Overview

What Is Elastic Volume Service?

Elastic Volume Service (EVS) offers scalable block storage for ECSs. With high reliability, high performance, and rich specifications, EVS disks can be used for distributed file systems, development and test environments, data warehouses, and high-performance computing (HPC) scenarios to meet diverse service requirements.

Just like the physical disks in local PC need to be installed before they can be used, EVS disks need to be attached to servers before they can be used. They cannot be used alone. You also need to partition and create file systems on them before they can be used for persistent data storage.

Figure 1 EVS architecture

A system disk runs the server OS. It is like drive C in a PC.
When a server is purchased, a system disk is automatically purchased and attached. You cannot purchase a system disk separately. The maximum size of a system disk is 1,024 GiB.
Data disks store the server data. They are like drive D, drive E, and drive F in a PC.
Data disks can be purchased during or after the server purchase. If you purchase data disks during the server purchase, the system will automatically attach the data disks to the server. If you purchase data disks after the server purchase, you need to manually attach the data disks. The maximum size of a data disk is 32,768 GiB.

Disk Types

There are the following types of EVS disks, each with different levels of I/O performance: Extreme SSD, General Purpose SSD V2, Ultra-high I/O, General Purpose SSD, High I/O, and Common I/O. EVS disk types differ in performance. Choose a disk type based on your requirements.

EVS disk types differ in performance. Choose a disk type based on your requirements.

For more information about EVS disk specifications and performance, see Disk Types and Performance.

Device Types

EVS disks have two device types, Virtual Block Device (VBD) and Small Computer System Interface (SCSI).

VBD:
When you create an EVS disk on the management console, Device Type of the EVS disk is VBD by default. VBD EVS disks support only simple SCSI read/write commands.
SCSI:
You can create EVS disks whose Device Type is SCSI on the management console. These EVS disks support transparent SCSI command transmission, allowing ECS OS to directly access underlying storage media. SCSI EVS disks support both basic and advanced SCSI commands.

For more information about how to use SCSI EVS disks, for example, how to install a driver for SCSI EVS disks, see Do I Need to Install a Driver for SCSI EVS Disks?

Device Types Supported by ECS

Device types supported by ECS are determined by the ECS type and scenarios.

**Table 1** Device types supported by ECS
Scenario	Device Type (System Disk)	Device Type (Data Disk)
Purchasing an ECS	General computing-plus C7: SCSI Memory-optimized M7: SCSI Bare metal ECS (with the additional identifier "physical"): SCSI Other: VBD	General computing-plus C7: SCSI Memory-optimized M7: SCSI Bare metal ECS (with the additional identifier "physical"): SCSI Other: VBD or SCSI
Attaching a disk to an existing ECS	N/A	General computing-plus C7: VBD or SCSI Memory-optimized M7: VBD or SCSI Bare metal ECS (with the additional identifier "physical"): SCSI Other: VBD or SCSI

Instructions for Disk Operations

**Table 2** Instructions for common disk operations
Scenario	Instruction
Creating a data disk along with an ECS	You can create one or more data disks when purchasing an ECS. These data disks are automatically attached to the ECS. After the ECS is purchased, you need to log in to the ECS and initialize the data disks. The billing mode of the data disks created along with the ECS is the same as that of the ECS. Adding a Disk to an ECS Initializing Data Disks
Adding a data disk (on the ECS console)	You can add data disks on the ECS console. These data disks are automatically attached to the ECS. After the disks are added, you need to log in to the ECS and initialize the disks. The billing mode of the data disks added on the ECS console is the same as that of the ECS. Adding a Disk to an ECS Initializing Data Disks
Adding a data disk (on the EVS console)	You can add data disks on the EVS console and manually attach the disks to the ECS. Then, log in to the ECS and initialize the data disks. The billing mode of the data disks purchased on the EVS console can be different from that of the ECS. Purchasing an EVS Disk Attaching an EVS Disk Initializing Data Disks
Creating a disk from a snapshot	You can create disks from a snapshot to quickly copy the snapshot data to disks. For details, see Creating a Disk from a Snapshot.
Expanding the disk capacity	If the capacity of an existing EVS disk is insufficient, whether it is a system disk or a data disk, you can expand it. The capacity expansion includes the following operations: expanding the disk capacity, and extending the partitions and file systems. For details, see Expanding the Capacity of an EVS Disk.
Changing the EVS disk type	EVS disks of different types have different performance. If the performance of an existing EVS disk cannot meet service requirements, you can change the type of the EVS disk.
Detaching and deleting an EVS disk	If an EVS disk is no longer needed, you can detach and delete it to save costs. Detaching an EVS Disk Unsubscribing from or Deleting an EVS Disk