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I was wondering if one could set up a RAID-5 array such that only some space of the third disk is used in the raid array and the left over space can be used as a non-raid volume.

I have three HDDs with 2TB, 2TB and 3TB. The 1TB on Disk 3 should still be usable but not redundant. (on an ubuntu server)

The following image shows what I'm trying to achieve: Raid Setup (orange = raid; green & blue = normal Volumes)

Is this possible or can disks which are part of a raid-array in their entirety only be used within the raid?

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Firstly don't use RAID 5 at all, it's very well documented how dangerous it is, especially with large slow, especially consumer, HDD's - it's been considered dangerous for over a decade now and it frankly upsets me seeing it still being an option on disk controllers and even in software.

Anyway what you want to achieve isn't usually possible with hardware disk controllers, you could probably make software RAID work like this but it wouldn't be advisable, I'm sure it'd be fragile and hard to rebuild when the inevitable disk failure happens.

Just buy the extra disks to run this as a R6 or R10, knowing you have a supportable and stable platform.

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  • That, and performance of the RAID will suffer greatly when an access to the second "partition" moves the heads away without the RAID controller knowing -- it will still schedule reads to that disk as normal, and wait for data to come back in a timely manner. Just ignore the wasted space, you'll get it back when the other disks have failed and been replaced with larger disks.
    – Simon Richter
    Dec 14, 2021 at 14:39
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Its possible with ZFS software raidz(use pass through or/jbod) - create two partition(eg. part1-2T & part2-1T) on 3T disk, and use part1 + remaining 2x2T to create raidz pool. warning: raidz has only fault tolerance level 1 & performance issue arise if system gets io heavy on both partitions of 3T device

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