红帽全球峰会16.2. RAID types
RAID can be implemented through firmware, dedicated hardware controllers, or software. Each type offers different benefits in terms of cost, performance, and features, allowing you to choose the best approach for your storage requirements.
The following are the possible types of RAID:
- Firmware RAID
- Firmware RAID, also known as ATARAID, is a type of software RAID where the RAID sets can be configured using a firmware-based menu. The firmware used by this type of RAID also hooks into the BIOS, allowing you to boot from its RAID sets. Different vendors use different on-disk metadata formats to mark the RAID set members. The Intel Matrix RAID is an example of a firmware RAID system.
- Hardware RAID
A hardware-based array manages the RAID subsystem independently from the host. It might present multiple devices per RAID array to the host.
Hardware RAID devices might be internal or external to the system. Internal devices commonly consist of a specialized controller card that handles the RAID tasks transparently to the operating system. External devices commonly connect to the system via SCSI, Fibre Channel, iSCSI, InfiniBand, or other high speed network interconnect and present volumes such as logical units to the system.
RAID controller cards function like a SCSI controller to the operating system and handle all the actual drive communications. You can plug the drives into the RAID controller similar to a normal SCSI controller and then add them to the RAID controller’s configuration. The operating system will not be able to tell the difference.
- Software RAID
A software RAID implements the various RAID levels in the kernel block device code. It offers the cheapest possible solution because expensive disk controller cards or hot-swap chassis are not required. With hot-swap chassis, you can remove a hard drive without powering off your system. Software RAID also works with any block storage, which are supported by the Linux kernel, such as SATA, SCSI, and NVMe. With today’s faster CPUs, Software RAID also generally outperforms hardware RAID, unless you use high-end storage devices.
Since the Linux kernel contains a multiple device (MD) driver, the RAID solution becomes completely hardware independent. The performance of a software-based array depends on the server CPU performance and load.
The following are the key features of the Linux software RAID stack:
- Multithreaded design
- Portability of arrays between Linux machines without reconstruction
- Backgrounded array reconstruction using idle system resources
- Hot-swap drive support
- Automatic CPU detection to take advantage of certain CPU features such as streaming Single Instruction Multiple Data (SIMD) support.
- Automatic correction of bad sectors on disks in an array.
- Regular consistency checks of RAID data to ensure the health of the array.
- Proactive monitoring of arrays with email alerts sent to a designated email address on important events.
Write-intent bitmaps, which drastically increase the speed of resync events by allowing the kernel to know precisely which portions of a disk need to be resynced instead of having to resync the entire array after a system crash.
注意The resync is a process to synchronize the data over the devices in the existing RAID to achieve redundancy.
- Resync checkpointing so that if you reboot your computer during a resync, at startup the resync resumes where it left off and not starts all over again.
- The ability to change parameters of the array after installation, which is called reshaping. For example, you can grow a 4-disk RAID5 array to a 5-disk RAID5 array when you have a new device to add. This grow operation is done live and does not require you to reinstall on the new array.
- Reshaping supports changing the number of devices, the RAID algorithm or size of the RAID array type, such as RAID4, RAID5, RAID6, or RAID10.
- Takeover supports RAID level conversion, such as RAID0 to RAID6.
- Cluster MD, which is a storage solution for a cluster, provides the redundancy of RAID1 mirroring to the cluster. Currently, only RAID1 is supported.
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