Disk Types and Performance

EVS disks are classified into the following types by I/O performance: Extreme SSD, Ultra-high I/O, General Purpose SSD, High I/O. EVS disks differ in performance and price. Choose the disk type most appropriate for your applications.

Extreme SSD EVS disks use the congestion control algorithms for Remote Direct Memory Access (RDMA) deployments. An extreme SSD disk can reach up to 1,000 MiB/s of throughput and extreme low single-channel latency.

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

**Table 1** EVS performance data
	Extreme SSD	Ultra-high I/O	General Purpose SSD	High I/O
Max. capacity (GiB)	System disk: 1,024 Data disk: 32,768	System disk: 1,024 Data disk: 32,768	System disk: 1,024 Data disk: 32,768	System disk: 1,024 Data disk: 32,768
Short description	Superfast disks for workloads demanding ultra-high bandwidth and ultra-low latency	High performance disks excellent for enterprise mission-critical services as well as workloads demanding high throughput and low latency	Cost-effective disks designed for enterprise applications with medium performance requirements	Disks suitable for commonly accessed workloads^f
Typical workloads	Database workloads Oracle SQL Server ClickHouse AI workloads	Transcoding services I/O-intensive workloads NoSQL Oracle SQL Server PostgreSQL Latency-sensitive applications Redis Memcache	Enterprise OA Medium-scale development and test environments Small- and medium-sized databases Web applications System disks	Common development and test environments
Max. IOPS^a	128,000	50,000	20,000	5,000
Max. Throughput^a (MiB/s)	1,000	350	250	150
Burst IOPS limit^a	64,000	16,000	8,000	5,000
Disk IOPS^c	Min. [128,000, 1,800 + 50 x Capacity (GiB)]	Min. [50,000, 1,800 + 50 x Capacity (GiB)]	Min. [20,000, 1,800 + 12 x Capacity (GiB)]	Min. [5,000, 1,800 + 8 x Capacity (GiB)]
Disk throughput^b (MiB/s)	Min. [1,000, 120 + 0.5 × Capacity (GiB)]	Min. [350, 120 + 0.5 × Capacity (GiB)]	Min. [250, 100 + 0.5 × Capacity (GiB)]	Min. [150, 100 + 0.15 × Capacity (GiB)]
Single-queue access latency^d (ms)	Sub-millisecond	1	1	1–3
API Name^e	ESSD	SSD	GPSSD	SAS

a: The maximum IOPS, maximum throughput, and burst IOPS limit are all calculated based on the sum of read and write operations. For example, maximum IOPS = read IOPS + write IOPS.

b: Take ultra-high I/O for example: The baseline throughput is 120 MiB/s. The throughput increases by 0.5 MiB/s for every one GiB added until it reaches the maximum throughput 350 MiB/s.

c: Take ultra-high I/O for example: The baseline IOPS is 1,800. The IOPS increases by 50 for every one GiB added until it reaches the maximum IOPS 50,000.

d: A single queue indicates that the queue depth or concurrency is 1. The single-queue access latency is the I/O latency when all I/O requests are processed sequentially. The values in the table are calculated with 4 KiB data blocks.

e: This API name indicates the value of the volume_type parameter in the EVS API. It does not represent the type of the underlying hardware device.

f: High I/O disks (except for those created in dedicated storage pools) are HDD-backed disks. They are suitable for applications with commonly accessed workloads. The baseline throughput of a high I/O disk is 40 MiB/s per TiB, and the maximum throughput of a high I/O disk is 150 MiB/s. If your applications have high workloads, it is recommended that you choose the disk types with higher specifications. Such types of disks are SSD-backed disks.

EVS disk performance is closely related with the data block size. An EVS disk can achieve either the maximum IOPS or maximum throughput depending on which one is reached first.

For data blocks of a small size, such as 4 KiB or 8 KiB, the disk can reach the maximum IOPS.
For data blocks of a large size, greater than or equal to 16 KiB, the disk can reach the maximum throughput.

The following uses an ultra-high I/O disk as an example. According to the formula, when the size of an ultra-high I/O disk is greater than or equal to 964 GiB, the disk theoretically can reach either the maximum IOPS 50,000 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. For details, see Table 2.

**Table 2** Maximum performance of an ultra-high I/O EVS disk
Data Block Size	Max. IOPS	Max. Throughput (MiB/s)
4 KiB	About 50,000	About 195
8 KiB	About 44,800	About 350
16 KiB	About 22,400	About 350
32 KiB	About 11,200	About 350

Disk IOPS Calculation Formula

Disk IOPS = Min. (Maximum IOPS, Baseline IOPS + IOPS per GiB x Capacity)

The following example uses an ultra-high I/O EVS disk with a maximum IOPS of 50,000.

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.

The following example uses an ultra-high I/O EVS disk with the IOPS burst limit of 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

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 28,800,000 (1,800 x 16,000).

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.

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.

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.

Figure 1 Burst capability diagram