RAID 5 Availability
What is RAID5 availability?
RAID 5 availability is a capability to provide uninterrupted data access in case of a disk failure. Let's consider two situations:
- you use a single disk + backup and the disk fails;
- you use a RAID 5 and one of the member disks fails.
In the first case a backup stores data but it is impossible to get the data immediately. At first you need to restore a data copy from the backup.
With a RAID 5 you can access data continuously, albeit slow.
What is hotswap?
In case of a disk failure some systems (for example PCs which use a software RAID) require a shutdown to replace a failed disk.
Therefore, all the work stops, which may be undesirable.
Hotswap is an ability of system to provide a disk replacement without power-off and shutdown.
What is hot spare?
Hot spare is an empty disk connected to a RAID controller in advance to replace a failed disk.
When a failure of one of the disks is detected, a controller starts creating copy of lost data on a hot spare drive immediately.
What is shared hot spare?
If there are several RAID 5 arrays, then one hot spare disk may be used for all arrays.
Usually it is enough to have just one disk because it is unlikely that a hot spare disk will be needed for several arrays simultaneously.
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