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  1. If one HDD fails, does the computer continue to work as if no HDD failed?

  2. If you plan to re-RAID 1 the HDD that didn't fail with a brand-new HDD, do you have to "take down" the computer to perform it, or can it be done while the computer is doing it's normal operation (e.g., a web server still up while re-RAID 1 occurs) ?

Note: The reason I ask this is why are some web hosts unavailable during "RAID rebuild" ?

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1. That's pretty much the main point of RAID. 2. Depends on if hot-swap is supported. –  Oliver Salzburg Aug 8 '12 at 9:57
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Which software and hardware are we talking about exactly? But yes, that's usually what RAID 1 is meant for. Hot swapping drives in a RAID is normal, if supported by the controller. –  slhck Aug 8 '12 at 9:57
    
It depends on the web host, but if they are using RAID5 for example, then while the disk is down, there is no fault tolerance. Disabling access during a rebuild reduces the risk of something going wrong during the rebuild. Also RAID 5 and 6 can be heavy on disk usage during a rebuild, and so this would impact performance, but also, writing to a disk during a rebuild will slow the rebuild. –  Paul Aug 8 '12 at 14:06

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up vote 4 down vote accepted

A RAID rebuild happens (for example) when a previously failed RAID member disk is replaced.

The reason a webhost could be unavailable (even though they shouldn't) during a RAID rebuild, is because they took the whole storage area offline while it is being rebuilt.

If they didn't do that, you could still happily write to that storage. That wouldn't be the worst thing in the world, but it would reduce rebuild performance (as some of the possible throughput is used up by you).
But when the RAID is in a degraded state, you want to get it back online as soon as possible.

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Redundant array of Independent Disks (RAID) storage technologies provide a way to secure information using one logical unit composed by distinct physical disk drives, allowing continued online status.

With RAID 1, data is written identically to two drives, and the array continues to operate as long as at least one drive is functioning.

Since RAID 1 operates as a mirror, replicating everything from HDD 1 to HHD 2, a RAID rebuild on one of this HDDs shouldn't cause the entire service to stop (controller dependent). If it does, is because the web host is intentionally stopping the entire storage or the RAID controller is unable to operate with only one drive.

Since drive capacities has grown at a much faster rate than transfer speed, the larger capacity drives may take hours, if not days, to rebuild. If the web host keeps the unit online, this operation gets increased. Putting the entire RAID unit offline to allow all the resources to be available to the rebuild process may be the best way to have everything back and running faster.

More detailed information at Wikipedia - RAID.

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Yes, if a RAID 1 HDD drive fails (most) RAID controllers will continue functioning with the remaining good drive, while posting some sort of alert.

On high-end equipment I've seen a technician simply pull-out a bad drive, put in a new good drive, all with the system running live. The performance was slightly degraded while the new drive was rebuilt but an end-user wouldn't be able to tell.

On consumer based RAID 1 gear you'd probably need to shut-down while you replace the drive but even then it should allow the system to function while the RAID 1 is being re-built.

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