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I have four 1TB hard drives and want to set them up in a RAID (5). Some calculations:

4x1000G = 3000G usable space
8x500G  = 3500G usable space
16x250G = 3750G usable space

Does this mean it will be more beneficial for me to partition my 4 drives into 250G parts and use a software RAID instead of hardware?

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3 Answers 3

up vote 4 down vote accepted

No, that would be horrible. If one physical drive fails, your whole array is gone (as that failure would take out at least 2 members of your raid array). Which is what you were trying to prevent in the first place.

edit: Please note, I wrote the following paragraph having a software raid setup in mind which might use devices directly (like /dev/sda) instead of partitions (like /dev/sda1). The former is what was causing me trouble.

But I would still recommend creating partitions on the drives (one partition spanning the whole drive) and using these partitions for the raid array (instead of the drives themselves). Otherwise, data might be written to the start of the disk, thus, confusing your system.
One of my servers suffers from me messing this up when I created the initial raid volume.

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its also handy to create one big partition which is slightly less than the drive size in case you buy a drive which is ever so slightly different in size. –  Sirex Jan 27 '12 at 11:51

This wont work. If you create multiple volumes on a drive, only one volume per drive can be in the raid array.

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If you go with making them on partitions, you may as well use RAID0 and get the full 4TB of space.

Each part of a RAID5 array needs to be on a separate physical disk - if you had 2 partitions on the same drive and that drive fails, you lose everything on the array.

If you are mostly concerned about storage space, and don't need full redundancy, you can use something like mhddfs (on linux) or windows drive extender on Windows home server. Those will make the drives appear to be a single drive, with all 4TB of space, but if a drive fails, you only lose what was on that drive. I do that on my home file server because it's mostly recorded TV and other files that I can replace easily.

If you want all your data to be able to survive a hard drive failure, create 1 partition on each drive and use those for the RAID5 array (or use the entire drives with a hardware raid controller). You lose a full drive worth of space, but you won't lose anything when a drive fails.

The reason for making a single partition is so that the drives don't appear to be empty to other operating systems and utilities - some will mess up a drive that looks like has no partitions.

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