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Does Oracle uses data guard and Rac must with ha/dr (vcs and vvr) ?

Created: 24 Apr 2011 • Updated: 25 Apr 2011 | 8 comments
Zahid.Haseeb's picture
This issue has been solved. See solution.

I want to use ha/dr for my application and its backend database which is oracle on a Linux platform (RHEL).My question is do i need Oracle Rac and Data Guard must if i already have vcs and vvr licenses included in my ha/dr ?

What my knowledge is that Oracle uses Oracle Rac as a vcs/cluster and Oracle data guard as vvr.

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mikebounds's picture

You are right in that Oracle uses Oracle Rac to make Oracle highly available (HA) (equiv of VCS) and Oracle uses Data Guard for replication to a DR site (equiv of VVR) and so, you have a few choices:

  1. Oracle RAC + Oracle Data Guard
  2. Oracle (non-RAC) + VCS + Oracle Data Guard
  3. Oracle (non-RAC) + VCS + VVR

Mixed into this is if you want to use SF - you can use SF or SFCFS with Oracle (non-RAC) and you can use SFRAC or Oracle filesystem with RAC.

Last year, SFCFS was the same price as SF, so many customer were using SFCFS + VCS + Oracle as a cheaper alternative to RAC.  However, I think VVR maybe more expensive than Oracle Data Guard (I think this is free with RAC), but if you already have VVR + VCS licence, then option 3 above makes sense.

On saying this, VVR will replicate a lot more data than Data Guard, because VVR replicates Oracle redo logs + archive logs + redo segments + Oracle Datafiles, where as Data Guard just replicates redo logs.  VVR has other advantages so if you have enought bandwidth then VVR is probably best option, but if you are short of bandwidth, then you should use Data Guard.

Mike

UK Symantec Consultant in VCS, GCO, SF, VVR, VxAT on Solaris, AIX, HP-ux, Linux & Windows

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Zahid.Haseeb's picture

Thanks Mike for your kind reply:

where as Data Guard just replicates redo logs.  

Data Guard just replicate redo logs then how the complete/consistent DB reached at DR site ?

VVR has other advantages

What are the key advantages are

 if you are short of bandwidth, then you should use Data Guard.

Offcourse when Data Guard will only replicate the redo log then definitely the replication data will be in minimum quantity.

Question: VVR perform replication on Block level.Means if any Block changes on primary site the Block will replicate on DR site. 

Does the Data Guard do the same. I mean that does the Data Guard also perform replication on Block level ?

 

I totally lost your below point. Kindly throw some more light on it:

Mixed into this is if you want to use SF - you can use SF or SFCFS with Oracle (non-RAC) and you can use SFRAC or Oracle filesystem with RAC.

Any comment will be appreciated. Mark as Solution if your query is resolved
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Zahid Haseeb

zahidhaseeb.wordpress.com

mikebounds's picture

If you use ODG, then you initially backup database and restore at DR site and then ODG just applies redo logs to DR database to keep up to date and there are various modes as to how and when updates are applied.

Advanages of VVR are:

  1. VVR can replicate files too, like Oracle binaries and config files and any other applications that may be running on the server, so that you only need one replication solution, not one for replicating Oracle data (ODG) and a different replication product for replicating files.
  2. Better solution for providing Firedrill which is the abilty to to test DR site while primary site is still running.  In VVR you take a Space-optimised snapshot (Copy-On-Write) and mount this snapshot to test database, whilst replication continues, whereas in ODG I believe you have to suspend replication while you test the database at the DR site (hence comprimising how up-to-date DR site is during the Firedrill test).
  3. If you loose connection to DR site, then when connection is restored I believe ODG switches to shipping completes log files, rather than individual transactions, so if you were to loose primary site during the "catch-up" and you had only shipped a partial redo log file, then this is of no use, whereas with VVR, the backlog is replicated from the SQL continously.
  4. VVR is more robust.  If you do operations the wrong way round with ODG, you have to resync from scratch and last time I worked with ODG a couple of years ago, we had to resync 2 or 3 times.  It is pretty hard to get into situation with VVR where you have to resync from scratch and for example, if you loose primary and so takeover at secondary, then to restore old primary with VVR is very easy, but the Oracle guy I was working with could not figure out how to do the same recovery when using ODG.

If you are using standard Oracle (not RAC), then you can use SF, or SFCFS - the advantages of SFCFS, is that you do not need to unmount, deport, import and mount to failover, just stop Oracle on one node and start Oracle on other node, so failover is quicker - almost as quick as RAC.

Mike

UK Symantec Consultant in VCS, GCO, SF, VVR, VxAT on Solaris, AIX, HP-ux, Linux & Windows

If this post has answered your question then please click on "Mark as solution" link below

Zahid.Haseeb's picture

Thanks mike for your kind input again and again. Your every post was too helpful and each post deserve vote which i have given you. I also need soome more queries please.

