Special topic: Log RAID
A very special topic in Transparent RAID is the “RAID Migration Log” configuration RAID type (or Log RAID).
The kernel Transparent RAID engine has no concept of files and only deals with storage devices.
A Log RAID is a RAID configuration that can use either disk image files or raw disk slices and presents itself as a storage device for the purpose of task logging.
Another benefit of using a RAID for logging is that the RAID behaves as a mirror if it contains more than one DRU (please ensure that each DRU is on a separate disk).
A Log RAID is nothing more than a log file abstracted such that it presents itself as a disk. The tRAID kernel engine only understands disks. It does not deal with files.
When configuring a Log RAID, you have the option of adding multiple DRUs (each DRU being a file image). If using more than one DRU in the Log RAID, you are setting up a mirrored log. Each disk image file will contain the same data, and you should ensure that each image file is on a separate disk or else it would be pointless.
Internally, tRAID presents Log RAIDs as N-way mirror disks to the kernel for it to write its logs to. You can have one or more DRU in that N-way mirror.
Log RAIDs need only be a few MBs in size. The rule of thumb is that you should make them the size of 1 MB * the number of disks in the array or larger. For instance, if you have 10 disks in your array, you would make the Log RAID 10MB large or larger.
There are a number of tasks for which logging the task executions enables the ability to resume such tasks in the case where they were aborted. All of these tasks make logging optional as logging can slow down the task.
Nonetheless, for greater safety, it is a good idea to log those operations when the option is available.
In this tutorial, we will create a Log RAID which is backed up by a single file acting as disk image file.
1. We first need to register a volume that will contain our disk image file.
The volume to use must be a volume on a disk that will not be used for Transparent RAID.
A good default is to use the C volume, but you can use any other volume on any other drive for as long as that drive will not be used for Transparent RAID.
2. Now that we have our C volume registered, we will create a disk image file on it by right-clicking it and selecting to “Create Disk Image File”.
3. Here, we enter the display name, name, and size of the file to create.
4. After the disk image file has been created, right-click on the “RAID Configurations” node and select to create a new RAID.
5. Select the RAID type to be “RAID Migration Log” and enter a display name.
6. The created RAID Log will show under the “RAID Configurations” node.
7. The screenshot below shows the 10 MB file created on the C drive.
8. Back to the Log RAID configuration, open it and select to add a UoR and add the created disk image file as DRU.
9. There is no initialization task for a Log RAID. As such, the typical control buttons at the bottom of the grid will not show like they show for a Transparent RAID configuration. The configuration is now complete.
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