Disk Types and Performance¶
EVS disks are classified based on the disk I/O performance. EVS disks differ in performance and price. You can choose whichever disk type that is the best fit 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.
High I/O: This type of EVS disks delivers a maximum IOPS of 3,000 and a minimum read/write latency of 6 ms. They are designed to meet the needs of mainstream high-performance, high-reliability applications, such as enterprise applications, large-scale development and test environments, and web server logs.
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.
Note
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¶
EVS performance metrics include:
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
Parameter | Common I/O | High I/O | General Purpose SSD | Ultra-high I/O | Extreme SSD |
|---|---|---|---|---|---|
IOPS per GiB/EVS disk | 1 | 2 | 12 | 50 | 50 |
Min. IOPS/EVS disk | 100 | 100 | 1,800 | 100 | 1,800 |
Max. IOPS/EVS disk | 1,000 | 3,000 | 20,000 | 20,000 | 128,000 |
Disk IOPS | Min. (1,000, 100 + 1 x Capacity) | Min. (3,000, 100 + 2 x Capacity) | Min. (20,000, 1,800 + 12 x Capacity) | Min. (20,000, 100 + 50 x Capacity) | Min. (128,000, 1,800 + 50 x Capacity) |
IOPS burst limit/EVS disk | 1,000 | 3,000 | 8,000 | 10,000 | 64,000 |
Max. throughput | 90 MiB/s | 150 MiB/s | 250 MiB/s | 350 MiB/s | 1,000 MiB/s |
Disk throughput (MiB/s) | 50 | Min. (150, 100 + 0.15 x Capacity) | Min. (250, 100 + 0.5 x Capacity) | Min. (350, 120 + 0.5 x Capacity) | Min. (1,000, 120 + 0.5 x Capacity) |
I/O read/write latency (single-queue) | 10 ms to 15 ms | 6 ms to 10 ms | 1 ms | 1 ms to 3 ms | Sub-millisecond |
EVS disk performance is closely related with the data block size:
If data blocks are all the same size, a disk can achieve either the maximum IOPS or maximum throughput depending on which one is reached first.
If data blocks are of different sizes, the maximum performance metric that a disk can achieve varies:
For small data blocks, such as 4 KiB or 8 KiB, a disk can reach the maximum IOPS.
For data blocks of a large size, 16 KiB or greater, a disk can reach the maximum throughput.
Table 2 uses an ultra-high I/O disk as an example. In theory, when the size of an ultra-high I/O disk is at least 460 GiB, the disk theoretically can reach either the maximum IOPS 20,000 or the maximum throughput 350 MiB/s. However, this is not the case in practice. The maximum IOPS and maximum throughput that a disk can reach also vary with the data block size.
Data Block Size (KiB) | Max. IOPS | Max. Throughput (MiB/s) |
|---|---|---|
4 | About 20,000 | About 78 |
8 | About 20,000 | About 156 |
16 | About 20,000 | About 312 |
32 | About 11,200 | About 350 |
Disk IOPS Calculation Formula¶
Disk IOPS = Min. (Maximum IOPS, Minimum IOPS + IOPS per GiB x Capacity)
Take an ultra-high I/O EVS disk with a maximum IOPS of 20,000 for example.
If the disk capacity is 100 GiB, the disk IOPS is calculated as follows: Disk IOPS = Min. (20,000, 100 + 50 x 100)
The disk IOPS is 5,100, the smaller of the two values (20,000 and 5,100).
If the disk capacity is 1,000 GiB, the disk IOPS is calculated as follows: Disk IOPS = Min. (20,000, 100 + 50 x 1,000)
The disk IOPS is 20,000, the smaller of the two values (20,000 and 50,100).
Disk Burst Capability and Principles¶
EVS disks have a burst capability. A small-capacity disk can surpass its official maximum IOPS for a short period of time. This IOPS applies to each disk individually.
Disks with burst capability are well-suited for speeding up server startup. In most cases, system disks are fairly small, so their basic IOPS is fairly low. For example, the IOPS of a 50-GiB ultra-high I/O disk without burst can only reach up to 2,600 IOPS (Min. (20,000, 100 + 50 x Capacity)). But with burst capability, its IOPS can burst up to 10,000.
The following example uses an ultra-high I/O EVS disk with the IOPS burst limit of 10,000.
If the disk capacity is 100 GiB, the disk has a maximum IOPS of 5,100, but it can burst to 10,000 IOPS in a certain duration.
If the disk capacity is 1,000 GiB, the disk has a maximum IOPS of 20,000. The disk maximum IOPS already exceeds its burst IOPS 10,000, 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
In the following example, a 100-GiB ultra-high I/O EVS disk is used, and the fixed burst duration is 1800s. Therefore, the number of initial tokens is 18,000,000 (1,800 x 10,000).
Token production rate: This rate equals the disk maximum IOPS, which is 5,100 tokens/s.
Token consumption rate: This rate is based on the I/O usage. Each I/O request consumes a token. The maximum consumption rate is 10,000 tokens/s, which is the larger value of the disk burst IOPS and the maximum IOPS.
Consumption principles
When tokens are consumed faster than they are produced, 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 3,673 seconds [18,000,000/(10,000 - 5,100)].
Reservation principles
When tokens are consumed more slowly than they are produced, the number of tokens increases accordingly, and the disk regains burst capability. In this example, if the disk is suspended for approximately 3,529 seconds (18,000,000/5,100), the token bucket will be filled up with tokens.
Note
As long as there are tokens in the token bucket, the disk has the burst capability.
Figure 1 shows the token consumption and reservation principles. The blue bars indicate the disk IOPS usage, the green dashed line represents the maximum IOPS, the red dashed line indicates the IOPS burst limit, and the black curve indicates the changes of the number of tokens.
As long as there are tokens, the disk IOPS can exceed 5,100 and can burst to up to 10,000, the IOPS burst limit.
When there are no more tokens, the disk loses the burst capability, and the disk IOPS can reach up to 5,100.
Any time the disk IOPS is less than 5,100, the number of tokens starts to increase, and the disk regains the burst capability.
Figure 1 Burst capability diagram¶
Performance Testing¶
For details about how to test the EVS disk performance, see How Do I Test My Disk Performance?.