Many people asked me what the correct order is when using hdbnsutil commands in the context of system replication.
If you have no clue what I am talking about you might want to press ALT+F4 or have a look into following how-to guide:
How to Perform System Replication for SAP HANA
Let me try to make it as clear as crystal…while keeping it simple.
ENVIRONMENT
SITEA=first and preferred data center
SITEB=second data center
PRIM=HANA system which acts as primary in system replication mode
SEC=HANA system which acts as secondary in system replication mode
INITIAL SETUP TO ENABLE SYSTEM REPLICATION BETWEEN SITEA AND SITEB
SITEA> hdbnsutil -sr_enable –name=SITEA (HANA system started, SITEA=PRIM)
SITEB> hdbnsutil -sr_register –name=SITEB –… (HANA system stopped, SITEB=SEC)
SITEB> sapcontrol -nr <instance> -function StartSystem (system replication will start when SITEB is completely started)
DISASTER IN SITEA OCCURS
SITEB> hdbnsutil -sr_takeover (SITEB=PRIM)
now recovery of A is performed and when done next step
SITEA> hdbnsutil -sr_register –name=SITEA –… (HANA system stopped, SITEA=SEC)
SITEA> sapcontrol -nr <instance> -function StartSystem (system replication will start when SITEA is completely started, HANA will determine if a complete sync is necessary or a delta will do)
FAIL BACK
Now SITEA=SEC SITEB=PRIM and systems are in sync. You might want to fail back to the original setup SITEA=PRIM and SITEB=SEC for various reasons
SITEA> hdbnsutil -sr_takeover (HANA system started, SITEA=PRIM)
SITEB> sapcontrol -nr <instance> -function StopSystem
SITEB> hdbnsutil -sr_register –name=SITEB –… (HANA system stopped, SITEB=SEC)
SITEB> sapcontrol -nr <instance> -function StartSystem (system replication will start when SITEB is completely started)
yes, this is the only way to fail back, also if you ask me 10 more times 😉
SIDE STORY: DISASTER IN SITEB OCCURS WHILE SITEB=SEC
Not much to do, once SITEB is recovered and restarted system replication will re-sync
Happy XMAS and bye for now,
Frank
PS: Executing this commands in the wrong order might lead into a HANA system which does not start anymore. So be careful! And I know what I am talking about after searching for hours to find the root cause and then figuring out it was created by myself.
Happy holidays, Frank.
I enjoyed seeing this:
I create several problems myself as well 🙂
Thanks Frank!
Would appreciate if you can write a blog regards to hdbnsutil -sr_unregister as the 1945676 – Correct usage of hdbnsutil -sr_unregister
is very confusing.
eg; in scenario 1, it said we need to stop secondary system in order to run hdbnsutil -sr_unregister, however, the commnad need to run whilst database is online.
also, running hdbnsutil -sr_unregister is not clearing the replication parameter in global.ini, and the status is “red” in Hana Studio -> Overview tab.
Cheers,
Nicholas Chang
Moreover, we also found out that even we switch off preload_column_tables for both primary and secondary instance, secondary instance size are almost identical to primary after sync completed.
Off topic…but did you also set the parameter global_allocation_limit with the correct value as explained in the guide How to Perform System Replication for SAP HANA ?
HI Frank,
Thanks for the reply. Yes we did.
Cheers,
Nicholas
The way sr_unregister is described in the note is the correct way! If you have issues doing it this way please create an OSS…
just for clarification, hdbnsutil -sr_unregister is run when 2ndary instance is shutdown? What’s the return message get? I’ve tested according to the note, run hdbnsutil -sr_unregister when 2ndary instance is down, the error message return with “hdbnsutil -sr_unregister can’t run on an offline db”
Please open a support message or a thread in SAP HANA and In-Memory Business Data Management .
We should not abuse the comment section of a blog for troubleshooting.
/Frank
Hi Frank,
I would like to configure system replication for scaleout systems.
siteA/siteB each with 3 systems.
Question is “Is it acceptable to deploy HANA with single virtual hostname for all HANA hosts?”
I will keep only one virtual host entry in global.ini
example: siteA
host1=saphanadb(v.host)
host2=saphanadb(vhost)
host3=saphanadb(vhost)
Site B:
host1=saphanadb(vhost)
host2=saphanadb(vhost)
host3=saphanadb(vhost)
Client can access via virtual hostname.
Is the above configuration is acceptable or not?
Please share your thoughts?
Thanks for your help.
Regards,
Mangilal D
Thanks for highlighting the steps. I was looking for a way for installed netweaver eg erp ehp to connect to secondary site once failover happens. Any configuration needed to do that?
Hi Frank,
We did takeover from site B to site A (which is primary and HA with worker and standby node) But after takeover, on Site A, no standby node available now, possibly all configuration came from Site B overwrite it. Did you see such situation anywhere? Any inputs will be highly appreciated.
Amit
Hi,
Silly question: is it possible to do a failover test and without stopping the primary server (and maybe without performing a takeover)?
A bit like doing a System Copy based on System Replication, but to “reinstate” the secondary when the test has finished.
In the System Admin Guide it states that you need to do a takeover in order to use the secondary as a copy, but does that not leave “meta-data” on the primary server saying that it is now a secondary? Or do you need to then apply SAP Note 1945676 to unregister the secondary (but, it is now a primary if you did a takeover). So, basically I’m still a bit confused, altough I have not yet read all the documentation.
Frank