RAID 10 Data Recovery Services

When Does a RAID 10 Array Become Unrecoverable?

Recovery in the second scenario depends on whether we can restore at least one drive in the failed pair. If a drive failed due to a burned TVS diode, a seized motor, or corrupted firmware, physical intervention (board repair, head swap, PC-3000 firmware reconstruction) can bring it back to a readable state. Once one member per pair is imaging, the array can be reconstructed.

Degraded RAID 10 arrays left running while a rebuild proceeds are at peak risk. If a second drive in the same mirror pair fails during the rebuild, the array drops offline. 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?

• Entropy delta scanning: Compressed and encrypted regions show near-uniform Shannon entropy. Plaintext database files, log structures, and filesystem metadata show characteristic non-uniform distributions. Chunk boundaries become visible where the entropy distribution shifts at exactly the same LBA on two members. Mirror partners show byte-identical entropy at the same LBA; striped partners do not.
• Filesystem boundary alignment: NTFS $MFT records are 1024 bytes each and chunk-aligned writes produce a clean break at every chunk boundary. The ext4 primary superblock sits at byte offset 1024, and on sparse_super layouts backup copies land at the start of block group 1 and every block group that is a power of 3, 5, or 7 (with the default 4 KiB block size, block group 1 begins at block 32768). XFS allocation group headers sit at the start of each AG and contain a self-referential AG number. ZFS uberblock arrays sit in the second half of each 256 KiB vdev label, after the boot block and nvlist configuration, and contain a TXG counter that monotonically increases across the label set. VMFS heartbeats occupy a fixed region near the start of the volume. Each anchor lands at the same offset on mirror partners and at chunk-spaced offsets on striped partners.
• mdadm software RAID 10: mdadm --examine /dev/sdX against each cloned image reports the superblock version (v0.90 trailer near the end of the device, v1.0 at the end, v1.1 at offset 0, v1.2 at 4 KiB from the start), the layout code (n2 near, f2 far, o2 offset), the chunk size in KiB, and the device role within the array. With those five fields we can reassemble the array on a vanilla Linux workstation with mdadm --assemble --readonly.
• 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, with a reserved region of roughly 32 MB growing inward from the end. HP Smart Array uses a Reserved 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 before falling back to entropy and filesystem-anchor inference.

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 probabilistic upper bound, which corresponds to an expected value of one URE per 12.5 TB of sustained reads. A full read of a 4 TB consumer member during rebuild is a statistically meaningful fraction of that expected interval on aging hardware. Real-world fleet error rates can exceed the published spec.
• Enterprise SAS URE probability: Enterprise drives spec one URE per 10^15 bits, an expected value of one URE per 125 TB. The ten-times-lower error rate is the dominant reason enterprise arrays last longer in service, not the spindle speed or the interface.
• 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.

The trap is leaving that degraded pair powered and serving I/O while a rebuild runs against it. A consumer SATA member specs one URE per 10^14 bits, an expected one error per 12.5 TB of sustained reads; an enterprise SAS member specs one per 10^15 bits, one per 125 TB. A full read of the lone surviving leg during a rebuild is a meaningful fraction of that interval on aging hardware, and every retry the controller forces accelerates head wear on the one drive you cannot afford to lose.

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.

We state the limit plainly. 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. The rest of the volume still reconstructs from the readable sectors and from the healthy pairs, so a depleted mirror is rarely a total loss, but the affected span is read to the limit of the one drive that remains. There is no diagnostic fee, and if we recover no data you owe nothing.

How Do We Arbitrate Members With Mismatched Event Counts or DDF Sequence Numbers?

Split-generation members are common after an interrupted rebuild. A background initialization, a consistency check, a patrol read, or a hot spare that was promoted late can advance one member's metadata while another sat offline; an auto-rebuild that started, stalled, and restarted against a moving target leaves the same divergence. The result is a pair whose two halves disagree about which one holds the last committed transaction.

For software RAID, mdadm --examine /dev/sdX against each cloned image reports the Events count and Update Time per member. The highest Events count was last in sync; a lower count fell out of the array earlier and must not be promoted as current. For hardware RAID, the DDF carries a sequence number the controller compares to decide which member is authoritative. The conceptual fields we arbitrate on look like this when read off a clone:

mdadm --examine /dev/sdc   # member A (cloned image)
          Events : 184273
     Update Time : last in sync

mdadm --examine /dev/sdd   # member B (cloned image)
          Events : 184019    # lower count: fell out earlier, do NOT promote
     Update Time : stale

The hazard is automation. A live auto-rebuild or a mdadm --assemble --force that ignores the event-count gap promotes the stale leg and overwrites the current data with the old generation. On the live array that write is irreversible. This is the same destructive path a RAID 5 forced assembly takes when it readmits a dropped member without checking its generation first.

