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Why array space decrease in S6800V2 storage

Issue Description
OceanStor S6800T has 56 600G SAS Disks, 120 4000G NLSAS disks, 12 600G HSSD. All drives are included to disk domain. Customer find that Device total capacity is 475816 GB, Total Domain Capacity is 446511 GB, Hot spare Space using Low Policy is 20139 GB.
Customer doubts that there are 9TB unused space: Calculating Total Capacity –Domain Capacity –Hot spare = 475816 - 446511 - 20139 =9166 (GB).
Customer asked to clarify, for what purpose that 9TB space used.
Solution
Domain capacity includes hot spare space. The capacity loss is not 9 TB but 28.6TB
1. Device total capacity
Total disks capacity is 56*600+120*4000+600*12=520800 GB
Loss of available hard disk capacity in OS and nominal capacity: hard drive manufacturers generally is in accordance with 1000 as the unit of progress (1KB = 1000byte, 1MB = 1000KB, 1GB = 1000MB), while the operating system is in accordance with the 1024 as a unit of Progress (1KB = 1024byte, 1MB = 1024KB, 1GB = 1024MB), and therefore the nominal capacity of the hard drive manufacturers converted to the operating system becomes smaller.
Then the total capacity is 520800*1000*1000*1000/1024/1024/1024=520800*93.13%=488145GB
Integration Capacity: Hard disk can be formatted as a sector size 520Byte or 512Byte, which is usually called the 520 disk and 512 disk. The same nominal size of the disk, after formatting, 520 disk spaces is different from 512 disk, the former space after formatting approximately 98.4% of the latter.
Then the total capacity is 488145*512/520=480635.
Reserved space: Each disk will reserve 512MB as front reserved space and 64MB back reserved space
Writehole space: To strengthen the raid data reliability, each disk individually 256M reserved space to ensure the reliability of business data.
Coffer Space: When the hard disk as a coffer disk, will use part of the space to store part of the business class data and configuration data (for 18000 and V3 speaking, this part of the space for 5G).
Then the total capacity is 480479-(Reserved space + Writehole space + Coffer space) ≈475816 GB
2. Total Domain Capacity
After all disks join as one disk domain, there are Raid2.0+ loss: