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I'm looking to buy a NAS, however am confused about scalability. I dont understand how a 4 disk NAS can have a max capacity of 2tb? why cant i replace the drives with 1tb drives? Or 2tb drives when they come out?

edit - thanks for the answers! i'm just investigating at the moment. Here is an example http://www.qnap.com/pro_detail_hardware.asp?p_id=108.

"HDD - 6 x 3.5" SATA I/II HDD, (up to 1.5TB HDD, the standard system is shipped without HDD)"

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I can see 2 problems here:

First possible problem:

The nas comes with 4 disks of 1 TB configured with RAID10 (or any other drive RAID combination that gives you 2 TB total storage) and isn't able to resize the RAID array size when larger disks are added run, don't walk away from the product - then there's nothing you can do about it but a host based migration

  • copy files away from the NAS
  • replace all disks
  • completeley re-initiate the RAID with the new drives

Second possible problem:

The device is perfectly capable from a feature point of view but has only certain disks supported, the largest supported disks available yield a config of a max. of 2 TB only. "Supported" in this context means disks listed on the compatibility list of the vendor, not actually supported from a technical point of view.

Another Guess:

Marketing speech. Often marketing will release a product with a folder which says "Up to n TB of storage supported" most times this refers only to the maximum disk size available at the time of the writing, but the device doesn't have a:

  • missing feature preventing you from actually migrating RAID levels
  • a hardware compatibility list preventing you from using larger disks (in terms of accepted support cases)

Can you update your question with the actual error message or are you right now just investigating options and the limits are from a product fact sheet?

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The problem is with address sizes. RAID controllers with 48-bit addressing only have enough physical numbers to allocate 2TB per array. Often you can get around this by creating multiple arrays on the same controller.

Newer (and more expensive) controllers have 60-bit addressing, which can make much larger arrays (but one day we'll run into issues that 60-bit is not big enough, and we'll go around in circles again).

Good news is that a 1TB drive is actually only 938gb (or thereabouts) so you can very safely install two of them without actually hitting the barrier.

The workaround is, if the controller supports it, create multiple 2TB arrays. We have a controller that has three 2TB RAID-5 arrays to span all 6TB of storage. But some NAS devices might not give you this option, in which case there's not much you can do...

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2TB disks are out, but I suspect that the maker of the NAS you're thinking of may not have certified their box for >500GB disks. There are lots of NAS box makers that support 1/1.5/2TB disks, go for one of them (I like Thecus myself)

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Also the file system on the device probably only supports 2 TB volumes as most 32bit platforms only support 2 TB volumes.

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NetGear purchased the Infrant's ReadyNAS line a couple of years ago and have been improving it ever since. Their ReadyNAS Pro line supports excellent expansion and performance. We're really liking their product line now.

http://www.netgear.com/Products/Storage.aspx?for=All

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