Planning the Available Capacity

The storage system stores service and system data. To ensure sufficient capacity for service data, the storage system capacity must be properly planned.

For details about the available capacity and purchased capacity, contact your local Huawei representative office or Huawei authorized distributor.

The evaluation results of the local Huawei representative office or Huawei authorized distributor supersede the considerations in this section.

The system data capacity is mainly consumed by file systems, hot spare space, and coffer disks. The actual available capacity for users is smaller than that provided by the storage system because the system consumes some capacities.

The capacities consumed by the storage system comprise several parts:

• Capacity used by parity data or mirrored data in a RAID group

  Table 3-1 lists the disk utilization of different RAID levels.

  Table 3-1 Disk utilization of different RAID levels

  RAID Level	Disk Utilization
  RAID 0	100%
  RAID 1	2Da: about 50% 4D: about 25%
  RAID 3	RAID 3 supports flexible configurations. Specifically, a RAID 3 policy allows data block and parity block policies ranging from 2D+1P to 13D+1P. The following examples show disk utilization of several configurations commonly used by RAID 3: 4D+1Pb: about 80% 2D+1P: about 66.67% 8D+1P: about 88.89%
  RAID 5	RAID 5 supports flexible configurations. Specifically, a RAID 5 policy allows data block and parity block policies ranging from 2D+1P to 13D+1P. The following examples show disk utilization of several configurations commonly used by RAID 5: 2D+1P: about 66.67% 4D+1P: about 80% 8D+1P: about 88.89%
  RAID 6	RAID 6 supports flexible configurations. Specifically, a RAID 6 policy allows data block and parity block policies ranging from 2D+2P to 26D+2P. The following examples show disk utilization of several configurations commonly used by RAID 6: 2D+2P: about 50% 4D+2P: about 66.67% 8D+2P: about 80% 16D+2P: about 88.89%
  RAID 10	50%
  RAID 50	(2D+1P) x 2: about 66.67% (4D+1P) x 2: about 80% (8D+1P) x 2: about 88.89%
  a: D indicates a data block. b: P indicates a parity block. NOTE: For a flexibly configured RAID policy xD+yP, the disk utilization is [x/(x + y)] x 100%.

• Capacity used by hot spare space

  To prevent data loss or performance deterioration caused by a member disk failure, a storage system uses hot spare space to take over data from the failed member disk. The following hot spare policies are supported:

  • High

    The capacity of one disk is used as hot spare space if the number of disks at a storage tier is equal to or fewer than 12. The hot spare space non-linearly increases as the number of disks increases.

  • Low

    The capacity of one disk is used as hot spare space if the number of disks at a storage tier is equal to or fewer than 25. The hot spare space non-linearly increases as the number of disks increases.

    Number of disks of which capacity is used as hot spare space in a low hot spare policy = Number of disks of which capacity is used as hot spare space in a high hot spare policy/2 (rounded up)

  • None

    The storage system does not provide any hot spare space. In the event a member disk in a disk domain fails, the storage system uses the free capacity in the disk domain for reconstruction. If the free capacity in the disk domain is insufficient, the storage system uses the unallocated capacity in storage pools for reconstruction. If reconstruction fails, the disk domain will change to the Degrade state, which will cause the read/write performance to deteriorate, affecting the storage system reliability.

  Table 3-2 describes how hot spare space changes for a single engine with the number of disks. The hot spare space changes at a storage tier are used as an example here. The hot spare space changes at different storage tiers are the same.

  Table 3-2 Changes of hot spare space for a single engine

  Number of Disks	Number of Disks of Which Capacity Is Used as Hot Spare Space in a High Hot Spare Policya	Number of Disks of Which Capacity Is Used as Hot Spare Space in a Low Hot Spare Policya
  (1, 12]	1	1
  (12, 25]	2
  (25, 125]b	3	2
  (125, 325]	4
  ……
  a: Huawei storage systems use RAID 2.0+ virtualization technology. Hot spare capacity is provided by member disks in each disk domain. Therefore, the hot spare capacity is expressed in the number of disks in this table. For example, if a disk domain is composed of 12 SSDs and the high hot spare policy is used, the hot spare space occupies the capacity of one SSD and the capacity is provided by member disks in the disk domain. If a disk domain is composed of 13 SSDs and the high hot spare policy is used, the hot spare space occupies the capacity of two SSDs. b: When the number of disks at a storage tier reaches 125, the storage tier uses the capacity of one disk in every 200 additional disks as the hot spare space in a high hot spare policy.

  • Number of Disks in the above tables refers to the number of same-type disks owned by a same engine. If you select disks from multiple engines to create a disk domain, calculate the number of disks used for hot spare space on each engine and sum up the values.

  • When you are creating a disk domain, ensure that the disks used to provide hot spare space are sufficient.
  • Hot spare space can be used for a specific disk domain only.
  • Common capacity changes of the hot spare space are listed in this section. The number of disks supported by a storage system and the capacity of their hot spare space are based on actual specifications.

