RMAN backup on NFS v3 takes too much time

Problem:

I was trying to test RMAN backup on Azure blob storage with NFS v3 access. I have noticed a huge delay even when backing up a small controlfile.

The same backup on disk took 7sec, but on NFS 15min. While watching IO on NFS mountpoint using nfsiostat I saw that initial 13min there was no IO at all and actual backup was taken only at the last minute.

Another interesting thing while watching netstat output was the following:

[root@rac1 dbbackup2]# netstat -na|grep 10.0.0.
tcp 0 0 10.0.0.5:875 10.0.0.16:2048 ESTABLISHED
tcp 0 1 10.0.0.5:54997 10.0.0.16:2049 SYN_SENT

You can also turn debug on before running backup in RMAN and check the output:

RMAN> RUN
{
  ALLOCATE CHANNEL disk1 DEVICE TYPE DISK FORMAT '/dataload/%U';
  debug on;
  BACKUP current controlfile;
  debug off;
}

Solution:

Disable dNFS.

$ cd $ORACLE_HOME/rdbms/lib
$ make -f ins_rdbms.mk dnfs_off

Retry RMAN backup, in my case it took only 1sec:

...
piece handle=/dbbackup/0q0rruhd_26_1_1 tag=TAG20220425T202309 comment=NONE
channel disk1: backup set complete, elapsed time: 00:00:01
Finished backup at 25-APR-22