Data Domain: Understanding Disk Reconstruction

Summary: This article provides an understanding of the Data Domain Disk Reconstruction process.

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Instructions

PURPOSE

Provide an understanding of the Data Domain (DD) Disk Reconstruction process.

APPLIES TO

  • All Data Domain systems
  • All Data Domain Operation Systems (DDOS) Releases

Data Domain systems use RAID 6 to ensure robust protection of customer data, even if there are multiple disk failures. Monitoring disk reconstruction status is essential when a disk fails. 

  • RAID 6 Protection: Supports multiple simultaneous disk failures without data loss (RAID 6 has double parity protection per disk group).

  • Reconstruction Rules:

    • A maximum of two disk reconstructions can occur simultaneously (across two disk groups).
    • Exception: If two disks fail within the same Disk Group (DG), only one reconstruction can proceed at a time. The second will begin only after the first completes.

  • Global Sparing Behavior:

    • Data Domain uses global spares across all external enclosures.
    • If all spares in one enclosure are consumed and not replaced, the system pulls a spare from another enclosure with the matching disk size.
    • This may result in a Disk Group spanning multiple enclosures. While it is ideal to keep RAID and spare disks within the same enclosure, this configuration does not impact performance.

  • Reconfiguration Guidance:

    • If global sparing has occurred, contact support to verify system health and assist with disk enclosure reconfiguration.

  • Controller Disk Behavior:

    • Head Unit (Controller) disks do not reconstruct to external shelf spare disks.

  • Best Practice:

    • Replace failed disks as soon as possible to maintain optimal protection and system stability.

Viewing Disk Reconstruction Status on Data Domain

To monitor disk reconstruction progress on a Data Domain system, use the following DD-CLI command:

# disk show state
OR
# storage show all

Command Output Includes:

    • Current disk state
    • Reconstruction activity
    • Disk group undergoing reconstruction
    • Percent completion of reconstruction

Special Case:

If a second disk failure occurs in the same disk group (for example, dg1 or dg2), the output indicates a reconstruction pending status. This means that the second reconstruction will not begin until the first has been completed.

For Example:  
sysadmin@hostname# disk show state
Enclosure   Disk
            1  2  3  4  5  6  7  8  9  10 11 12 13 14 15
---------   ---------------------------------------------
1           s  .  .  .
2           .  .  R  .  .  .  .  .  .  .  .  .  .  .  F
3           F  .  .  .  .  .  .  .  .  .  .  .  .  .  R
4           .  s  .  F  .  .  .  .  .  .  .  .  .  .  .
---------   ---------------------------------------------

Legend       State                          Count
------       ----------------------------   -----
.            In Use Disks                   44
s            Spare Disks                    1
R            Spare (reconstructing) Disks   2
F            Failed Disks                   3
------       ----------------------------   -----
Total 49 disks

Disk      Disks                        Progress   Remaining
Group   Reconstructing                 (%)        (minutes)
-----       --------------            --------    ---------
dg1           2.3                      0             238
dg2           3.15                    99             10
dg3        (Pending)
-----       --------------            --------    ---------

Additional Information

 

Affected Products

Data Domain

Products

Data Domain, Data Domain
Article Properties
Article Number: 000011134
Article Type: How To
Last Modified: 26 Sep 2025
Version:  7
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