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What does Raid state Online, Ready or Foreign means? What other state can it be?

What needs to be done in such state? I think Online is the only safe state.

What are the DO's and Don't during clearing its state or replacing drives (or when should we replace drives).

What to do during multiple disk failures or in a specific state? 

Raid 0 - 1 drive fails, everything is lost.
Raid 1 - 1 drive fails, data is still safe. 
Raid 5 - 1 drive fails, data is still safe but if 2 drive fails - everything is lost.
Raid 6 - 2 drive fails, data is still safe but if 3 drive fails - everything is lost.
Raid 10 - 1 drive failure, data is still safe. 2 mirrored drive failure - everything is lost. 2 drive failed from different pair - data is still safe.

But what if in Raid 10, 2 Drives are in Foreign state and 2 Drives in Ready state?

What does those state mean? And what needs to be done?

user630702
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1 Answers1

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From this answer Server Fault 718220

The "foreign state" means that the newly inserted disks carry an existing raid configuration (possibly once created on another server).

You may either:

"clear" this state on each individual disk (this action will destroy data on these disks),

or

"import" this foreign RAID configuration (assuming that all disk involved in the RAID configuration are present) to recover the whole disk configuration.

"ready" means the drive can be added to the array

Dave M
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  • And to round out the answer online means the disk is part of an array. And the array has no degraded members. Foreign can also occur if a drive is physically removed and reinserted in a chassis without properly marking it Bad/down first. Though that happens more on Hardware base trade than software. – Rowan Hawkins Mar 27 '20 at 23:42