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About _disable_txn_alert undocumented parameter - at least as for Oracle 11g win32.

This parameter is connected with ktsmgd_ global variable.

This variable is actually bitmap.
Default value is 0.
Bits which are checked within Oracle 11g processes: 1, 2, 4, 8, 0x10, 0x20, 0x40, 0x80, 0x100, 0x200.

0x1: if it is set, ktuilqa() function will not execute.
0x2: if it is set, ktrsiosa() function will not execute.
0x4: if set: write some debug info to trace file using ksdwrf() function.
0x8: if set: write some debug info to trace file using ksdwrf() function.
0x10: if set, ktsmguuf() function will not execute.
0x20: if set: write some debug info to trace file using ksdwrf() function.
0x40: if set: write some debug info to trace file using ksdwrf() function.
0x80: affect code flow in ktsmgfru() function.
0x100: affect code flow in ktsmgru(), ktsmgruarr(), ktsmghlth(), ktsmgadv() functions.
0x200: affect code flow in ktsmgmql(), ktsmgaex(), ktsmgmsz(), ktsmgbpr(), ktsmgru(), ktsmgruarr(), ktsmghlth(), ktsmgadv() functions.

Thus, turn on trace flags will be: 0x4 + 0x8 + 0x20 + 0x40 = 0x6C or 108 decimal.
Or turn on all flags: 0x1 + 0x2 + 0x4 + 0x8 + 0x10 + 0x20 + 0x40 + 0x80 + 0x100 + 0x200 = 0x3FF or 1023 decimal.

Note: ksdwrf() function write trace information to files usually named <SID>_<process_name>_<thread_id>.trc, at the folder which is usually defined by background_dump_dest parameter.

Update: this command is also can be used for viewing state of ktsmgd_ variable: oradebug dumpvar SGA ktsmgd_

Update: Article where Yong Huang make use of this parameter.