Storage Redundancy Components

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There are many other components that help keep the storage system secure and reliable. Some of these include:

Hot Spares

A hot spare drive is a drive that contains no data and acts as a standby in case a drive fails. The hot spare drive adds another level of redundancy in the RAID Module. If a drive of similar or a smaller capacity fails, the hot spare automatically takes over on behalf of the failed drive until the failed drive can be replaced with another drive. Once the failed drive is replaced, the hot spare automatically returns to a spare-standby status. Depending on how many hot spares are configured, a LUN (logical unit) could remain optimal and still have several failed drives with each one being covered by a hot spare. This method helps to keep the RAID 1, 3, or 5 LUNs functional.

Hot Plug

A hot-plug component means that it is electrically safe to remove or add the component while the machine is still running. Typically, the system must be rebooted before the hot-plug component is configured into the system.

Hot Swapping

A hot-swap component can be installed or removed by simply pulling the component out and putting in the new one. The system will either automatically recognize the component change and reconfigure itself as necessary or will require user interaction to configure the system. In both cases a reboot is not required. All hot-swappable components are also hot pluggable, although not all hot-pluggable components are hot-swappable. The device being removed or reinstalled is the only device that is not operational during this process.

Multiple RAID Controllers and Storage Processors

RAID Controllers are the units in a storage array that control the RAID operations. They present one or more virtual devices or LUNS to the host system and distribute the I/O operations to the member disks. With the help of a RAID utility, the physical disks are grouped and formed into Logical Units (LUNS). The LUN is the basic storage unit presented to the host system for its use.

It is also responsible for maintaining the mapping between the virtual device and individual disks in the array. The existence of a RAID controller makes a JBOD array into a RAID Storage Array. RAID Controllers are also used in parallel access arrays for improving the I/O performance. Any failure in the RAID controller cripples the total functionality of the storage array.

Many of the storage arrays offer intelligent storage processors, which control the RAID activity on the array. The storage processor is the RAID controller for the storage array. In many arrays, there is a local cache to handle the data movement along with storage processors. Dual caches or battery-backed caches help with protection and provide additional redundancy.

Power Management

One of the most important and unanticipated threats is the loss of power to the entire storage unit. After providing many levels of redundancy such as multiple paths, multiple controllers, and adequate RAID levels for disk drives, if adequate power supply redundancy is neglected, the whole storage unit is still at risk of losing its functionality.

Therefore, it must be equipped with redundant power supplies. Each power supply needs to have the required connections and wattage to supply power to the entire storage system. Where possible, use different power circuits for different power supplies. This way the risk of losing storage unit functionality in the entire cluster would be minimized.

High Availability with SAN

SAN-based storage systems focus on high availability and high performance. Storage switches are the main building blocks for the implementation of the SAN infrastructure. SAN incorporates many redundant hardware components to avoid disruption to I/O activity. Even though the storage system is part of the SAN infrastructure, it can still have multiple connections to the host system, thus providing extra redundancy. These storage-to-host connections can be active or on standby with the capability of automatic failover to an alternate path.

Mirroring is another technique used in achieving high availability. SAN enables the efficient method of mirroring the data on a peer-to-peer basis across the fabric. Mirroring is commonly used to deploy remote sites within a SAN environment.

Use of FC switches provides an extremely reliable infrastructure. Many of the switches have hot-pluggable components, such as optic modules, which can do single port replacements, dual CPUs, and redundant power supplies. The dual fabric nature of the SAN provides the application with an alternate path to the data and also contributes to high availability. The dual fabric nature also eliminates the failure point in the data path.