Why is the disk size of NAS reduced after RAID configuration completes?
Posted by Alan Huang on 02 February 2012 02:38 PM
Depends on the RAID type, the available space is different. 
* When configuring two disks as RAID-1, the disk size usable is equal to the size of one disk.
* When configuring N disks as RAID-5, the disk size usable is equal to the size of (N-1) disks.
* When configuring N disks as RAID-6, the disk size usable is equal to the size of (N-2) disks.
Hard disk manufacturer calculates disk size as 1M=1000,000 while computer calculates actual disk capacity as 1M=1024x1024. When a manufacturer marks a disk size as 2TB (disk size: 2TB=2,000GB=2,000,000MB=2,000,000,000KB=2,000,000,000,000), the actual usable disk capacity could be 2,000,000,000,000/1024/1024/1024/1024 = 1.81TB. Therefore when configuring 8x 2TB disks as RAID 5, the total capacity will be 1.81TB x (8-1)=12.73TB.
QNAP NAS store configuration and system files on the hard disk. It will take about 1.5GB storage space on each disks.
After formatted with ext3/ext4 file system, the file system take about 1% to 2% of total volume space. 

Applied Models: All NAS Series
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Comments (1)
01 May 2012 01:01 AM
We had this come up in tech support in #centos two weeks ago and spent 6 hours nurnsig a user [tasked to be a sysadmin by his PHB] through several single point of failure' errors. I meant to write about it still need to, I guess, as there are lots of OTHER ways to hose a network and lose access to dataBest regards Russ