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​This is a summary of common questions regarding ​​SnapView ​​Snapshot. We are going to provide 10 Q&A sets for your quick reference to better understand how does SnapView Snapshot work.​

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​Q1: What is SnapView?​

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​A1: SnapView is a licensed local storage data protection software installed on EMC CLARiiON and VNX storage system, which provides you with the capabilities of creating snapshot and clone for a particular source LUN for testing, backup, replication, data analysis, data recovery, etc..​

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​Q2: What is the basic operating principle of SnapView snapshot?​

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​A2: Snapshot is created to maintain the status of a piece of point-in-time (PIT) data for a particular LUN. To achieve this PIT goal, SnapView leverages Copy-On-First-Write (COFW) mechanism to preserve the data that is going to be changed at the first time to another persistent location from the source LUN, this location is called Reserved LUN. After the reservation, the old data can then be changed by the write I/O. Then, the snapshot points to the old data in the reserved LUN so that it is able to find it when you need to access the PIT data.​

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​Q3: What would happen when I stop a snapshot session?​

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​A3: Sessions are used to track data changing, which is based on COFW. Stopping session means to stop tracking, this would make the corresponding snapshot useless because all of the data in the reserved LUN for this snapshot will be cleaned up.​

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​Q4: What does “activate to a snapshot” mean? Why do we​​ need this​​ procedure?​

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​A4: We have talked about snapshot session which is used for data tracking. To be able to access the snapshot data, we must present a virtual LUN to the host. For this reason, we must let the snapshot know which PIT copy of data does it want to be presented to the host and this is what we called “Activating to a snapshot session.”​

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​Q5: Once the data of a snapshot was changed, is it possible to roll back​​ to that ​​PIT data?​

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​A5: Yes, we could roll back to the original PIT status by "Deactivating" the snapshot, this will clean up all the writes to the snapshot.​

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​Q6: How to apply the changes to the snapshot to its source LUN?​

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​A7: Perform a roll back operation with the snapshot being activated, this will apply the changes to the snapshot to its source LUN.​

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​Q7: Is there any impact after reserved LUN being completely consumed?​

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​A7: (1) New sessions cannot be started. (2) The session that exhausted the reserved LUN pool would be stopped. Which means the corresponding snapshot will be useless to the host. ​

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​Q8: What would happen if the roll back operation was disrupted due to SP reboot or LUN trespass?​

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​A8: Snapshot has a checkpoint capability, which means the roll back operation will continue from the last point once the operation was recovered. ​

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​Q9: Is the source LUN accessible during roll back?​

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​A9: Yes, please note that the source LUN here refers to the LUN PIT data of snapshot being reverted back to, rather than the original status of the source LUN. ​

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​Q10: Is there any official document for reference regarding SnapView snapshot?​

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​A10: Yes, please refer to < ​​EMC CLARiiON SnapView snapshots and Snap Sessions Knowledgebook– A Detailed Review​​ >. ​

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​Author: Steve Zhou​

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​iEMC APJ​

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