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If RAID-1 is an exact mirror copy, why must you do a "rebuild" on a RAID-1 drive after the primary drive fails? Will the system suffer from any downtime if the primary drive fails?

Are there any hot-backup options RAID levels available that can withstand a drive failure without having to rebuild or suffer from any downtime?

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closed as not a real question by womble, John Gardeniers, dyasny, Khaled, ewwhite May 13 '12 at 7:53

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You're misunderstanding how RAID works. Mulaz gave a very good answer as to what might happen if the RAID didn't rebuild and the risk associated with such an occurrence. In addition, a rebuild generally doesn't incur any downtime. –  joeqwerty May 13 '12 at 2:01
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If you don't do a rebuild how do you think the data will be copied to the new drive? –  John Gardeniers May 13 '12 at 6:25
    
Thanks guys for the rude comments, obviously if I knew ahead of time the answer I wouldn't have asked. But I know better now than to ask questions here. StackOverflow used to be the place to go to ask newbie questions--now it's full of "RTFM" Linux geeks. –  Cyan Lite May 13 '12 at 14:51
    
@CyanLite This isn't Stack Overflow. If you're thinking of the network of sites, that's Stack Exchange. Regardless, each site has an FAQ, please at least glance at it so you understand the site. You seem to have the misconception that you are part of this site's audience. This site it for professional system administrator (et al) only. Thank you! –  Chris S May 14 '12 at 0:26
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2 Answers

You don't have to rebuild, you can run it in a "degraded" (just one disk) state.

But ofcourse, if the other drive fails too, you lose all your data - usually you have raid to prevent that, so there's no point in having raid with just one drive, and not rebuilding it.

You can have raid1 with three drives, so when one fails, you don't need to rebuild anything, since you still have raid1 with two drives... but if two disk fail, you still have one disk, and no redundancy again.

ps: raid is not(!) backup!

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I think you're confused about the purpose of RAID. The rebuilding occurs after the replacement of a failed drive. Data from the healthy drive is copied to the new disk.

And our local RAID technology question: What are the different widely used RAID levels and when should I consider them?

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