Mastering RAID Fault Tolerance: Consistency, Hot Spare, Rebuild & More

1 Consistency Check

For RAID levels with redundancy (RAID 1,5,6,10,50,60), the controller can perform consistency checks, comparing data on disks with their redundant copies and automatically repairing mismatches while logging errors. RAID 0 lacks redundancy and therefore does not support consistency checks.

2 Hot Spare

The hot‑spare feature is provided by hot‑spare disks and emergency backup.

Hot Spare Disk

A hot‑spare is an idle disk that automatically replaces a failed member disk in a RAID group and rebuilds the data onto it. The spare must have equal or greater capacity and the same media type and interface as the member disks.

Two types of hot spares are supported:

Global hot spare – shared by all configured RAID groups on the controller; multiple global spares can be defined.

Local hot spare – dedicated to a specific RAID group; each group can have one or more local spares.

Hot spares are only used with RAID groups that have redundancy (RAID 1,5,6,10,50,60) and replace only disks on the same controller.

Emergency Backup

If a RAID group with redundancy experiences a disk failure and no hot spare is assigned, an idle disk on the controller will automatically take over the failed member and rebuild the data, preventing data loss. The replacement disk must have capacity and media type equal to the member disks.

3 RAID Reconstruction

When a disk fails, the controller can reconstruct the data onto a new disk. Reconstruction is available only for RAID levels with redundancy (RAID 1,5,6,10,50,60). If a global or local hot spare is configured, it is used automatically; otherwise reconstruction starts after a new disk is inserted. The reconstruction rate (CPU usage) can be set from 0 % to 100 %.

4 Virtual Disk Read/Write Policies

Read Policy

The controller supports two read strategies:

Read‑ahead (e.g., “Always Read Ahead”, “Read Ahead”, “Ahead”): data is prefetched into cache, reducing seek time and improving read speed. This requires power‑loss protection; a faulty super‑capacitor may cause data loss.

Non‑read‑ahead: data is read only when a read command is received.

Write Policy

Write‑Back: data is first written to cache and flushed to the virtual disk later, improving write performance. Requires power‑loss protection.

Write‑Through (direct write): data is written directly to the virtual disk without caching; works without power‑loss protection but offers lower write speed.

Write‑Back with BBU: when a Battery Backup Unit is present and healthy, writes use cache; otherwise the controller switches to write‑through automatically.

Write‑Back Enforce: forces write‑back even if the controller lacks a capacitor; not recommended because data may be lost on unexpected power loss.

5 Data Power‑Loss Protection

Enabling the controller’s cache boosts write performance, but data in cache is lost if the system loses power. A super‑capacitor module can protect the cache by powering it long enough to write cached data to NAND flash during a power outage.

Super‑Capacitor Calibration

The controller automatically calibrates the super‑capacitor through a three‑stage charge‑discharge cycle to maintain a stable voltage range. During calibration the write policy switches to “Write‑Through” to ensure data integrity, which may reduce performance.

6 Disk Striping

Striping distributes I/O load across multiple physical disks, improving parallelism and throughput. Data is divided into stripes and written across disks, allowing concurrent access to different parts of a file. Key concepts include stripe width (number of disks), RAID‑group stripe size, and individual disk stripe size.

7 Disk Mirroring

Mirroring, used in RAID 1 and RAID 10, writes identical data to two disks, providing 100 % redundancy. If one disk fails, data remains available without interruption, though it doubles the required storage capacity.

8 Foreign Configuration

A foreign configuration appears when a newly installed disk contains RAID metadata from another controller, when a controller is replaced, or after hot‑plugging a disk that carries existing RAID information. Administrators can import, delete, or ignore these configurations based on the current environment.

9 Disk Power‑Saving

The controller can spin down idle SAS or SATA disks to save energy. Disks and idle hot spares enter a low‑power state and are awakened when needed for operations such as RAID creation, hot‑spare activation, or reconstruction.

10 Disk Pass‑Through (JBOD)

Enabling JBOD allows the controller to forward commands directly to attached disks without creating a virtual RAID volume, facilitating direct OS installation or management software access to the raw disks.