Redirect on write

Spectrum Accelerate uses the Redirect-on-Write (ROW) mechanism.

The following items are characteristics of using ROW when a write request is directed to the master volume:
  1. The data originally associated with the master volume remains in place.
  2. The new data is written to a different location on the disk.
  3. After the write request is completed and acknowledged, the original data is associated with the snapshot and the newly written data is associated with the master volume.

In contrast with the traditional copy-on-write method, with redirect-on-write the actual data activity involved in taking the snapshot is drastically reduced. Moreover, if the size of the data involved in the write request is equal to the system's slot size, there is no need to copy any data at all. If the write request is smaller than the system's slot size, there is still much less copying than with the standard approach of Copy-on-Write.

In the following example of the Redirect-on-Write process, The volume is displayed with its data and the pointer to this data.
Figure 1. The Redirect-on-Write process: the volume's data and pointer
The Redirect-on-Write process: the volume's data and pointer
When a snapshot is taken, a new header is written first.
Figure 2. The Redirect-on-Write process: when a snapshot is taken the header is written first
The Redirect-on-Write process: when a snapshot is taken the header is written first
The new data is written anywhere else on the disk, without the need to copy the existing data.
Figure 3. The Redirect-on-Write process: the new data is written
The Redirect-on-Write process: the new data is written
The snapshot points at the old data where the volume points at the new data (the data is regarded as new as it keep updating by I/Os).
Figure 4. The Redirect-on-Write process: The snapshot points at the old data where the volume points at the new data
The Redirect-on-Write process: The snapshot points at the old data where the volume points at the new data
The metadata established at the beginning of the snapshot mechanism is independent of the size of the volume to be copied. This approach allows the user to achieve the following important goals:
Continuous backup
As snapshots are taken, backup copies of volumes are produced at frequencies that resemble those of Continuous Data Protection (CDP). Instant restoration of volumes to virtually any point in time is easily achieved in case of logical data corruption at both the volume level and the file level.
Productivity
The snapshot mechanism offers an instant and simple method for creating short or long-term copies of a volume for data mining, testing, and external backups.