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.
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
Parameter |
High I/O |
Ultra-high I/O |
---|---|---|
IOPS per GiB/EVS disk |
6 |
50 |
Max. IOPS/EVS disk |
5,000 |
33,000 |
Baseline IOPS/EVS disk |
1,200 |
1,500 |
Disk IOPS |
Min. [5,000, 1,200 + 6 x Capacity (GiB)] |
Min. [33,000, 1,500 + 50 x Capacity (GiB)] |
IOPS burst limit/EVS disk |
5,000 |
16,000 |
Max. throughput |
150 MiB/s |
350 MiB/s |
API name
NOTE:
This API name is the value of the volume_type parameter in the EVS API. It does not represent the type of the underlying hardware device. |
SAS |
SSD |
Typical scenarios |
Mainstream applications requiring high performance and high reliability, such as large-scale development and test environments, web server logs, and enterprise applications. Typical enterprise applications include SAP applications, Microsoft Exchange, and Microsoft SharePoint. |
Read/write-intensive workloads that demand ultra-high I/O and throughput, such as distributed file systems used in HPC scenarios or NoSQL and relational databases used in I/O-intensive scenarios. Typical databases include MongoDB, Oracle, SQL Server, MySQL, and PostgreSQL. |
Calculating Disk IOPS Limit
To calculate the IOPS limit of a disk, obtain the smaller value of the following two values:
- Max. IOPS/disk
- Baseline IOPS/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. (33,000, 1,500 + 50 x 100)
The disk IOPS limit is 6,500, the smaller of the two values (33,000 and 6,500).
- If the disk capacity is 1,000 GiB, the disk IOPS limit is calculated as follows:
Disk IOPS limit = Min. (33,000, 1,500 + 50 x 1,000)
The disk IOPS limit is 33,000, the smaller of the two values (33,000 and 51,500).
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 have small capacities. For example, the IOPS of a 50-GiB ultra-high I/O disk without burst capability can reach only 4,000 (1,500 + 50 x 50). If the disk has burst capability, its IOPS can burst up to 16,000. 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 reach only 4,300 (1,800 + 50 x 50). 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 6,500, but it can temporarily burst to 16,000 IOPS.
- If the disk capacity is 1,000 GiB, the disk has a maximum IOPS of 33,000. 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 6,500 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 16,000 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 3,032 seconds [28,800,000/(16,000 - 6,500)].
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 4,431 seconds (28,800,000/6,500), 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.
- As long as there are tokens, the disk IOPS can exceed 6,500 and can burst to up to 16,000, the IOPS burst limit.
- When there are no more tokens, the disk loses the burst capability, and the disk IOPS can reach up to 6,500.
- Anytime the disk IOPS is less than 6,500, the number of tokens starts to increase, and the disk regains the burst capability.
Performance Testing
For details about how to test the EVS disk performance, see How Do I Test My Disk Performance?.
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