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Cluster : Storage > Disks : Understanding disks : About degraded mode
About degraded mode
When a disk fails, Data ONTAP can continue to serve data, but it must reconstruct the data from the failed disk using RAID parity. When this happens, the affected RAID group is said to be in degraded mode. The performance of a storage system with one or more RAID groups in degraded mode is decreased.
A RAID group goes into degraded mode in the following scenarios:
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A single disk fails in a RAID4 group.
After the failed disk is reconstructed to a spare, the RAID group returns to normal mode.
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One or two disks fail in a RAID-DP group.
If two disks have failed in a RAID-DP group, the RAID group goes into double-degraded mode.
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A disk is taken offline by Data ONTAP.
After the offline disk is brought back online, the RAID group returns to normal mode.
Note: If another disk fails in a RAID-DP group in double-degraded mode or a RAID4 group in degraded mode, data loss could occur (unless the data is mirrored). For this reason, always minimize the amount of time a RAID group is in degraded mode by ensuring that appropriate hot spares are available.

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