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I have this question since (I think) 3 years ago, but since I was never actually planning to build server, I just shrug it off. But tonight, I literally can't sleep because of this burning question.

Lets say I have 3x 2TB disk. According to Wikipedia, RAID 5 requires at least three disks, and with this setup I would get 4TB capacity for the RAID.
Now, lets replace one of the 2TB disk with 2x 1TB disk. Now I have 2x 2TB disk and 2x 1TB disk. Is it possible to RAID 0 the 1TB disk (to get 2TB), then RAID 5 all of them?
To make it worse, lets replace another 2TB disk with 4x 500GB disk, and then RAID 0 them. Now I want to make RAID 5 out of (RAID 0 2x 1TB), (RAID 0 4x500GB), and 2TB disk.

And yes, I already know the RAID 0 disadvantages where when one of the disk fails, all data is lost. But then, this is RAID 5 on top. assuming the 500GB fails, I could just slap another 500GB disk there, and then rebuild everything from the RAID 5. Right? Right?

To make it even worse, lets replace one of the 1TB disk with 2x500GB disk, then RAID 0 them, then RAID 0 them again with the 1TB to make 2TB, and so on ...

Is it even possible? Is it worth it?
Now to make it even worse, lets run all the disk from USB 2.0 :(

Rinaldo Jonathan
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  • This question is literally about RAID. Not sure why it gets downvoted. At least tell me why – Rinaldo Jonathan Oct 09 '21 at 06:35
  • because it is only possible to have one raid level. If you have 3 Disk then they have to be the same size as the rest – djdomi Oct 09 '21 at 08:14
  • @djdomi but there's RAID 10 .... The disk was RAID 0, then the RAID 0 was RAID 1'ed again. My question was (probably) more like RAID 50, but with different disk size on the RAID 0. But it should have same size (4x500GB RAID 0 is still 2TB, then I want to add this RAID 0 to existing RAID 5 with 2TB disk) – Rinaldo Jonathan Oct 09 '21 at 08:38
  • raid10 ist just one raid level, only in explaining its two. Raid 50 is even a other level – djdomi Oct 09 '21 at 08:43

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And yes, I already know the RAID 0 disadvantages where when one of the disk fails, all data is lost. But then, this is RAID 5 on top. assuming the 500GB fails, I could just slap another 500GB disk there, and then rebuild everything from the RAID 5. Right? Right?

Be very specific with your recovery procedure. Replace the physical disk in the RAID 0 array. However RAID 0 cannot be rebuilt, there will be corruption striped in it. Remove and re-add the replaced RAID 0 member of the RAID 5 array, not the physical drive.

Introduces a little operational complexity. Beginner how to repair an array articles are focused on physical drives, not nested arrays. Also, not all array software can do RAID 05. Linux md software raid should allow arbitrary nested, possibly some enterprise arrays are capable as well.

Now I want to make RAID 5 out of (RAID 0 2x 1TB), (RAID 0 4x500GB), and 2TB disk.

Theoretically you can, but in practice don't do that.

Any member of RAID 0 failing is a problem. Odds of success decrease as more disks are added. Say a certain drive model can expect one unrecoverable error in 1e14 bytes read. RAID 0 with 4x of these means all need to independently achieve this with 99.999999999999% chance of success. Multiply the probabilities together, and pretty soon you get 13 nines of durability rather than 14.

Which is a part of a redundant array, yes, but this introduces unnecessary risk of degrading that array. Your layout is less reliable than if it was a traditional 7 disk RAID 6. Further, double parity is superior, maintains fault tolerance even during rebuild.

Admittedly the 05 of varying sizes is more space efficient. Although I would argue that the operational simplicity of all drives the same size is worth the cost of sourcing them.

John Mahowald
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  • THIS WAS THE INFORMATION I WAS LOOKING FOR. So it was possible, but not recommended. This turns from saving little bucks from using leftover disks, to massive headache later on. Thanks! – Rinaldo Jonathan Oct 11 '21 at 05:24
  • so, correct me if I'm wrong, but is the RAID I want to build from (RAID 0 2x 1TB), (RAID 0 4x500GB), and 2TB disk named RAID 05? – Rinaldo Jonathan Oct 11 '21 at 05:29
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    05 is something I made up consistent with the convention of the first digit being the array type of the physical disk. Contrast to RAID 10, which you will actually see in the real world: several mirror sets, then stripe across those. – John Mahowald Oct 11 '21 at 21:17