Most external storage devices provide support for different RAID (Redundant Array of Independent Disks) levels. The most commonly used high availability hardware RAID levels in VLDB environments are RAID 1 and RAID 5. Though less commonly used in VLDB environments, other high availability RAID levels can also be used.
This section contains the following topics:
RAID 1 is a basic mirroring technique. Every storage block that has been written to storage is stored twice on different physical devices as defined by the RAID setup. RAID 1 provides fault tolerance because if one device fails, then there is another, mirrored, device that can respond to the request for data. The two write operations in a RAID 1 setup are generated at the storage level. RAID 1 requires at least two physical disks to be effective.
Storage devices generally provide capabilities to read either the primary or the mirror in case a request comes in, which may result in better performance compared to other RAID configurations designed for high availability. RAID 1 is the simplest hardware high availability implementation but requires double the amount of storage needed to store the data. RAID 1 is often combined with RAID 0 (striping) in RAID 0+1 configurations. In the simplest RAID 0+1 configuration, individual stripes are mirrored across two physical devices.
RAID 5 requires at least 3 storage devices, but commonly 4 to 6 devices are used in a RAID 5 group. When using RAID 5, for every data block written to a device, parity is calculated and stored on a different device. On read operations, the parity is checked. The parity calculation takes place in the storage layer. RAID 5 provides high availability for a device failure because the device's contents can be rebuilt based on the parities stored on other devices.
RAID 5 provides good read performance. Write performance may be slowed down by the parity calculation in the storage layer. RAID 5 does not require double the amount of storage but rather a smaller percentage depending on the number of devices in the RAID 5 group. RAID 5 is relatively complex and consequently, not all storage devices support a RAID 5 setup.