> For the complete documentation index, see [llms.txt](https://linux-tutorial-cli.gitbook.io/linux-cli-tutorial/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://linux-tutorial-cli.gitbook.io/linux-cli-tutorial/txt-files/file-systems-cocepts/lpic3-306/proc-mdstat.md).

# /proc/mdstat

The `/proc/mdstat` file in Linux is used to monitor the status of **RAID arrays** managed by the **MD (Multiple Devices)** subsystem, which includes software RAID configurations (such as RAID 0, 1, 5, 6, 10, etc.). It provides a live, dynamic view of the state of the RAID arrays, including whether the arrays are operational, degraded, or in the process of rebuilding.

### Key Information Provided by `/proc/mdstat`:

1. **RAID Device Status**:
   * Displays the status of each RAID array (e.g., whether it’s active, degraded, or in the process of rebuilding or reshaping).
2. **RAID Level**:
   * Shows the RAID level of the array (e.g., RAID 1, RAID 5, RAID 10).
3. **Number of Devices**:
   * Displays the number of devices that make up the RAID array and their current state (e.g., `active`, `spare`, `failed`).
4. **Resync Progress**:
   * If the RAID array is being rebuilt (in the process of resyncing), it shows the progress of the resynchronization operation, typically in percentage.
5. **Write Intent Bitmap**:
   * For certain RAID levels (like RAID 1), it may display whether a write intent bitmap is used. This can help improve the speed of rebuilds by tracking areas of the array that have been modified.
6. **Array Health**:
   * Shows the overall health of the RAID array, including any potential failures or if a disk is marked as "faulty" or "missing."

### Example Output of `/proc/mdstat`:

```bash
$ cat /proc/mdstat
Personalities : [raid1] [raid5] [raid6] [linear] [multipath]
md0 : active raid1 sda1[0] sdb1[1]
      1953514496 blocks super 1.2 [2/2] [UU]
      bitmap: 0/15 pages [0KB], 65536KB chunk

md1 : active raid5 sdc1[0] sdd1[1] sde1[2]
      7816121600 blocks super 1.2 level 5, 64k chunk, algorithm 2 [3/3] [UUU]
      bitmap: 0/15 pages [0KB], 65536KB chunk
```

### Explanation of the Output:

1. **`Personalities`**:
   * Shows the RAID levels supported by the kernel (e.g., RAID1, RAID5, RAID6). It indicates the possible configurations for software RAID.
2. **`md0`**:
   * This is the name of the RAID array (in this case, `md0`).
   * **Status**: `active raid1` — The array is actively using RAID 1 (mirroring).
   * **Devices**: `sda1[0] sdb1[1]` — The array consists of two devices (`sda1` and `sdb1`), with both devices marked as "active" (`[UU]`).
   * **Blocks**: The array has `1953514496` blocks of data storage.
   * **Health**: `[UU]` — Indicates both devices are online and functioning properly.
3. **`md1`**:
   * This is another RAID array (`md1`).
   * **Status**: `active raid5` — The array is actively using RAID 5 (striped with parity).
   * **Devices**: `sdc1[0] sdd1[1] sde1[2]` — The array consists of three devices (`sdc1`, `sdd1`, and `sde1`).
   * **Health**: `[UUU]` — Indicates all three devices are active and healthy.
   * **Resync**: If the RAID array were in a degraded state or being rebuilt, you'd see a progress indicator (e.g., `10% complete`).

### Key Indicators:

* **\[UU]**: All devices are active and functioning (RAID is healthy).
* **\[U\_]**: One device is active, and another is in a degraded state (e.g., missing or failed).
* **\[\_\_]**: Devices are missing or failed.
* **Resync**: If the RAID is rebuilding or synchronizing after a failure, this field shows the progress of the resync operation (e.g., `Resyncing` or `Rebuild in progress` with a percentage).

### Other Information:

* **Bitmap**: Shows if a bitmap is being used for the array, which helps optimize rebuilds by keeping track of changes.
* **Chunk Size**: The chunk size of the RAID (especially important for RAID 5/6 arrays).

### Use Cases for `/proc/mdstat`:

1. **Monitoring RAID Status**: `/proc/mdstat` is commonly used to check the health and status of software RAID arrays. It is a quick way to verify if an array is degraded or has failed.
2. **RAID Rebuild Monitoring**: When a disk in a RAID array fails, it needs to be replaced. `/proc/mdstat` allows you to monitor the progress of the rebuild operation, which can take a significant amount of time depending on the size of the array and the disk speed.
3. **Troubleshooting RAID Issues**: By examining the output of `/proc/mdstat`, you can identify if a device is missing or failed, helping you take corrective actions such as replacing the failed disk.

### Conclusion:

The `/proc/mdstat` file is an essential tool for Linux administrators managing software RAID arrays. It provides real-time status and health information about RAID devices, which can help in troubleshooting and monitoring the RAID subsystem. Always monitor `/proc/mdstat` after making changes to your RAID configuration (e.g., adding, removing, or replacing disks) to ensure the array remains healthy and performs optimally.