If a disk is temporarily disconnected from its RAID group and then reconnected, the RAID group must synchronize all of its data. This process may take a long time. If the RAID group has a bitmap then only changes that were made after the disk was disconnected need to be synchronized, greatly speeding up the process.
A disk can become temporarily disconnected in the following situations.
A disk is accidentally removed from the NAS while the NAS is powered on.
The NAS unexpectedly shuts down because of a hardware or software error.
A user presses the power button for 10 seconds or disconnects the power cable while the NAS is powered on.
You can only create bitmaps for RAID 1, RAID 5, RAID 6, and RAID 10 groups.
Enabling a RAID bitmap may slightly decrease the read and write performance of the RAID group.
A bitmap improves synchronization time only if the same disk is disconnected then reconnected. Having a bitmap does not improve synchronization time when a new disk is added to the RAID group.