I have a 4-bay NAS with two 2TB hard drives in a RAID 1 configuration. I was planning to upgrade it with 2 additional 4TB drives to a RAID 10 configuration.
Now the new capacity I will receive with the additional 2 drives is 4TB and not 6TB. How come?
I thought RAID10 means stripe + mirror, so I had figured that it will stripe the 2TB + 4TB drive and mirror those to the other two drives.
A RAID calculator confirms this behavior. Is the reason that the RAID level tries to ensure that at any time two drives can fail and therefore the maximum size per volume is 2TB?
When I replace the two 2TB drives (one by one) with 4TB versions, will it be able to expand to 8TB?
Best Answer
If you stripe the 2TB and the 4TB drive you get 4TB capacity in your logical volume. If you mirror the 2TB and the 4TB drive you get 2TB capacity.
I don't know of any implementations that will allow you to use the mixed size drives the way you seem to want to. What NAS is it?
Yes, if you upgrade to 4x4TB drives you will get 8TB capacity in RAID 10.
Here's an exercise to illustrate. Suppose you have a 512Kb drive and a 1024Kb drive and you are using RAID 0 with 128Kb stripe size. You write the stripes alternately to the two disks. After writing 4 stripes to each disk, the smaller one is at capacity so you stop writing. Your array is at capacity. At this point you have 512Kb wasted space on the larger disk...