MajorLinux
6592eb4fea
- Fixed 4 broken markdown links (bad relative paths in See Also sections) - Corrected n8n port binding to 127.0.0.1:5678 (matches actual deployment) - Updated SnapRAID article with actual majorhome paths (/majorRAID, disk1-3) - Converted 67 Obsidian wikilinks to relative markdown links or plain text - Added YAML frontmatter to 35 articles missing it entirely - Completed frontmatter on 8 articles with missing fields Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
114 lines
3.3 KiB
Markdown
114 lines
3.3 KiB
Markdown
---
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title: "mdadm — Rebuilding a RAID Array After Reinstall"
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domain: linux
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category: storage
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tags: [mdadm, raid, linux, storage, recovery, homelab]
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status: published
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created: 2026-04-02
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updated: 2026-04-02
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---
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# mdadm — Rebuilding a RAID Array After Reinstall
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If you reinstall the OS on a machine that has an existing mdadm RAID array, the array metadata is still on the disks — you just need to reassemble it. The data isn't gone unless you've overwritten the member disks.
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## The Short Answer
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```bash
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# Scan for existing arrays
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sudo mdadm --assemble --scan
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# Check what was found
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cat /proc/mdstat
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```
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If that works, your array is back. If not, you'll need to manually identify the member disks and reassemble.
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## Step-by-Step Recovery
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### 1. Identify the RAID member disks
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```bash
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# Show mdadm superblock info on each disk/partition
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sudo mdadm --examine /dev/sda1
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sudo mdadm --examine /dev/sdb1
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# Or scan all devices at once
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sudo mdadm --examine --scan
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```
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Look for matching `UUID` fields — disks with the same array UUID belong to the same array.
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### 2. Reassemble the array
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```bash
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# Assemble from specific devices
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sudo mdadm --assemble /dev/md0 /dev/sda1 /dev/sdb1
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# Or let mdadm figure it out from superblocks
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sudo mdadm --assemble --scan
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```
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### 3. Verify the array state
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```bash
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cat /proc/mdstat
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sudo mdadm --detail /dev/md0
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```
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You want to see `State : active` (or `active, degraded` if a disk is missing). If degraded, the array is still usable but should be rebuilt.
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### 4. Update mdadm.conf so it persists across reboots
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```bash
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# Generate the config
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sudo mdadm --detail --scan | sudo tee -a /etc/mdadm.conf
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# Fedora/RHEL — rebuild initramfs so the array is found at boot
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sudo dracut --force
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# Debian/Ubuntu — update initramfs
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sudo update-initramfs -u
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```
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### 5. Mount the filesystem
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```bash
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# Check the filesystem
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sudo fsck /dev/md0
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# Mount
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sudo mount /dev/md0 /mnt/raid
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# Add to fstab for auto-mount
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echo '/dev/md0 /mnt/raid ext4 defaults 0 2' | sudo tee -a /etc/fstab
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```
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## Rebuilding a Degraded Array
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If a disk failed or was replaced:
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```bash
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# Add the new disk to the existing array
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sudo mdadm --manage /dev/md0 --add /dev/sdc1
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# Watch the rebuild progress
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watch cat /proc/mdstat
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```
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Rebuild time depends on array size and disk speed. The array is usable during rebuild but with degraded performance.
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## Gotchas & Notes
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- **Don't `--create` when you mean `--assemble`.** `--create` initializes a new array and will overwrite existing superblocks. `--assemble` brings an existing array back online.
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- **Superblock versions matter.** Modern mdadm uses 1.2 superblocks by default. If the array was created with an older version, specify `--metadata=0.90` during assembly.
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- **RAID is not a backup.** mdadm protects against disk failure, not against accidental deletion, ransomware, or filesystem corruption. Pair it with rsync or Restic for actual backups.
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- **Check SMART status on all member disks** after a reinstall. If you're reassembling because a disk failed, make sure the remaining disks are healthy.
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Reference: [mdadm — How to rebuild RAID array after fresh install (Unix & Linux Stack Exchange)](https://unix.stackexchange.com/questions/593836/mdadm-how-to-rebuild-raid-array-after-fresh-install)
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## See Also
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- [snapraid-mergerfs-setup](snapraid-mergerfs-setup.md)
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- [rsync-backup-patterns](../../02-selfhosting/storage-backup/rsync-backup-patterns.md)
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