# /proc/sys/dev/raid

The `/proc/sys/dev/raid` directory in Linux provides a way to access and modify RAID (Redundant Array of Independent Disks) settings at runtime. These settings control various aspects of how the RAID subsystem behaves on your system, including parameters related to RAID devices and configurations.

### Key Files in `/proc/sys/dev/raid/`

Below are some of the common files found in `/proc/sys/dev/raid` and their purposes:

1.  **`/proc/sys/dev/raid/speed_limit_max`**

    * Controls the maximum speed at which the RAID subsystem can rebuild arrays or resynchronize devices. This setting helps prevent RAID operations from consuming excessive CPU or disk bandwidth.

    Example:

    ```bash
    cat /proc/sys/dev/raid/speed_limit_max
    1000
    ```

    * **Adjusting this value**: You can modify the rebuild speed limit to reduce the load on the system or speed up the process based on your needs.

    ```bash
    echo 10000 > /proc/sys/dev/raid/speed_limit_max
    ```
2.  **`/proc/sys/dev/raid/speed_limit_min`**

    * Similar to `speed_limit_max`, this setting controls the minimum speed for rebuilding or resynchronizing RAID arrays. It sets a lower limit to avoid RAID operations running too slowly.

    Example:

    ```bash
    cat /proc/sys/dev/raid/speed_limit_min
    100
    ```

    * **Adjusting this value**: It can be modified to ensure the RAID rebuild speed is above a certain threshold.

    ```bash
    echo 200 > /proc/sys/dev/raid/speed_limit_min
    ```
3.  **`/proc/sys/dev/raid/stripe_cache_size`**

    * Controls the size of the stripe cache for RAID devices. This cache helps optimize the read/write performance of RAID arrays by holding frequently accessed data.

    Example:

    ```bash
    cat /proc/sys/dev/raid/stripe_cache_size
    128
    ```

    * **Adjusting this value**: The cache size can be increased or decreased depending on your system’s memory resources and performance requirements.

    ```bash
    echo 256 > /proc/sys/dev/raid/stripe_cache_size
    ```
4.  **`/proc/sys/dev/raid/array_state`**

    * Displays the current state of RAID arrays on the system, such as whether they are rebuilding, idle, or in a degraded state.

    Example:

    ```bash
    cat /proc/sys/dev/raid/array_state
    degraded
    ```
5.  **`/proc/sys/dev/raid/raid1`**

    * Controls the settings related to RAID 1 (mirroring) devices, such as write intent bitmap settings, which can affect how the array is rebuilt when a disk failure occurs.

    Example:

    ```bash
    cat /proc/sys/dev/raid/raid1
    write_intent_bitmap
    ```
6.  **`/proc/sys/dev/raid/devices`**

    * Controls the maximum number of devices that a RAID array can have. This can be adjusted if you're working with RAID levels that support multiple devices.

    Example:

    ```bash
    cat /proc/sys/dev/raid/devices
    16
    ```

    * **Adjusting this value**: You can increase or decrease the number of devices that can be used in the array.

    ```bash
    echo 32 > /proc/sys/dev/raid/devices
    ```
7.  **`/proc/sys/dev/raid/disk_state`**

    * Displays the current state of disks in a RAID array, including whether they are online, offline, or marked as faulty.

    Example:

    ```bash
    cat /proc/sys/dev/raid/disk_state
    online
    ```
8.  **`/proc/sys/dev/raid/health`**

    * Displays the overall health of the RAID subsystem, including the health status of individual RAID devices.

    Example:

    ```bash
    cat /proc/sys/dev/raid/health
    healthy
    ```

### Conclusion

The `/proc/sys/dev/raid` directory provides critical parameters for managing RAID behavior on a Linux system. By adjusting these values, you can fine-tune RAID operations such as rebuild speed, disk caching, and the number of devices in an array. It's essential for system administrators managing RAID configurations to understand these settings to ensure optimal performance and fault tolerance in the RAID subsystem.

As with any system configuration, it's crucial to make changes carefully and understand the implications, as improper settings could affect performance or the reliability of your RAID arrays.
