Below errors in alert.log after database goes down.
ORA-00603: ORACLE server session terminated by fatal error
ORA-27504: IPC error creating OSD context
ORA-27300: OS system dependent operation:sendmsg failed with status: 105
ORA-27301: OS failure message: No buffer space available
ORA-27302: failure occurred at: sskgxpsnd2
kgefec: fatal error 0
As you have upgraded ODA from 22.214.171.124.0 to 126.96.36.199.0, There is a major change in the OS as well as kernel version which is 2.6.39-400 to 4.1.12-61.
This issue faced mainly due to OS memory fragmentation. If you clear the OS cache or reboot the server – as fragmentation is gone – You are not going to see this issue.
This is nothing related to RDBMS. This is related to OS level Memory page allocation to processes.
Please refer to the below MOS doc to fix this issue,
Oracle Linux: ORA-27301:OS Failure Message: No Buffer Space Available ( Doc ID 2041723)