What a Failed RAID 5 Rebuild Means
RAID 5 distributes parity across all drives to survive one drive failure. A rebuild failure occurs when the array loses one drive, begins regenerating data onto a replacement, and encounters a second error before regeneration completes. The array is now worse off than after the original failure.
- 1.One drive in the array fails or goes offline. The array enters degraded mode.
- 2.The controller serves data by computing the missing drive's contribution from parity on each read request.
- 3.An administrator inserts a replacement drive (or a hot spare activates). The controller begins rebuild: reading every sector of every surviving drive and XORing the results onto the replacement.
- 4.A second drive reports an Unrecoverable Read Error or fails outright. The controller cannot XOR the stripe where the error occurred.
- 5.The rebuild aborts. The array transitions from degraded to failed.
Example: A 4-drive RAID 5 with 8TB drives loses drive 2. The admin inserts a replacement. At 73% completion, drive 4 returns a read error on sector 14,722,091,008. The controller cannot compute the stripe because two sources are now missing (the original failed drive and the sector with the URE). The rebuild halts and the controller marks the array as failed.
Stop. Do Not Attempt Another Rebuild.
After a rebuild failure, the first correct action is inaction. Do not retry the rebuild, do not swap drives between slots, do not run filesystem repair tools, and do not power the system on without a plan. Every additional operation risks overwriting recoverable data.
- 1.Power down the server or NAS cleanly if the OS allows it.
- 2.Do not remove any drives from their current slots.
- 3.Label each drive with its physical bay number (bay 0, bay 1, etc.). This is critical for offline reconstruction when controller metadata is damaged or unavailable.
- 4.Record the RAID controller model, firmware version, and any error messages from the management interface.
- 5.Do not run fsck, chkdsk, xfs_repair, or any filesystem repair utility. These tools assume the block device is consistent. On a broken array, they interpret parity errors as filesystem corruption and delete valid directory entries.
Example: A storage admin sees a rebuild failure on a Dell PowerEdge with a PERC H740. They select "Force Online" in the PERC configuration utility. The controller begins writing reconstructed parity to the surviving drives. Because the original rebuild was 73% complete, the forced-online operation mixes partially rebuilt parity with original degraded-state parity. The volume mounts, but 15% of files return read errors. The directory entries for those files now point to corrupted stripe data that was consistent before the force operation.
How Forcing a Stale Drive Online Destroys Parity
A stale drive contains data from before it was removed from the array. Forcing it back online causes the controller to recalculate parity using outdated blocks, silently corrupting every stripe that received writes while the drive was absent.
- 1.RAID 5 parity for each stripe is the XOR of all data blocks in that stripe.
- 2.When a drive goes offline, the controller stops including it in parity calculations and continues serving I/O using parity reconstruction.
- 3.Writes that occur while the drive is offline update the remaining drives but leave the offline drive unchanged.
- 4.If the stale drive is forced back in, the controller XORs its outdated blocks with current blocks. The resulting parity is wrong for every modified stripe.
- 5.Reads from affected stripes return silently corrupted data. The corruption is invisible until a parity scrub or until an application encounters garbage output.
Example: A 5-drive RAID 5 serving a database. Drive 3 loses its SATA connection for 4 hours. During those hours, the database writes 200GB of transactions across all stripes. The admin reconnects drive 3 and forces it online without a rebuild. Every database page updated during those 4 hours now contains an XOR mismatch. The database reports B-tree corruption on the next integrity check.
Why Large-Drive Rebuilds Fail
Rebuilding a degraded high-capacity RAID 5 array places extreme, sustained I/O load on the remaining aging drives. This intensive read operation drastically increases the risk of a secondary mechanical failure or encountering an unforeseen latent sector error before the parity calculation can complete.
- 1.A RAID 5 rebuild reads every sector of every surviving drive to regenerate the failed drive's data.
- 2.Drives from the same manufacturing batch tend to accumulate similar wear. If one drive has failed, the remaining members are statistically closer to failure themselves.
- 3.Rebuild times on large arrays can exceed 24 hours, during which the array has zero remaining fault tolerance and all surviving drives experience sustained sequential I/O stress.
- 4.Any latent sector error or mechanical failure on a surviving drive during this window halts the rebuild and crashes the array.
This is the core reason storage engineers consider RAID 5 inadequate for drives larger than 2TB. For RAID 5 data recovery, the mechanical stress of a full rebuild on aging drives is the primary risk factor.
Example: A NAS with four 10TB consumer drives in RAID 5. One drive fails. The rebuild must read 30TB across the surviving three drives under sustained sequential I/O. During this prolonged operation, a surviving drive encounters a latent sector error. The NAS reports "Repair failed" and the volume transitions to a crashed state.
Degraded vs Failed: Two Different Problems
A degraded array has lost one drive but continues operating with parity intact; all data remains computable. A failed rebuild is a different state: partial parity has been written to the replacement drive, and the pre-rebuild data on surviving drives may be partially overwritten.
Degraded Array
- ●One drive missing, parity intact
- ●All data computable on-the-fly via XOR
- ●Performance reduced; no tolerance for a second failure
- ●Recovery is straightforward: image surviving drives and reconstruct offline
Failed Rebuild
- ●Replacement drive contains partial data
- ●Controller may have updated parity on surviving drives during the partial rebuild
- ●Array may not import or assemble at all
- ●Recovery requires careful analysis of which stripes were modified during rebuild
Example: An LSI MegaRAID 9361-8i has a 6-drive RAID 5. Drive 2 fails Monday morning. Rebuild starts at 10 AM. At 3 PM (approximately 50% complete), drive 5 goes offline. The controller aborts the rebuild. The admin removes the replacement drive and clears the foreign configuration. The controller re-imports drives 1, 3, 4, 5, and 6 as degraded. But during the 5-hour rebuild, the controller wrote partial parity updates to drives 1, 3, 4, and 6. The pre-rebuild degraded state has been partially overwritten, and the RAID data recovery now requires forensic analysis of which stripes were modified.
Frequently Asked Questions
Why do RAID 5 rebuilds fail?
Rebuilding a degraded high-capacity RAID 5 array places extreme, sustained I/O load on the remaining aging drives. This intensive read operation increases the risk of a secondary mechanical failure or encountering a latent sector error before the parity calculation can complete. The second most common cause is a complete second drive failure during the rebuild, triggered by increased I/O load and thermal stress on drives that have been running in a degraded array for hours or days.
Can data be recovered after a failed RAID 5 rebuild?
In most cases, yes. Professional recovery bypasses the RAID controller entirely by imaging each physical drive individually through write-blocked connections. The original RAID geometry (stripe size, drive order, parity rotation pattern) is reconstructed in software from the drive images, allowing data extraction without relying on the controller state or metadata.
Is RAID 5 still safe for large drives?
Storage engineers and enterprise vendors increasingly recommend against RAID 5 for drives larger than 2TB. The sustained I/O stress of a full rebuild on aging, high-capacity drives makes single-parity protection insufficient at modern drive capacities. RAID 6 (dual parity), RAID 10 (mirrored stripes), or ZFS raidz2 provide better fault tolerance for 4TB and larger drives.
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