How we select the freshest member safely

1. Arbitrate on cloned images first. Event counts and DDF sequence numbers are read from the clones, never from the live drives, so the comparison itself writes nothing to the originals.
2. Select the freshest copy per pair. The member with the highest Events count or sequence number is chosen for each pair; the stale one is set aside rather than readmitted.
3. Assemble read-only. Software arrays are brought up with mdadm --assemble --readonly; hardware arrays are assembled with Data Extractor Express RAID Edition reading the raw sequence fields off the clones.
4. Redo without cost when a guess is wrong. Because every step runs against clones, a wrong member selection can be reversed and re-run without consuming any of the surviving members.

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.

Controller-Specific Metadata Layout

Dell PERC (H730 / H740 / H755): DDF anchored at the last LBA

PERC silicon is rebadged LSI MegaRAID with Dell firmware. It writes a Dell variant of the SNIA DDF (Disk Data Format) record set anchored at the absolute last LBA of each member drive, with the reserved DDF region (roughly 32 MB) growing inward from the end. When a PERC controller fails and the drives are reseated into a replacement PERC of a different generation, the new controller flags the array as Foreign Config in the boot-time BIOS interface. Accepting the import rewrites the controller NVRAM from the on-disk DDF and preserves member ordering across PERC generations as long as the replacement firmware supports the source DDF dialect. Clearing the Foreign Config wipes the DDF from the trailing sectors and destroys the geometry map. If foreign-config status appears, photograph the BIOS screen, shut the server down, and pull the drives for cloning before any further action.

HP Smart Array (P408i / P816i): RIS at the start of each drive, not DDF

HP Smart Array uses a proprietary Reserved Information Sector (RIS) written at the start of each member drive, in reserved physical sectors independent of any OS partitioning. This is not DDF and is not parseable by third-party DDF utilities. The on-drive RIS records member position, stripe size, mirror pairing, and array geometry. Smart Array defaults to a 256 KiB chunk size, not the 64 KiB default seen on Dell PERC. Standard destriping tools that assume 64 KiB will return scrambled data on a Smart Array image. The controller ships with a Flash-Backed Write Cache (FBWC) or Battery-Backed Write Cache (BBWC). If the controller dies with dirty writes in the cache module, those writes never reach the member drives. A degraded BBWC or supercap means in-flight writes are lost; treat the array as a pre-existing-state recovery rather than a current-state recovery.

LSI MegaRAID (9361 / 9460 / 9560): DDF with span definitions

LSI MegaRAID writes a SNIA DDF record on each member that includes virtual drive group definitions, span definitions for RAID 10 / 50 / 60, and mirror-set pairing identifiers. The DDF anchor header sits at the absolute last LBA of each drive, with the reserved region growing inward from the end, which means cloning to a replacement drive even one LBA smaller than the original truncates the metadata. Forensic triage runs against cloned images, not live drives: storcli /c0 show all and MegaCli64 -PDList -aALL executed against a sacrificial test rig populated with the clones reveal the same metadata the live controller would surface, without risking a write to the originals. The DDF on member drives is authoritative; the controller NVRAM is a cache. When the two disagree, the on-disk DDF wins.

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. Common values are 64 KiB (default on Dell PERC), 128 KiB, 256 KiB (default on HP Smart Array), and 1 MiB depending on controller and workload tuning.

Foreign config

The status flag a hardware RAID controller raises when it boots and finds member drives carrying a configuration the controller does not recognize as its own. Accepting the import writes the on-disk configuration into controller NVRAM. Clearing or re-initializing destroys it.

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 (around 32 MB) growing inward from the end. Dell PERC writes a Dell variant of DDF to the same trailing region.

RIS (Reserved 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.

Span

A subgroup of drives within a nested RAID configuration. A RAID 10 array is one span per mirror pair; RAID 50 and RAID 60 use span counts to express the parity grouping.

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; enterprise drives spec one per 10^15. During a RAID 10 rebuild, a URE on the surviving mirror member silently corrupts the new mirror at that LBA.