Beware of loopback MTU size with RAC on Oracle Linux 7

I recently had to troubleshoot a hung instance in a 2 node RAC system. 4 months earlier, the system was reinstalled in a rolling fashion due to the requirement of Linux Upgrade from Oracle Linux 5 to Oracle Linux 7. This was required because of lack of certification for a storage migration to an AllFlash Storage. The system has been stable when running with Oracle Linux 5 for several years. Around 4 months after the reinstallation, one node got hung with and traces showed these error messages:

Mon Feb 26 08:53:37 2018
skgxpvfynet: mtype: 61 process 15801 failed because of a resource problem in the OS. The OS has most likely run out of buffers (rval: 4)
Errors in file /u01/app/oracle/diag_p/diag/rdbms/prod/PROD2/trace/PROD2_ora_15801.trc (incident=480004):
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
Incident details in: /u01/app/oracle/diag_p/diag/rdbms/prod/PROD2/incident/incdir_480004/PROD2_ora_15801_i480004.trc

This was strange because OS /proc/meminfo was showing huge amounts of free memory for this physical host with 512GB of RAM.

[root@node17 ~]# cat /proc/meminfo 
zzz ***Mon Feb 26 08:53:09 CET 2018
MemTotal:       528028424 kB
MemFree:        14593828 kB
MemAvailable:   78305772 kB
Buffers:        28009752 kB
Cached:         46896496 kB
SwapCached:            0 kB
Active:         22627436 kB
Inactive:       66945168 kB
Active(anon):   14315300 kB
Inactive(anon):  2105748 kB
Active(file):    8312136 kB
Inactive(file): 64839420 kB
Unevictable:      363996 kB
Mlocked:          364020 kB
SwapTotal:      33554428 kB
SwapFree:       33554428 kB
Dirty:               404 kB
Writeback:             0 kB
AnonPages:      15768480 kB
Mapped:           709896 kB
Shmem:            945384 kB
Slab:            2462644 kB
SReclaimable:    2092232 kB
SUnreclaim:       370412 kB
KernelStack:       28336 kB
PageTables:       395568 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:    87853440 kB
Committed_AS:   22087384 kB
VmallocTotal:   34359738367 kB
VmallocUsed:     1106260 kB
VmallocChunk:   34085810172 kB
HardwareCorrupted:     0 kB
AnonHugePages:         0 kB
CmaTotal:          16384 kB
CmaFree:               0 kB
HugePages_Total:   204800
HugePages_Free:    20214
HugePages_Rsvd:     6266
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:    36827756 kB
DirectMap2M:    78444544 kB
DirectMap1G:    423624704 kB

Oracle Support then referred to this MOS Note:

ORA-27301: OS Failure Message: No Buffer Space Available / ORA-27302: failure occurred at: sskgxpsnd2 ( Doc ID 2322410.1 )

It turned out that on systems with a lot of physical memory and on Oracle Linux 7, the MTU size of loopback adapter lo0 has to be reduced from the default value of 64k to 16436 bytes to avoid memory fragmentation in RAC. The note also mentioned that the parameter vm.min_free_kbytes should be set to physmem * 0,004 * (here 2). This requirement was already considered during installation of the system, so the only thing missing was the MTU size.

I was very surprised that neither Cluster Verification Utility (CVU), nor orachk in most recent version did catch this problem at the point of installation. In my opinion, if default value of MTU size of loopback interface on Oracle Linux 7 has the potential to cause an outage, then this must be pre-checked by CVU at installation time or at least integrated into orachk. Unfortunately, this is not the case. It seems that in July we will know if the on-prem release 18.3.0 eventually will catch and enforce this configuration requirement during installation time.

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