RAID (Redundant Array of Independent Disks) is a technology that uses multiple physical disks as a single logical disk. Its main purpose is to improve data reliability and disk access speed. It has become an almost essential component in modern server systems.
Basic Principles of RAID
RAIDs are broadly classified into hardware RAID and software RAID. Hardware RAID is implemented using a dedicated controller. The operating system recognizes the RAID configuration as a single disk, and the RAID controller handles all operations. It offers high reliability and performance but comes at a higher cost.
Software RAID is implemented directly by the operating system. Linux supports it through the md (multiple devices) driver. It can be configured without the need for separate hardware, but it has the disadvantage of consuming CPU resources.
Features of RAID Levels
RAID 0 (Striping)
- Stores data across multiple disks in a striped manner
- Improves performance linearly with the number of disks
- Capacity = disk size × number of disks
- Single disk failure results in complete data loss
RAID 1 (Mirroring)
- Duplicates the same data on multiple disks
- Improves read performance, write performance remains the same
- Capacity = single disk size
- Data remains safe even if one disk fails
RAID 5 (Parity)
- Stores data and parity information in a distributed manner
- Requires a minimum of 3 disks
- Capacity = disk size × (number of disks - 1)
- Can recover from single disk failures
- Write performance is somewhat degraded
RAID 6 (Double Parity)
- Stores two copies of parity information
- Requires a minimum of 4 disks
- Capacity = disk size × (number of disks - 2)
- Can recover from dual disk failures
- Write performance is further degraded compared to RAID 5
RAID 10 (RAID 1+0)
- A combination of RAID 1 and RAID 0
- Offers both high performance and reliability
- Requires a minimum of 4 disks
- Capacity = total disk capacity ÷ 2
- Can recover from multiple disk failures under certain conditions
Considerations for Choosing RAID
The importance of data is the most critical criterion for choosing a RAID level. For mission-critical databases, it is recommended to use RAID 10 or RAID 6, which offer high reliability and performance. On the other hand, for data that can be recovered, such as temporary data or cache, RAID 0, which focuses on performance, is sufficient.
The primary usage of the system is also an important consideration. Database servers require both fast reads and writes, making RAID 10 the most suitable choice. Web servers, on the other hand, are mostly read-intensive, so cost-efficient RAID 5 or RAID 6 can be good alternatives.
Cost should also be carefully considered. RAID 1 or RAID 10 provide excellent performance and reliability, but they utilize half of the disk capacity for mirroring. RAID 5 or RAID 6 use parity to utilize capacity more efficiently, but they have the disadvantage of relatively lower write performance.
Conclusion
RAID is not a replacement for backups. RAID is a technology to guard against disk hardware failures; it does not protect against issues like data deletion or corruption. Therefore, a regular backup policy is still necessary.
RAID rebuild time is also an important consideration. RAID 5 or RAID 6 configured with large-capacity disks can take several days to rebuild. During the rebuild period, not only will system performance be degraded, but the risk of another disk failure is also increased, requiring special attention.
To mitigate these risks, it is recommended to prepare a hot spare (spare disk). In case of disk failure, the hot spare is automatically substituted and rebuild starts immediately, minimizing system downtime.
RAID is an indispensable element in modern server systems. Choosing and configuring the appropriate RAID level can significantly improve the stability and performance of the system. However, it is important to always keep in mind that RAID alone does not provide complete data protection, and a comprehensive data protection strategy must be established.