RAC with Continuos Access

[Miguel Palacios]

We just want a confirmation about a particular Oracle RAC installation

1) We are planning to install Oracle 10g R2 RAC with 2 nodes on HP-UX Itanium, they will be connected to a storage (HP EVA XP).
2) We need a standby site, but this site will no te be a RAC installation, we just need a single instance installation.

We are discussing the ways to replicate the RAC database:
First Choice: Use Data Guard with a primary RAC database and a standby No-RAC single instance database.

Second Choice: Use HP Continuos Access to replicate primary SAN box with standby SAN box.

Questions:

1) Is there any certified procedure to convert from RAC database to single instance? I think we will need this if we decide to implement the second choice because we are replicating a RAC database and on the standby site there will not be RAC software. Maybe the oracle database will need to be converted from RAC to single instance.

2) How to mix Data Guard functionalities with SAN replication features (like HP continuos access?

[Duncan Edmonstone]

Miguel,

1) Not that I'm aware of- I expect you would simply configure the standby site to simply behave as one of the RAC instances (i.e. still RAC, but effectively with an instance down).

2) I wouldn't try and mix these two types of functionality in the database *at all*, as there is no connection between the two. You might use Dataguard for the database, and then use CA for the some non-database flat files that also need to be replicated - that might make sense, but otherwise both products should be used on their own or not at all (i.e. all Dataguard or all CA - not a combination of the two).

HTH

Duncan

[Ben Dehner]

More focusing on your second question, Continuous Access (CA) vs Data Guard (DG), I think Duncan had a good point -- you should not try to mix them. Each has different advanatages, you will want to consider carefully. I am not familiar with CA in particular, but in general:

DG, as a product, is essentially free. It is bundled in with the Oracle licensing. CA is presumably an additional product that you will need to purchase. On the other hand, you need to have a host covered by an Oracle license at the standby site. For CA, you don't need to have a host running a database at all at the standby site until you do a fail-over. If you plan on using the standby for reports or backups or somesuch, then it really doesn't matter.

CA will get all of your data, including flat files, binaries, etc. DG will get only the specifcally configured database data. Whether this is good or bad depends on the environment. If you have lots of test databases you don't want to replicate, then DG is better. If you have multiple production databases you do want to replicate, then CA is better because you don't have to have the invididual DG configuration per database.

DG allows for a reversible transition from primary to standby; a "switchover", in Oracle terms. I'm not sure if CA, or any array-based replication, can do that. Usually you need to fail back by manually re-copying the data back to the original site. Most array features allow for an updateable snapshot or clone at the target site, but not a truely reversible transition.

DG is storage agnostic. As long as you have the disk available to the standby host, it doesn't care who's disk it is. CA is presumably vendor specific -- you MUST have another EVA at the standby site. Again, I am unfamiliar with CA, but most disk replication software only works with "like" disk. Most cross-vendor SAN utilities are designed for migration, not high availability. If you are buying new equipment, this may not matter, but if you are using existing devices, or working with an outsourcer for the standby site, you may not have full control over the hardware platforms, and this might be important.

Both DG and CA will require you to run HP-UX hosts on the primary and standby side. If all the systems are Itanium, then it's a wash. If you're mixing Itanium and PA-RISC, then you can't use CA to replicate the Oracle binaries, because the Oracle installations aren't binary compatible. This probably isn't of much concern.

Most array-based replication requires some additional disk overhead for using replication features. You will need additiona disk in both systems beyond the data requirements.

Oracle 10G data guard might have additional features that you'll want to investigate. The array-based replication might also have some good features. It is not an easy question to answer, but here's a few thoughts.

[skt_skt]

Dont mix data guard and CA for replication.Best idea is use DG for DB and CA for FS replication.

CA is additional product and need to be purchased in terms of TB capacity.

DG is better even though you need to have multiple DG need to be setup for each database. Also even if you would like to go with CA , i would say keep different CA pair for each database.That give the flexibility to work/effect only one DB/CA pair at time.No other group will be effected.

CA works with similar capacity disks. For CA we need a stable link between the sight.Becase the link break often leaves the CA pair in error state and you may need to sync it a fresh.

Also always keep the same/similar h/w, SAN, OS at both side and that would make the recovery faster in case of a real DR.

CA does not require any additional disk over head. But it makes load on the network during replication.