An active object in a RAID region is one that is actively used by the region and contains data or parity information. When creating a new RAID region, all the objects selected from the main available-objects panel will be active objects. Linear-RAID and RAID-0 regions only have active objects, and if any of those active objects fail, the region is unavailable.
On the other hand, the redundant RAID levels (1 and 4/5) can have spare objects in addition to their active objects. A spare is an object that is assigned to the region, but does not contain any live data or parity. Its primary purpose is to act as a "hot standby" in case one of the active objects fails.
In the event of a failure of one of the child objects, the MD kernel driver removes the failed object from the region. Because these RAID levels provide redundancy (either in the form of mirrored data or parity information), the whole region can continue providing normal access to the data. However, because one of the active objects is missing, the region is now "degraded."
If a region becomes degraded and a spare object has been assigned to that region, the kernel driver will automatically activate that spare object. This means the spare object is turned into an active object. However, this newly active object does not have any data or parity information, so the kernel driver must "sync" the data to this object. For RAID-1, this means copying all the data from one of the current active objects to this new active object. For RAID-4/5, this means using the data and parity information from the current active objects to fill in the missing data and parity on the new active object. While the sync process is taking place, the region remains in the degraded state. Only when the sync is complete does the region return to the full "clean" state.
You can follow the progress of the sync process by examining the /proc/mdstat file. You can also control the speed of the sync process using the files /proc/sys/dev/raid/speed_limit_min and /proc/sys/dev/raid/speed_limit_max. To speed up the process, echo a larger number into the speed_limit_min file.
As discussed above, a spare object can be assigned to a RAID-1 or RAID-4/5 region when the region is created. In addition, a spare object can also be added to an already existing RAID region. The effect of this operation is the same as if the object were assigned when the region was created.
If the RAID region is clean and operating normally, the kernel driver will add the new object as a regular spare, and it will act as a hot-standby for future failures. If the RAID region is currently degraded, the kernel driver will immediately activate the new spare object and begin syncing the data and parity information.
For both RAID-1 and RAID-4/5 regions, use the "addspare" plug-in function to add a new spare object to the region. The only argument is the name of the desired object, and only one spare object can be added at a time. For RAID-1 regions, the new spare object must be at least as big as the region, and for RAID-4/5 regions, the new spare object must be at least as big as the smallest active object.
Spare objects can be added while the RAID region is active and in use.
If a RAID-1 or RAID-4/5 region is clean and operating normally, and that region has a spare object, the spare object can be removed from the region if you need to use that object for another purpose.
For both RAID-1 and RAID-4/5 regions, use the "remspare" plug-in function to remove a spare object from the region. The only argument is the name of the desired object, and only one spare object can be removed at a time. After the spare is removed, that object will show up in the Available-Objects list in the EVMS user interfaces.
Spare objects can be removed while the RAID region is active and in use.
In RAID-1 regions, every active object has a full copy of the data for the region. This means it is easy to simply add a new active object, sync the data to this new object, and thus increase the "width" of the mirror. For instance, if you have a 2-way RAID-1 region, you can add a new active object, which will increase the region to a 3-way mirror, which increases the amount of redundancy offered by the region.
The first process of adding a new active object can be done in one of two ways. First, the "addactive" plug-in function adds any available object in EVMS to the region as a new active object. The new object must be at least as big as the size of the RAID-1 region. Second, if the RAID-1 region has a spare object, that object can be converted to an active member of the region using the "activatespare" plug-in function.