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TB Hard drives in Synology Disk Station DS409

Posted on 2009-06-29
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Last Modified: 2013-11-14
Hello experts,

I have purchased a synology ds409 from newegg.com and four 1.5 TB hard drives.

The hard drives show up as 1.36TB, and when i formatted them into a single RAID5 volume, i only have 4TB total on the volume.

Where did the other 2TB of space go? i know some went to the raid config, but why does only 1.36 show up? That's like 140 x 4 = 560GB's.. thats a ton of drive space that is missing.
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Question by:jaysonfranklin
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by:simsjrg
ID: 24738296
That's about right then. Essentially with RAID 5 you get the combined total drive capacity -1 disk so...

4 x 1.5TB = 6TB - 1.5TB = 4.5TB. This is the RAW capacity so the ~4TB formatted capacity you are seeing is correct.
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by:jaysonfranklin
ID: 24738722
So RAID 5 uses 1 disk for the striping and parity, but the space for this is distributed over an entire disk?

why is the raw capacity for a 1.5 tb drive only 1.36? That's 140GB which is a lot of space and more than my internal hdd on my computer..
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simsjrg earned 500 total points
ID: 24738836
>> So RAID 5 uses 1 disk for the striping and parity, but the space for this is distributed over an entire disk?

Not exactly... the parity is spread over all of the drives that if any one drives fails you can recover without and data loss.

>> why is the raw capacity for a 1.5 tb drive only 1.36? That's 140GB which is a lot of space and more than my internal hdd on my computer..

This goes back to the fact that manufacturers sell drives and market a GB as being 1000MB when in fact it's 1024MB. I believe it's somewhere in the market of ~74MB or ~75MB you "lose" per GB.

I wish I could find the site to reference for that...
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Expert Comment

by:simsjrg
ID: 24747758
Anything else we can help you with regarding this?
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by:jaysonfranklin
ID: 24749465
guess not.. didn't know if you were still trying to locate the link. No worries. Thanks a bunch for your help.
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by:oidz
ID: 25078170
Another reason for the loss of space is that the Synology NAS needs to install it's 'OS' onto the drives in order for it to function. This creates a series of partitions which swallows up a reasonable amount of space. It's usually around the 2GB per disk level, so you would be looking at losing another 10GB or so there.
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