RAID5 Frequently Asked Questions

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Various Questions

RAID 5 Capabilities

Is it possible to create a RAID 5 using removable hard drives?

RAID 5 on removable hard drives is considered a bad idea, although it is not quite clear as to why. In Linux or MacOS, it is possible to build a RAID (using mdraid driver) from whatever media you want including USB floppy drive. Windows requires the use of dynamic disks to build a RAID, and that's where the problem is - removable hard drives cannot be dynamic. So it is impossible to create a RAID on disks connected via USB in a regular Windows installation.

However, there is a patch which deceives Windows and represents removable hard drives as non-removable. Then you can feed Windows these "non-removable" disks and create a RAID.

If I have a RAID 5 which doesn't contain an operating system, what will happen if I reinstall operating system on a PC to which the RAID is connected?

In case of a hardware RAID, the array data becomes accessible as soon as all the required drivers are installed.

In case of a software RAID:

  • in Linux driver mdraid can often recognize the array automatically. Sometimes it is needed to enter the command
    sudo mdadm --assemble --scan
  • in Windows with dynamic disks it is sometimes required to open Disk Management and use Import Foreign Disks, and the array will then be accessible. The same actions are necessary when transferring the array between the PCs.

Can I add disks to a RAID5 array?

The answer depends on a particular implementation. Some high-end controllers and mdraid (and therefore, most NASes) allow to change a size, an array type, and the number of disks without losing data. This process is called reshaping. Generally, it is required to backup the original data, configure a new array, and then restore data from the backup.

If I take out one of the member disks from the RAID5 on-the-fly, what would happen? If I then put the disk back to the array, would the array work as before?

If you pull the disk out from the RAID5 on-the-fly, array will still continue to work, but the performance will degrade. If you return the disk back to the array after a while, the rebuild process is initiated. During the rebuild all the data on the returned disk is overwritten with new parity and data blocks.

In this situation, rebuild is needed because there is a discrepancy between the array and the removed disk. Since it is not possible to identify what exactly is mismatched, rebuild of the entire array is needed.

Is it possible to expand the existing RAID 5 array should the need arise?

If it is not required to add more disks to the array, you can just replace the disks one by one with new larger disks and rebuild the array after each replacement. Once all the old RAID 5 member disks are replaced with the new ones, you can expand the filesysten into the unallocated space.

See also

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