RAID 10 Data Recovery Services

Should You Rebuild a Degraded RAID 10 Array?

A degraded RAID 10 array is at its highest risk when you leave it running during a rebuild. Power down a degraded array and contact a recovery service before initiating a rebuild on questionable hardware.

How Do We Detect Chunk Size and Mirror Pairing From Imaged Members?

• Hardware controller anchors: On hardware RAID 10, the controller writes metadata to known locations on each member. LSI MegaRAID and Dell PERC use a DDF variant anchored at the last LBA of each drive. HP Smart Array uses a RAID Information Sector in reserved physical sectors at the start of each drive, independent of any OS partitioning. We parse those structures off the cloned images.

What Are the URE Risks During a RAID 10 Rebuild?

• Consumer SATA URE probability: Manufacturer specifications list one URE per 10^14 bits read as a worst-case floor, meaning the drive is guaranteed to be no worse than that error rate, and real drives commonly perform far better than the guaranteed spec. Dividing the bytes read during a rebuild by the 12.5 TB that 10^14 bits represents gives the worst-case URE count the spec permits, not an expected number of errors. A full read of a 4 TB consumer member during a degraded rebuild is still a large enough fraction of that worst-case interval that the risk is real and scales with member capacity and drive age.
• Silent corruption window: If the surviving mirror member returns a bad sector during rebuild and the controller does not surface the error to the host, the new mirror inherits the corruption. The array then reports healthy while data is wrong. Some controllers issue a medium error and abort the rebuild; others retry, succeed on a random read, and proceed. Behavior is firmware-specific and must be characterized per controller model before any live rebuild.
• SMR drive eviction during rebuild: Shingled magnetic recording (SMR) consumer drives stall during sustained sequential writes when the CMR cache zone overflows. The kernel or the controller interprets a 30 to 60 second stall as a dead drive and ejects it, taking the rebuild offline mid-flight. SMR drives must never be placed into a RAID 10 array.

Our policy on a degraded RAID 10 with anything more than a clean single-drive failure is to refuse the live rebuild path. We power down the array, image every member with ddrescue or PC-3000 Portable III or PC-3000 Express through a write-blocker, then reconstruct the virtual array offline from the cloned images.

The original drives are never written to during recovery, which means a failed reconstruction can be redone with a different chunk-size guess or a different mirror-pair assumption without burning any of the surviving members.

What Happens When the Last Surviving Mirror Member Develops Bad Sectors?

This is a different failure than a dead pair, where both members are gone, and different than a URE on a member whose twin is healthy. When a twin survives, the controller or our offline reconstruction reads the lost LBA off the good member, and nothing is lost. When the pair is already down to one drive, the surviving leg is the only copy of that span, so a single bad sector in user data has no fallback. The math is the same one in our RAID array recovery work: redundancy is gone the moment a span drops to one member, and every read after that point is unprotected.

How we triage a depleted mirror

1. Power down immediately. A degraded pair under live rebuild I/O is the worst place the surviving leg can be. We stop reads to the original drive before it degrades further.
2. Write-blocked multi-pass imaging. The lone leg is cloned with DeepSpar Disk Imager and PC-3000 Portable III or PC-3000 Express through a write-blocker. The first pass grabs every easily readable sector fast; later passes return to the bad regions with adjusted timeouts and head-map selection to recover the maximum LBAs around damaged sectors before the heads wear out.
3. Selective head-map recovery. Imaging is ordered to read the most valuable regions first and to skip and revisit bad zones, so a head that fails partway through has already pulled the priority data rather than dying on a slow linear sweep.
4. Virtual reconstruction from the best image. PC-3000 and DeepSpar image and extract sectors; the array is then assembled in software from the best clone, with the surviving mirrors of every healthy pair filling their spans normally.

Here is the limit. If the only surviving member of a span has unreadable LBAs inside user data, those exact LBAs are gone; no twin and no parity exists to regenerate them. We still reconstruct the rest of the volume from the readable sectors and the healthy pairs. On the affected span, we read as much as the one remaining drive will give us. There's no diagnostic fee, and if we don't recover any data, there's no recovery fee.

How Much Does RAID 10 Recovery Cost?

Enterprise RAID Controllers We Recover From

Software RAID configurations (Linux mdadm, Windows Storage Spaces, ZFS) store their metadata in standardized superblock locations. These are easier to parse than hardware controller formats but still require correct identification of mirror pair groupings and stripe chunk sizes during reconstruction.

Rebuild ordering, in our lab

For every hardware controller above, the rebuild order is the same: clone first, parse second, assemble virtually third. We never run a foreign import on a live array. We never let the original controller write to a degraded member. If the customer has already accepted a foreign import and the result is unstable, we still image the members in their current state and reconstruct offline. The risk surface of a live rebuild on a multi-disk failure exceeds any time savings from skipping the imaging step.

RAID 10 Terminology Reference

The terms below are used throughout this page and across vendor documentation. Mixing them up is the single most common reason a recovery attempt destroys data that could otherwise have been retrieved.

Mirror set

Two member drives that hold byte-identical copies of the same stripe segment. A 4-drive RAID 10 has two mirror sets; a 12-drive RAID 10 has six.

Stripe set

The group of mirror sets that data is interleaved across. In RAID 10 the stripe set contains all mirror pairs, with each successive chunk written to the next mirror set in sequence.

Chunk size

The number of contiguous bytes written to one mirror set before the controller moves to the next.

DDF (Disk Data Format)

The SNIA-standardized on-disk RAID metadata format. LSI MegaRAID writes a SNIA-compliant DDF with its 512-byte Anchor Header at the absolute last LBA of each member, the reserved region growing inward from the end; MegaRAID reserves 512 MB for it. Dell PERC writes a Dell variant of DDF to the same trailing region.

RIS (RAID Information Sector)

HP's proprietary RAID metadata layout for Smart Array controllers, written at reserved physical sectors at the start of each member drive, independent of any OS partitioning. Not DDF and not interchangeable with DDF parsers.

Virtual drive group

The controller-level abstraction that maps a set of physical members to a single logical block device presented to the operating system. The OS sees one device; the controller knows the underlying member layout.

Write-back cache (FBWC / BBWC)

A controller-side memory buffer that acknowledges writes to the host before the data lands on the member drives. A flash-backed (FBWC) or battery-backed (BBWC) cache holds dirty data through power loss until the data is flushed. A failed cache module loses in-flight writes regardless of the array state.

URE (Unrecoverable Read Error)

A sector that returns a read error despite ECC retries. Consumer drives spec one URE per 10^14 bits read.