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I just received a HP Microserver. I've bought 2 2TB drives and I'm trying to RAID 0 them with the 250GB drive that the microserver comes with. When I RAID the 2 2TB drives together I get 4TB, but if I try and RAID all three, I get 750GB. This doesn't seem logical to me. Could someone explain this for me? Thanks

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migrated from stackoverflow.com Jun 25 '12 at 14:34

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The RAID 0 will only take the maximum capacity of the smaller of the disks :

2TB (considered as 250GB) + 2TB (considered as 250GB) + 250GB = 750 GB

Why ? Because the RAID 0 needs to split each data in 3 and give it to each disk at the same time, increasing the overal I/O speed . Thus, if you were writing the data over 750 GB, 2 chunk of the data could be written on the first 2 disks of 2TB, but there would be no space on the 250GB left for the system to write on it. And No, RAID 0 doesn't handle a multiple size array given if disk space is available (like, from 0 to 750GB, it would write 3x faster, and then, between 750GB and 4250GB it would write 2x faster.

The solution is

  • to buy another 2TB (or more, but as this case, if you buy a 3TB, only it's first 2TB would be used in that array ...) to replace the 250GB
  • keep the current 250GB if you want a x3 performance on the raid and don't want to buy another disk
  • let the raid with 2TB and 2 disks to have x2 faster performances and good size without buying a new disk

(x2, x3 are just theoritical IO perf increase, but it's around that in general)

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Another option is use something other than Raid 0 that allows different sized disks. Off the top of my head Drobo's BeyondRAID comes to mind, it allows you to mix and match disks. You wont get raid 0 performance, but it allows you to mix and match drives. – Scott Chamberlain Jun 25 '12 at 14:57
    
Yes, in fact, it is oftenly possible to use JBOD (Just a Bunch Of Disks) to aggregate as a big device. But you won't get any of raid 1 or 0 effects : no speed increase, no saves/mirroring. If one of the disk fail, you lose all the data on this disk. If the filesystem isn't too bad, you will lose the data that are between 2 underlying devices and no more. But you might have to scan the others devices to find the old files. – Dolanor Jun 26 '12 at 12:40

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