Disk Types and Performance
EVS disks are classified based on the disk I/O performance. EVS disks differ in performance and price. Choose the disk type most appropriate for your applications.
Application Scenarios
- Common I/O: This type of EVS disks delivers a maximum IOPS of 1,000. They are suitable for applications that require large capacity, a medium read/write speed, and fewer transactions, such as enterprise office applications and small-scale test environments.
- Ultra-high I/O: This type of EVS disks delivers a maximum IOPS of 20,000 and a minimum read/write latency of 1 ms. They are excellent for read/write-intensive applications that require super-high I/O and bandwidth, such as distributed file systems in HPC scenarios or NoSQL/relational databases in I/O-intensive scenarios.
- Extreme SSD: This type of EVS disks delivers a maximum IOPS of 128,000. They are designed for workloads demanding super-high bandwidth and super-low latency, such as Oracle databases and AI applications.
- General Purpose SSD: This type of EVS disks delivers a maximum IOPS of 20,000. They are suitable for workloads requiring high throughput and low latency, such as enterprise OA, development and testing, web server logging, containers, and high-performance system disks.
If an Extreme SSD disk is attached to a BMS, it can reach a maximum IOPS of 128,000. If it is attached to an ECS, it can reach a maximum IOPS of 100,000 due to I/O queue limitations.
EVS Performance
- IOPS: Number of read/write operations performed by an EVS disk per second
- Throughput: Amount of data read from and written into an EVS disk per second
- Read/write I/O latency: Minimum interval between two consecutive read/write operations on an EVS disk
Calculating Disk IOPS Limit
To calculate the IOPS limit of a disk, you need to obtain the smaller value of the following two values:
- Max. IOPS/EVS disk
- Baseline IOPS/EVS disk + IOPS per GiB x Disk capacity
- If the disk capacity is 100 GiB, the disk IOPS limit is calculated as follows:
Disk IOPS limit = Min. (50,000, 1,800 + 50 x 100)
The disk IOPS limit is 6,800, the smaller value between 50,000 and 6,800.
- If the disk capacity is 1,000 GiB, the disk IOPS limit is calculated as follows:
Disk IOPS limit = Min. (50,000, 1,800 + 50 x 1,000)
The disk IOPS limit is 50,000, the smaller value between 50,000 and 51,800.
Disk IOPS Calculation Formula
Disk IOPS = Min. (Maximum IOPS, Minimum IOPS + IOPS per GiB x Capacity)
If the disk capacity is 100 GiB, the disk IOPS is calculated as follows: Disk IOPS = Min. (50,000, 1,800 + 50 × 100)
The disk IOPS is 6,800, the smaller value between 50,000 and 6,800.
If the disk capacity is 1,000 GiB, the disk IOPS is calculated as follows: Disk IOPS = Min. (50,000, 1,800 + 50 × 1,000)
The disk IOPS is 50,000, the smaller value between 50,000 and 51,800.
Disk Burst Capability and Principles
EVS disks have burst capability, which allows a small-capacity disk to surpass its maximum IOPS within a certain period of time. This IOPS applies to individual disks.
Disks with burst capability are well-suited for speeding up server startup. In most cases, system disks have small capacities. For example, the IOPS of a 50-GiB ultra-high I/O disk without burst capability can only reach up to 4,300, calculated as follows: IOPS = Min. (50,000, 1,800 + 50 x Capacity). If the disk has burst capability, its IOPS can burst up to 16,000.
- If the disk capacity is 100 GiB, the disk has a maximum IOPS of 6,800, but it can burst to 16,000 IOPS in a certain duration.
- If the disk capacity is 1,000 GiB, the disk has a maximum IOPS of 50,000. The disk maximum IOPS already exceeds its burst IOPS 16,000, and the disk does not use the burst capability.
The following describes the burst IOPS consumption and reservation.
A token bucket is used to handle burst I/O operations. The number of initial tokens in the bucket is calculated as follows:
Number of initial tokens = Burst duration x IOPS burst limit
- Token production rate: This rate equals the disk maximum IOPS, which is 6,800 tokens/s.
- Token consumption rate: This rate is calculated based on the I/O usage. Each I/O request consumes a token. The maximum consumption rate is 16,000 tokens/s, which is the larger value between the disk burst IOPS and maximum IOPS.
Consumption principles
When the token consumption rate is greater than the production rate, the number of tokens decreases accordingly, and eventually the disk IOPS will be consistent with the token production rate (the maximum IOPS). In this example, the disk can burst for approximately 3,130 seconds [28,800,000/(16,000 - 6,800)].
Reservation principles
When the token consumption rate is smaller than the production rate, the number of tokens increases accordingly, enabling the disk to regain the burst capability. In this example, if the disk is suspended for approximately 4,235 seconds (28,800,000/6,800), the token bucket will be filled up with tokens.
As long as there are tokens in the token bucket, the disk has the burst capability.
- When the number of tokens is greater than zero, the disk IOPS can exceed 6,800 and has the capability to reach 16,000, the IOPS burst limit.
- When the number of tokens is zero, the disk does not have the burst capability, and the maximum IOPS is 6,800.
- When the disk IOPS is less than 6,800, the number of tokens starts to increase, and the disk can regain the burst capability.
Performance Test Method
For details about how to test the EVS disk performance parameters, see How Can I Test My Disk Performance?.
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