If you use ODG, then you initially backup database and restore at DR site and then ODG just applies redo logs to DR database to keep up to date

This means that if God forbidden our DR site completely lost or crashed then we have to ship again the backup of database and restore at DR site again then the Data Guard will start replicating the redo logs. Agreed ?

Better solution for providing Firedrill which is the abilty to to test DR site while primary site is still running.  In VVR you take a Space-optimised snapshot (Copy-On-Write) and mount this snapshot to test database, whilst replication continues, whereas in ODG I believe you have to suspend replication while you test the database at the DR site

You mean that VVR has a built-in snapshot tool or VVR has a capability that can take snapshot of Oracle which we can mount on any test Oracle Server ? and

As far as Oracle data guard is concern it does not have any tool/capability of taking snapshot ? agreed ?

If you loose connection to DR site, then when connection is restored I believe ODG switches to shipping completes log files, rather than individual transactions, so if you were to loose primary site during the "catch-up" and you had only shipped a partial redo log file, then this is of no use, whereas with VVR, the backlog is replicated from the SQL continously.

You mean that if the connection lost between Primary and DR meanwhile the redo log will be changed at the primary site and after few minutes/hours let suppose the connection will be UP between primary and DR site so the changes of redo log (while the time of connection were lost ) will not just replicate and the whole redo logs will be replicated from very scratch(means complete redo logs will replicate from beginning) via Data Guard. You mean that?

Any comment will be appreciated. Mark as Solution if your query is resolved
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Zahid Haseeb

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mikebounds's picture

See answers below:

 

This means that if God forbidden our DR site completely lost or crashed then we have to ship again the backup of database and restore at DR site again then the Data Guard will start replicating the redo logs. Agreed ?

Yes, if completely lost.  If it crashes and you take over at other site, the Oracle Guy I worked with a couple of years ago, could not figure out how and if you could resume replication, so did a restore from a backup.

You mean that VVR has a built-in snapshot tool or VVR has a capability that can take snapshot of Oracle which we can mount on any test Oracle Server ? and

As far as Oracle data guard is concern it does not have any tool/capability of taking snapshot ? agreed ?

The snapshot is built into Volume Manager, but requires an Enterprise license to use.  It is also fully integrated into VCS, so that there is a wizard that creates a SG that when you online, takes a snapshot, mounts it and starts the database on the snapshot.  My understanding for Oracle (but I am not an Oracle expert) is that you can take a checkpoint in Oracle, suspend replication and test against actual database, then after test, roll back to checkpoint and resume replication.  I don't know of any other method using solely Oracle tools.  This process is NOT automated in VCS as it is for VVR.

You mean that if the connection lost between Primary and DR meanwhile the redo log will be changed at the primary site and after few minutes/hours let suppose the connection will be UP between primary and DR site so the changes of redo log (while the time of connection were lost ) will not just replicate and the whole redo logs will be replicated from very scratch(means complete redo logs will replicate from beginning) via Data Guard. You mean that?

No.  The usual replication mode for ODG is to replicate transactions, as oppose to shipping complete log files so that the database is up-to-date and not a log file behind.  If you loose connection, then you cannot resume using replication of transactions, so you have to ship redo log files to catch-up and then go back to transaction replication, once up to date.  I also believe Oracle switches to log shipping mode if the database gets busy and the bandwidth is not sufficient or the secondary cannot apply transactions quick enough as Oracle has no where to store transactions, whereas VVR uses the SRL.

Mike 

UK Symantec Consultant in VCS, GCO, SF, VVR, VxAT on Solaris, AIX, HP-ux, Linux & Windows

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Zahid.Haseeb's picture

Update:

What is the good thing i concluded is that the primary work of any Replication software is to transfer the data from primary site to DR/Secondary Site. If this feature is not good enough and other features are good enough then the Replication Software is useless.

 

What i have noticed is that the VVR need around 10mb bandwidth for replication. And if we see the Oracle Replication is may need only 1mb to replicate the data.

Any comment will be appreciated. Mark as Solution if your query is resolved
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Zahid Haseeb

zahidhaseeb.wordpress.com

Zahid.Haseeb's picture

Mikebounds:

 

  1. Oracle (non-RAC) + VCS + Oracle Data Guard
  2. Oracle (non-RAC) + VCS + VVR

 

Does Oracle can run Active/Active in the above scenerio ?

Any comment will be appreciated. Mark as Solution if your query is resolved
__________________
Thanks in Advance
Zahid Haseeb

zahidhaseeb.wordpress.com

mikebounds's picture

With Oracle (non-RAC), an Oracle database can only run on one node in the cluster, but you can of course have 2 different databases, one on each node.  Only RAC lets you run the same databases from more than one node. 

Mike

UK Symantec Consultant in VCS, GCO, SF, VVR, VxAT on Solaris, AIX, HP-ux, Linux & Windows

If this post has answered your question then please click on "Mark as solution" link below