskfs (LU-3606). I guess that's a good sign that codes are using fallocate! 
However, we only have a few clients running 2.14 in production, and I'm now worried that after we upgrade the whole cluster (1,800+ nodes), this will lead to an extremely high rate of these messages on the servers. Would it be possible to update Lustre 2.12 LTS to print those when debugging only? for better compatibility between 2.12 servers and 2.14 clients.
Apr 09 09:26:39 oak-io6-s2 kernel: LustreError: 91410:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST012d: no handlers for opcode 0x16 Apr 09 09:26:39 oak-io6-s2 kernel: LustreError: 91410:0:(tgt_handler.c:617:tgt_handler_find_check()) Skipped 6 previous similar messages Apr 09 09:26:39 oak-io5-s1 kernel: LustreError: 187145:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST00f2: no handlers for opcode 0x16 Apr 09 09:26:39 oak-io5-s1 kernel: LustreError: 187145:0:(tgt_handler.c:617:tgt_handler_find_check()) Skipped 15 previous similar messages Apr 09 09:26:39 oak-io6-s1 kernel: LustreError: 46667:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST0124: no handlers for opcode 0x16 Apr 09 09:26:39 oak-io6-s1 kernel: LustreError: 46667:0:(tgt_handler.c:617:tgt_handler_find_check()) Skipped 14 previous similar messages Apr 09 09:26:40 oak-io5-s2 kernel: LustreError: 272212:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST00ed: no handlers for opcode 0x16 Apr 09 09:26:40 oak-io5-s2 kernel: LustreError: 272212:0:(tgt_handler.c:617:tgt_handler_find_check()) Skipped 6 previous similar messages Apr 09 09:26:40 oak-io6-s2 kernel: LustreError: 14583:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST012b: no handlers for opcode 0x16 Apr 09 09:26:40 oak-io6-s2 kernel: LustreError: 14583:0:(tgt_handler.c:617:tgt_handler_find_check()) Skipped 2 previous similar messages Apr 09 09:26:40 oak-io5-s1 kernel: LustreError: 186541:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST00fc: no handlers for opcode 0x16 Apr 09 09:26:40 oak-io5-s1 kernel: LustreError: 186541:0:(tgt_handler.c:617:tgt_handler_find_check()) Skipped 1 previous similar message Apr 09 09:26:41 oak-io5-s2 kernel: LustreError: 272212:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST00fb: no handlers for opcode 0x16 Apr 09 09:26:42 oak-io4-s1 kernel: LustreError: 259539:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST0096: no handlers for opcode 0x16 Apr 09 09:26:42 oak-io2-s1 kernel: LustreError: 55897:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST0048: no handlers for opcode 0x16 Apr 09 09:26:42 oak-io2-s1 kernel: LustreError: 55897:0:(tgt_handler.c:617:tgt_handler_find_check()) Skipped 1 previous similar message Apr 09 09:26:43 oak-io3-s2 kernel: LustreError: 229599:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST0071: no handlers for opcode 0x16 Apr 09 09:26:43 oak-io6-s1 kernel: LustreError: 210680:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST0128: no handlers for opcode 0x16 Apr 09 09:26:43 oak-io4-s1 kernel: LustreError: 62400:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST00ac: no handlers for opcode 0x16 Apr 09 09:26:43 oak-io5-s1 kernel: LustreError: 55531:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST00d0: no handlers for opcode 0x16 Apr 09 09:26:43 oak-io5-s1 kernel: LustreError: 55531:0:(tgt_handler.c:617:tgt_handler_find_check()) Skipped 3 previous similar messages Apr 09 09:26:44 oak-io5-s2 kernel: LustreError: 272148:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST00f1: no handlers for opcode 0x16 Apr 09 09:26:44 oak-io5-s2 kernel: LustreError: 272148:0:(tgt_handler.c:617:tgt_handler_find_check()) Skipped 1 previous similar message Apr 09 09:26:44 oak-io3-s1 kernel: LustreError: 1095:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST0076: no handlers for opcode 0x16 Apr 09 09:26:45 oak-io6-s2 kernel: LustreError: 150751:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST012f: no handlers for opcode 0x16 Apr 09 09:26:45 oak-io6-s2 kernel: LustreError: 150751:0:(tgt_handler.c:617:tgt_handler_find_check()) Skipped 1 previous similar message Apr 09 09:26:45 oak-io3-s2 kernel: LustreError: 233125:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST008f: no handlers for opcode 0x16 Apr 09 09:26:45 oak-io6-s1 kernel: LustreError: 149884:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST0132: no handlers for opcode 0x16 Apr 09 09:26:45 oak-io6-s1 kernel: LustreError: 149884:0:(tgt_handler.c:617:tgt_handler_find_check()) Skipped 4 previous similar messages Apr 09 09:26:46 oak-io4-s2 kernel: LustreError: 51373:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST00ad: no handlers for opcode 0x16 Apr 09 09:26:47 oak-io4-s1 kernel: LustreError: 259545:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST00b0: no handlers for opcode 0x16 Apr 09 09:26:48 oak-io2-s1 kernel: LustreError: 65922:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST004c: no handlers for opcode 0x16 Apr 09 09:26:49 oak-io5-s2 kernel: LustreError: 272129:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST00e5: no handlers for opcode 0x16 Apr 09 09:26:49 oak-io5-s2 kernel: LustreError: 272129:0:(tgt_handler.c:617:tgt_handler_find_check()) Skipped 4 previous similar messages Apr 09 09:26:49 oak-io5-s1 kernel: LustreError: 53823:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST00fe: no handlers for opcode 0x16 Apr 09 09:26:49 oak-io5-s1 kernel: LustreError: 53823:0:(tgt_handler.c:617:tgt_handler_find_check()) Skipped 2 previous similar messages Apr 09 09:26:51 oak-io6-s2 kernel: LustreError: 183056:0:(tgt_handler.c:617:tgt_handler_find_check()) oak-OST012b: no handlers for opcode 0x16 Apr 09 09:26:51 oak-io6-s2 kernel: LustreError: 183056:0:(tgt_handler.c:617:tgt_handler_find_check()) Skipped 3 previous similar messages