Online Expansion vs Offline Expansion
The Transparent RAID platform is extremely powerful in that almost all operations can be executed while the array is either online or offline. An array can be expanded while the array is live and being accessed by other applications.
Doing things while the array is online provides great convenience and eliminates down times.
Nonetheless, it is best to execute all RAID Expansion operations while the array is offline to minimize interferences and execute the process at the greater I/O speed possible. There is a performance penalty to executing the expansion operation while the array is online. You should do an online RAID Expansion only if you really cannot afford any down time.
Expanding an array (adding a new data drive to an existing array)
In this tutorial we will execute an Online RAID Expansion as to cover the most complex scenario.
1. Click on “Add Unit of Risk (UoR)” to add a new disk to the array.
2. When doing an Online Expansion, you are given a warning notice. Make sure to read and understand it.
Adding a disk containing existing data to an array while the array is online carries a bit of a risk. That risk arises in the event that another disk fails while the RAID Expansion is in progress and if the array was published with the “Read-Only On Failure” property set to false (which is the default). In such a case, you will may be able to successfully recover the entire failed disk. The “Read-Only On Failure” property, when true, makes it that a disk become read-only if it fails. By defaults, disks are readable/writable even on failure.
So, yes, all in all, the risk here is small (as many things will need to go wrong at once), but you should pay attention and understand it.
Adding a disk containing no data (choosing to erase the disk) is relatively safe as no parity data is changed. If an online expansion fails in this particular case, the RAID will still remain safe and properly protected. If a disk fails during an Offline Expansion, that disk will be recoverable.
Offline RAID Expansion operations are safer for both cases where a disk containing data or not containing data is being added.
For this tutorial, we are choosing to add DRU3 to the array, choosing to keep the existing data, and to log the task to a Log RAID.
3. After the RAID Expansion completes, the UI will update with the new configuration state.
If doing an Online Expansion, the array must be stopped and restarted for the newly added disk to become available for use.
As such, Online RAID Expansion minimizes down time (to just a few seconds as opposed to hours), but does not completely eliminate it.
It is a good idea to run the Verify+ operation on the array after completing the RAID Expansion.
Note that the Verify+ operation will fail when executed while the array is still online and if an Online Expansion was done but the array was not restarted yet as required above. This is because the newly added drive is not available yet for online use until the array is restarted post an Online RAID Expansion.
If that happens, restart the array and re-execute the verification task.
Also, if you chose to format and erase the new disk, you will need to initialize and create a volume on the new disk through your OS disk manager.
So, after the Expansion has completed:
1. Stop array (if it was an online expansion and to complete the expansion)
2. Disable both TCQ and SWO if enabled.
3. Start the array to bring it online (you will need to operate on the tRAID disk)
4. Using your OS disk manager, find the tRAID disk corresponding to the newly added disk and initialize it
5. Create a volume on the tRAID disk
6. Re-enable TCQ and SWO if desired
7. Run the Verify & Sync task on the array. Alternatively, you can run the Verify+.
If you chose to expanding while keeping the existing data:
1. Stop the array (if it was an online expansion and to complete the expansion)
2. Start the array to bring it back online.
3. Run the Verify & Sync task on the array. Alternatively, you can run the Verify+.
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