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Storage Vendors do provide Storage based mirroring (Hitachi ShadowImage, IBM TrueCopy, EMC SDRF etc). These have to be carefully setup to ensure that mirroring is correctly time-ordered. If a write to LUN A occurs before another write to LUN B, these two writes must be replicated in the same sequence.
Oracle controlfiles and archivelogs of redo logs may need to be handled separately in a DR situation.
OS based mirroring is more vulnerable to errors. It mirrors two LUNs using OS services.
Ask your client if the mirroring he has implemented has been certified by the OS and Storage vendor to work for Oracle databases.
Hemant K Chitale
i dont know exactly what strategy he is following for data.but know that he is following some OS level replication.If he is your client, you should understand from them on how they are actually doing this and your client is the appropriate party for this question to be asked.
There is different kind replication available which can be cheaper than using Oracle data guard. A lot of products are available which do data mirroring in real time by keeping your database (plus oracle software) mirrored on a separate storage. In case of failure of Primary, the mirrored image is available on the storage which can be accessed by other server by mounting this mirrored storage there.
This is just a simgle topology I explained, there are "n" number of posibilities available.