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# Why array space decrease in S6800V2 storage

Publication Date:  2015-01-30 Views:  49 Downloads:  0
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:
Metadata capacity losses: OceanStor metadata capacity accounted for 5.2%;
Each disk 1% space is reserved to even disks (make sure all present disk space can be used).
Total Domain Capacity = 475816*(1-5.2%)*(1-1%) ≈ 446511 GB
3. Total capacity loss
Total capacity Loss = Device total capacity - Total Domain Capacity  = 475816-
PS: Is OceanStor storage Raid2.0+ disk utilization very low?
Huawei RAID2.0 + technology uses two layers of virtualization software architecture, in order to facilitate flexible and efficient data management, set aside a certain capacity (accounting for only about 5%), but it is often misunderstood as disk utilization not high, while ignoring the storage efficiency for users meaningful. It is by means of the unique and innovative architecture, Huawei storage to achieve the industry's highest storage efficiency. For example, by means of a data block as a unit of resource management, global load balancing and fast reconstruction, remodeling rate up to 20 times; the upper virtualization software through Smart Series intelligent allocation of system resources, greatly enhance resource utilization .
For example, SAS hard disk using RAID5 (8D + 1P) of RAID policy, the capacity utilization rate is about 83.42%, only 5% lower than the 88.89% of traditional RAID.

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