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I'm setting up software RAID on Ubuntu Server 10.04; specifically RAID10. It's asking for the number of active devices for the array and the number of spare devices.

I know RAID10 is two mirrored pairs; RAID0 for two sets of RAID1 arrays (correct me if I'm wrong).

I would just like to know if all the devices (I'm using 4) are considered active, or if I should be setting it as 2 active and 2 spare.

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Keep in mind that a Linux software RAID10, isn't exactly a RAID10, it can be setup really weird in various configurations. – Zoredache May 9 '11 at 19:15
Didn't know that... it does say that the "4-drive example is identical to a standard RAID-1+0 array". So does that mean software RAID is going to be an okay choice in this case? – user29600 May 9 '11 at 19:43
The software RAID runs just fine, though if you are worried about high-performance, you may need a hardware RAID controller. Do some testing after you have the software RAID setup to see if it meets your requirements. I use the software RAID on many servers, but they aren't extremely critical. – Zoredache May 9 '11 at 20:12
Will do, thanks for the tip. – user29600 May 9 '11 at 22:05
up vote 1 down vote accepted

In a RAID configuration, a spare or hot spare is an extra drive that at array construction stores nothing. If at some time in the future, an active drive fails, md (or whatever RAID controller you are using) will automatically start restoring data from the active drives to one spare drive in order to replace the spare drive.

If you have a RAID 10 of five 1TB disks (four active and one spare), you will only have 2TB total capacity. This is because you have a stripe of 2TB, which is mirrored so your 3rd and 4th disks are used up, and the fifth disk does nothing until one of the four fails.

Wikipedia RAID#Hot Spares

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All 4 are active. Spares are referring to Hotspares, which take over in the event of a disk failure.

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Spares are usually on-line drives not currently part of the RAID set. If one of the drives fails, the RAID set can automatically rebuild using a spare.

In your example, all are active.

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