APC RAID Subsystem SCSI-SATA II Informações Técnicas descarregar pdf (Página 142)

Advanced Functions

5-24

5.4.3 How to Use Snapshots

• Overview

To make the most use of the snapshot function, proper planning and configuration is of essence. Below is a list of related

tasks grouped into three phases:

Phase 1: Planning

1. Identify the source volumes that need to have snapshots.

2. Define how many snapshots will be taken and how long they will exist.

3. Allocate the disk space and RAID attributes of the secondary volumes.

4. Allocate the disk space and RAID attributes of the spare COW volume.

Phase 2: Configuration

5. Create volumes of the secondary volumes.

6. Select a secondary volume for each source volume (primary volume).

7. Set up snapshot options, like overflow alert.

8. Build snapshot automation scripts and conduct trail runs (optionally).

Phase 3: Creating and Utilizing Snapshots (Manually or Scripted)

When taking a snapshot for a source volume, the tasks below are required:

9. Stop all the write access to the LUN of the primary volume.

10. Flush the cached data on the memory of your host systems.

11. Create the snapshot volume.

12. Resume the write access to the LUN of the primary volume.

13. Export the snapshot volume to a LUN for your applications (optionally)

14. Copy the data from the snapshot volume to other places (optionally)

15. Delete the snapshot volumes (optionally) to avoid running out of space on the secondary volumes.

The tasks in the phase 1 and 2 are done only once when you set up a RAID system or when you create a new LUN. They

could also be parts of your RAID system reconfigurations. The tasks in phase 3 are very likely to be repeated periodically

when a snapshot is needed.

• Planning for the Secondary Volumes

When planning your storage resources, you have to reserve sufficient free capacity for the secondary volumes. In

addition to the space reserved for the RAID controller to build lookup tables, the capacity reserved for the secondary

volumes depends on how much data could be modified on the primary and snapshot volumes during the life time of the

snapshot volumes. If you keep the snapshot volumes longer, it is more likely that more data will be modified. A

commonly-used capacity of a secondary volume is 20% of the source volume.

However, not all write commands would consume space of the secondary volume. For single block on the primary and

snapshot volume, the copy operation and space allocation on the secondary volume is performed only at the very first

time when a write command hits the block. As a result, if write commands tend to hit the same blocks, you may consider

using a smaller secondary volume.

Another consideration in estimate reserved capacity is that because the COW operations are done by chunks, consisting

of multiple consecutive sectors, more space is required than the actual data being modified.

If the space of a secondary volume is fully occupied, the data on the snapshot volumes will be corrupted. Be aware of

applications that would change huge amount of data on a source volume, like video recording and file system

defragmentation. If all the data will be changed, ensure that the secondary volume’s capacity is set to 105% of the size of

the source volume.

You will also properly set the RAID attributes of the secondary volume depending on your performance and reliability

requirements for the snapshot volumes. It is advised that the secondary volumes are located on the different disk group

of the primary volumes, such that the COW operation and the I/O access to the snapshot volume can be done more

efficiently.

Note

1. Set spare COW volumes to avoid data loss in the snapshot volumes when the space of the

secondary volume is overflow.

2. Expand secondary volume or spare COW volume to accommodate more differential data

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APC RAID Subsystem SCSI-SATA II Informações Técnicas Página 142

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