Hardware-Based Mirroring

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 Mirroring

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 Mirroring

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.