What to Do When a RAID 5 Rebuild Fails

• 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.If the second drive failed outright, the array is below parity tolerance and drops to failed. If it was a single read error, the outcome is controller-specific: legacy and low-end controllers abort the rebuild, while modern Dell PERC and LSI/Broadcom MegaRAID puncture that stripe and Linux mdadm logs it to its Bad Block Log, both continuing with only that stripe lost.

• 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• 4.If the stale drive is forced back in, the controller XORs its outdated blocks with current blocks.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.

• 1.Consumer drives (WD Red, Seagate IronWolf non-Pro) specify a URE rate of 1 in 10¹⁴ bits read. That equals roughly 1 unrecoverable error per 12.5 TB of sequential reads.
• 2.Enterprise drives (Seagate Exos, WD Ultrastar) specify 1 in 10¹⁵ bits, or roughly 1 error per 125 TB.
• 3.A 4-drive RAID 5 with 14 TB consumer drives rebuilds by reading 3 surviving members sequentially: 3 x 14 TB = 42 TB total reads.
• 4.Against the worst-case 1-in-10¹⁴ spec, reading 42 TB means reading 3.36 × 10¹⁴ bits, so the spec implies a worst-case upper bound around one URE per rebuild pass. Most drives read well past the spec, so this is a ceiling, not a schedule, and on the bench the dominant driver is mechanical: the long full-surface read pass pins marginal same-batch survivors at full load until one fails.

These numbers explain why RAID 5 is no longer recommended for drives above 2 TB in production environments. RAID 6 (dual parity) or RAID 10 (mirrored stripes) tolerate a single URE during rebuild without losing the array. For existing RAID 5 deployments, regular mdadm --action=check or controller-level patrol reads surface latent errors before they are discovered during the high-stakes rebuild window.

Left-Asymmetric (Forward Parity)

Parity starts on the last drive for stripe 0 and shifts one position to the left for each subsequent stripe. Data blocks fill the remaining positions left-to-right, skipping the parity position. Used by Linux mdadm (--layout=left-asymmetric) and some older Adaptec controllers.

Left-Symmetric (Default for mdadm)

Parity rotates in the same left-shifting pattern, but data blocks continue from where the previous stripe left off rather than restarting at position 0. This is the default for Linux mdadm and produces better sequential read performance because consecutive data blocks are distributed more evenly across drives.

Right-Asymmetric (Backward Parity)

Parity starts on the first drive and shifts one position to the right for each stripe. Common on Dell PERC (MegaRAID-based), LSI MegaRAID, and HP SmartArray controllers. This is the most frequently encountered rotation in enterprise hardware RAID.

Right-Symmetric (Backward Symmetric)

The right-shifting complement of left-symmetric. Data blocks continue from the previous stripe's ending position. Less common in practice; found on some Areca and 3ware controllers.

1. Each member drive is connected individually to a PC-3000 Portable III or PC-3000 Express through a write-blocked hardware channel. Sector-level images are created, including any drives with bad sectors (sectors with read errors are logged and filled with a pattern marker).
2. The technician inputs the detected array geometry: number of member drives, stripe size (identified via hex analysis), parity rotation algorithm, and drive order (determined from bay labels and parity block positions).
3. PC-3000 DE RAID assembles a virtual disk image by reading sectors from each drive image in the correct stripe order. Parity blocks are used to reconstruct data from any single missing or damaged drive.
4. The output is a single contiguous disk image that represents the original logical volume. This image is mounted read-only, and the recovered file system is verified before copying data to the client's destination media.

The entire process operates on images, not live drives. If the first parameter guess is wrong, the geometry is adjusted and reconstruction runs again without risk. For arrays where the original drive order is unknown, PC-3000 provides an auto-detection mode that tests candidate orderings against known file system signatures.