• Capacity used by coffer disks

  Part of the coffer disk space is used to store critical system data, including configuration data and system logs. The rest of the coffer disk space is used to store service data.

  For details about capacity used by coffer disks, see "Coffer Disk" in the product description specific to your product model and version.

• Capacity used by file systems and volume management software on the application server

  File systems and volume management software on the application server may occupy capacities in the storage system. The actually occupied capacities depend on the deployment of applications on the application server.

• WriteHole capacity

  WriteHole is used to resolve inconsistent data stripe verification caused by certain operations before I/Os are delivered to disks. Each disk reserves a 256 MB space as WriteHole capacity.

• Capacity used by system information

  The system information occupies 577 MB per disk.

• Metadata capacity

  Each disk reserves 0.6% of its total capacity as metadata capacity, and reserves 2% as metadata backup capacity.

• Capacity reserved for improving system performance and disk balance

  Each disk reserves 1% of its total capacity to improve system performance and disk balance. When 1% of the disk total capacity is smaller than 2 GB, 2 GB capacity is reserved.

• Integrated capacity

  When disks are being formatted, if the size of a sector is 520 bytes, the sector uses 8 bytes to store parity data. If the size of a sector is 4160 bytes, the sector uses 64 bytes to store parity data. The integrated capacity usage is about 98.46% (512/520 or 4096/4160).

Without considering the hot spare capacity consumption, you can use the following formula to calculate RAID 2.0+ disk capacity usage: RAID 2.0+ disk capacity usage = [1 – Metadata space – (1 – Metadata space) × Metadata backup space] × (1 – Disk space reserved for load balancing) x Integrated capacity usage = [1 – 0.6% – (1 – 0.6%) × 2%] × (1 – 1%) × 98.46% ≈ 94.95%

The disk capacity defined by disk manufacturers is different from that calculated by operating systems. As a result, the nominal capacity of a disk is different from that displayed in the operating system.

• Disk capacity defined by disk manufacturers: 1 GB = 1,000 MB, 1 MB = 1,000 KB, 1 KB = 1,000 bytes.
• Disk capacity calculated by operating systems: 1 GB = 1,024 MB, 1 MB = 1,024 KB, 1 KB = 1,024 bytes.

Formulas are for reference only. The disk capacity displayed on DeviceManager is considered most accurate.

Available Capacity Calculation Method

The following uses an example to explain how to calculate the allowed available capacity. Three valid digits are retained after the decimal point.

Assume that forty-eight 600 GB SAS disks will be added to the storage system, including four coffer disks and the hot spare policy and RAID policy are configured to Low and RAID 6 (8D + 2P) respectively. The allowed available capacity is calculated as follows:

1. 600 GB is the nominal capacity provided by the disk manufacturer. Use the following method to convert this capacity to one that can be identified by the storage system:

   600 GB x 1000 x 1000 x 1000/1024/1024 = 572204.590 MB

2. Minus the WriteHole capacity:

   572204.590 MB – 256 MB = 571948.590 MB

3. Minus the capacity used by system information:

   571948.590 MB – 577 MB = 571371.590 MB

4. Minus the metadata capacity:

   571371.590 MB x (1 – 0.6%) = 567943.361 MB

   The storage system reserves 0.6% of each disk's space as metadata space. It dynamically allocates metadata space as services increase. The actual services prevail. In this example, 0.6% is used.

5. Minus the metadata backup capacity:

   567943.361 MB x (1 – 2%) = 556584.494 MB

6. Minus the capacity reserved for improving system performance and disk balance:

   556584.494 MB x (1 – 1%) = 551018.649 MB

7. Minus the integrated capacity:

   551018.649 MB x 98.46% = 542532.962 MB

8. Minus the hot spare space capacity. Because the hot spare policy of the storage system is set to Low, capacity of two disks is used as hot spare space capacity. Therefore, the remaining capacity is as follows after the hot spare space capacity is deducted:

   542532.962 MB x (48 – 2) = 24956516.250 MB

   Equals to 24956516.250 MB/1024/1024 = 23.800 TB

9. Minus the capacity used by coffer disks:

   23.800 TB – 4 x 5 GB = 23.781 TB

   In this example, the capacity used by each coffer disk is calculated as 5 GB. For details about capacity used by coffer disks, see "Coffer Disk" in the product description specific to your product model and version.

10. Because the RAID policy of the storage system is RAID 6 (8D+2P), the disk utilization is 80%. Therefore, the allowed available capacity is:

    23.781 TB x 80% = 19.025 TB

In this example, the allowed available capacity is 19.025 TB.

The preceding available capacity is for reference only. The capacity displayed on DeviceManager prevails.

The evaluation results of the local Huawei representative office or Huawei authorized distributor supersede the considerations in this section.