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I have a NAS (Dlink Sharecenter DNS320).

I have 1 hard drive at the moment - 1 TB.

I will receive a 2TB hard drive later this week.

If I set the NAS with 2 drives (RAID), 1 TB and 2 TB I'll get 1 TB of space which is mirrored across the two drives right?

So, if I then went out and bought a 2 TB hard drive to replace the 1 TB hard drive will I have to wipe and repartition the hard drives? Or will I be able to just swap the 1 TB for the 2 TB and magically have 2 TB of space?

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  • You think you might have a total of 1 TB of data if you set up 3 TB's worth of hard drives in RAID? Let me doubt that. RAID won't be "wasting" space. Even if it does total data redundance and as such 1 TB of actual data takes 2 TB, you still have 1 TB left. And if you're using RAID 0, as far as I know, you won't be losing any space at all. The drives will just be "fused", looking like you have one 3 TB hard drive, while probably increasing their performance.
    – Ariane
    Commented Jan 8, 2013 at 16:24

2 Answers 2

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It depends on how the RAID is set up.

Some, including many hardware cards will do this:

  • Stripe (RAID 0) over both drives, just concattenating both drives: 3 TB usable.
  • Stripe (RAID 0) over both drives: Using 1TB on both drives: 2 TB usable.
  • Mirror (RAID 1) over both drives: 1 TB usable.

Some (such as mdadm RAID using in Linux and on many SOHO NAS devices) allow you to do this:

1 TB drive:    1TB data in mirror
2 TB drive:    1TB data in mirror    +  1 TB data as just a simple volume

I know your NAS does both striping and mirroring, but not the precise details.

[Edit]

1) Since the default formatting is in ext3 it is probably using some form of GNU/Linux.

2) Page 32 of the manual seems to allow using part of a drive as part of a mirror.

enter image description here

[Edit 2]

So, if I then went out and bought a 2 TB hard drive to replace the 1 TB hard drive will I have to wipe and repartition the hard drives? Or will I be able to just swap the 1 TB for the 2 TB and magically have 2 TB of space?

If you replace the old drive: Yes, you will have to repartition and you will have 2 TB.

If you just add the second drive in addition to the existing 1TB drive, then you can:

  1. Add second volume so you have a 1TB and a 2TB volume on the NAS.
  2. Or backup all data, reformat the NAS as a 1TB mirror and a 1TB JBOD.
    (Then copy all data back. The data on the mirror should survive even if a single drive fails).
  3. Or backup the data. Add the second drive and configure it as a 3TB stripe. This will be faster, but you will loose all data if any drive fails. And the speed gain is probably negligible because the network will be the slowest link.
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  • This seems pretty concise and useful. Thank you! :) I think I understand now.
    – Thomas Clayson
    Commented Jan 9, 2013 at 10:18
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The first thing to know is which RAID do you want to use:

1) RAID 0 (Striping): It provides improved performance and additional storage but no fault tolerance. In this case, you will end with a 3TB volume. If you wanted to swap in the future your 1TB HDD for a 2TB HDD, you will need to copy the information, since the RAID won't replicate itself.

2) RAID 1 (Mirroring): Data is written identically to two drives, thereby producing a "mirrored set". In this case, you will end with a 1TB volume. If you wanted to swap in the future your 1TB HDD for a 2TB HDD, the RAID should be able to replicate the information of the 2TB HDD in the new one and you could also expand the current volume from 1TB to 2TB.

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  • Ah I didn't realise there were two versions. I was talking about RAID 1 where there is redundancy. You're saying that I should be able to just expand the volume to 2tb if there is 2tb available?
    – Thomas Clayson
    Commented Jan 9, 2013 at 10:15
  • No. If you mirror then you are keeping two copies on disk. One copy on each disk. And because of that you are limited to the size of the smallest disk. --- This link explains it all with nice pictures: serverfault.com/questions/339128/…
    – Hennes
    Commented Jan 9, 2013 at 13:53

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