What are the differences among different write strategies for LUN creation?

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When creating a LUN, you can customize the write strategy to write through, write back with mirroring, write back without mirroring, mandatory write back with mirroring, or mandatory write back without mirroring.
The differences among these write strategies are as follows:
? Write through: Data is directly written to hard disks in the storage system; therefore, the hard disks are accessed during each write operation. This strategy features high reliability but is deficient in performance.
? Write back: Data is written to cache firstly, and during the I/O idle hours, data is written onto hard disks; therefore, the hard disks are not accessed during each write operation. This operation features high performance and contains risks in reliability.
? Write back with mirroring: Data is written to the local cache and the peer cache at the same time.
? Write back without mirroring: Data is directly written to the local cache.
? Mandatory write back with mirroring: Data is written to the local cache and the peer cache at the same time by force.
? Mandatory write back without mirroring: Data is directly written to the local cache by force.

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Feature of the LUN write through policy
1. After data is written to the controller, the controller needs to write the data to the disk and then feed back the acknowledgement information to the upper-layer host. 2. The reliability of data written through is increased and the performance is low.

What are the differences between a full LUN copy and an incremental LUN copy?
Both copy data from the source LUN to target LUNs. Their difference is: Full LUN copy: It copies all data from the source LUN to target LUNs each time. Incremental LUN copy: When it is started for the first time, it requires an initial synchronization during which all data is copied from the source LUN to target LUNs. For subsequent times, it copies only the changed data from the source LUN to target LUNs.

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