EX200 Create and configure file systems Practice Question
A system has two 500GB disks in a RAID1 (mirror) using mdadm. One disk fails. After replacement, what is the correct procedure to restore redundancy?
⚠ Common exam trap
Watch out — candidates often assume adding a new disk directly with 'mdadm --add' is sufficient, overlooking the critical prerequisite of having an identical partition table on the replacement disk, which is a common oversight in RAID recovery procedures.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use sfdisk to copy partition table from /dev/sda to /dev/sdb, then 'mdadm --manage /dev/md0 --add /dev/sdb1'
When replacing a failed disk in a RAID1 array managed by mdadm, the new disk must have a partition table that matches the surviving disk. Using sfdisk to copy the partition table from /dev/sda to /dev/sdb ensures that the partition layout (e.g., partition type 0xFD for Linux RAID autodetect) is identical, which is required before adding the partition (e.g., /dev/sdb1) to the array with mdadm --manage --add. Simply adding the raw disk without a proper partition table would fail because mdadm expects a partition with the correct RAID superblock and partition type.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Run 'mdadm --manage /dev/md0 --add /dev/sdb'
Why it's wrong here
The mdadm mirror is composed of partitions, not raw disks, so adding /dev/sdb as a whole disk would make it a non-conforming member and fail to integrate with /dev/sda1. You must first partition /dev/sdb with an identical layout (e.g., using sfdisk) and then add /dev/sdb1 to the array. Without a proper partition entry of type 'fd' (Linux RAID autodetect) or matching UUID, mdadm will reject the device.
- ✗
Remove the failed disk with 'mdadm /dev/md0 --fail /dev/sdb1' then add new.
Why it's wrong here
Since the original /dev/sdb1 is already marked as failed and removed from the array, executing '--fail' again is redundant and may even return an error. The recovery procedure should skip straight to copying the partition table from the surviving disk and then using '--add' to enroll the new /dev/sdb1. Failing a device that is no longer active does not help rebuild the mirror; it only wastes time.
- ✗
Run 'mdadm --assemble --scan' to rebuild the array automatically.
Why it's wrong here
'mdadm --assemble --scan' only activates an existing array by reading superblocks from member disks; it cannot restore a failed disk or trigger a rebuild. If the array is already running (as implied by the failure of one mirror member), assembly is not needed at all. This command would not partition the new disk, and it would simply return the current array to active state without replacing the missing member.
- ✓
Use sfdisk to copy partition table from /dev/sda to /dev/sdb, then 'mdadm --manage /dev/md0 --add /dev/sdb1'
Why this is correct
Use 'sfdisk' to clone the partition table from /dev/sda to /dev/sdb, ensuring the new disk has a matching partition layout with the correct RAID partition type and UUIDs. After that, 'mdadm --manage /dev/md0 --add /dev/sdb1' enrolls the freshly partitioned partition as a spare, causing the array to start rebuilding the mirror data. This sequence properly introduces the new disk into the existing RAID1 without disrupting the active array.
Quick reference
RAID Level Comparison
| RAID Level | Min Disks | Fault Tolerance | Read | Write | Usable Capacity |
|---|---|---|---|---|---|
| RAID 0 | 2 | None | Excellent | Excellent | 100% |
| RAID 1 | 2 | 1 disk | Good | Moderate | 50% |
| RAID 5 | 3 | 1 disk | Good | Moderate | 67–94% |
| RAID 6 | 4 | 2 disks | Good | Lower | 50–88% |
| RAID 10 | 4 | 1 disk per mirror | Excellent | Good | 50% |
RAID is not a backup strategy — it protects against disk failure but not against accidental deletion, ransomware, or site-level events.
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