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Products such as Synology offer something called Synology Hybrid RAID

http://www.synology.com/us/products/features/RAID.php

This RAID type allows you to make best use of your disks available by using all the disk space available as long as at least two disks share the same increased size where a typical RAID setup would simply "throw away" the extra space

I would like to build a NAS with 4 disks available. I will begin by populating it with 3 X 3TB to give me 6TB usable. By the time I have filled this 6TB I imagine that 4TB disks will have come down in price, so at this stage I would add a 4th 4TB disk to give me an additional 3TB of space. When I next run out of space I will change one of the original 3TB disks with a 4TB disk giving me an additional 1TB of space.

This is not possible with a typical RAID configuration, only with these "hybrid RAID" types

I am wondering if I can acheive a similar "hybrid RAID" with Ubuntu? or another linux distro?

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I am wondering if I can acheive a similar "hybrid RAID" with Ubuntu? or another linux distro?

Yes, you can using a combination of LVM and software raid (md).

I will begin by populating it with 3 X 3TB to give me 6TB usable.

OK, so you start with 3 disks in RAID 5 giving you 6 TB.

so at this stage I would add a 4th 4TB disk

On this, create TWO partitions - 3TB and 1TB. As you stated, this will initially give a 4-disk RAID5 with 9 TB of space.

When I next run out of space I will change one of the original 3TB disks with a 4TB disk giving me an additional 1TB of space.

Again, on this disk, create two partitions of 3TB and 1TB. Rebuild the RAID5 array onto the 1st partition. Then use mdadm to create a RAID-1 array from the two 1TB partitions on disks 3 and 4. Add this array to your logical volume, and voila! you have 10 TB of redundant storage.

Rinse and repeat :)

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thanks, can I horizontally expand the RAID 1 array when I get a fouth 4TB disk into a second RAID 5 array? –  stueng Jun 4 '12 at 15:54
    
you can convert RAID 1 to 5 or 5 to 6 if needed with mdadm. Also, RAID 5 only needs 3 disks. You can also add an additional disk to increase the number of drives in a RAID 1/5/6 with mdadm, though extra drives in a raid 1 is simply another mirror, but for raid levels 5 and 6 they give you extra storage when you add the extra disk. –  BeowulfNode42 Nov 7 '13 at 10:51
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