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Reviewed-by: Muller, Martin <martin.muller@t-systems.com>
Co-authored-by: Zaoxu, Li <lizaoxu@huawei.com>
Co-committed-by: Zaoxu, Li <lizaoxu@huawei.com>

2025-01-02 07:41:33 +00:00

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Notes and Constraints

This section describes the constraints on using EVS.

Specifications

**Table 1** Specifications
Resource Type	Item	Description
Disk performance	Major disk performance metrics include IOPS, throughput, and latency.	Different types of EVS disks have different performance. For details, see Disk Types and Performance.

Security

**Table 2** Security
Item	Description
Disk encryption	The encryption attribute of a disk cannot be changed after the disk is created. If you use an encrypted disk to create a backup, the backup generated will be an encrypted backup. You cannot modify the encryption attribute of the backup. If you use an encrypted disk to create a snapshot, the snapshot generated will be an encrypted snapshot. You cannot modify the encryption attribute of the snapshot.

Quotas

You can log in to the console to view default quotas. You can to apply for a larger quota if needed.

**Table 3** Quotas
Resource Type	Default Quota
Tags	10
Disks	Default quotas vary depending on regions. See the quotas shown on the console.
Disk capacity (GB)
Snapshots

Operations

**Table 4** Operations
Scenario	Item	Description
Disk creation	Maximum number of disks that can be created at a time	100
	Disk creation from snapshot	Batch disk creation from a snapshot is not supported. A disk created from a snapshot has the same device type (SCSI or VBD), encryption attribute, AZ, region, and disk type as the snapshot's source disk. A snapshot whose name starts with autobk_snapshot_vbs_, manualbk_snapshot_vbs_, autobk_snapshot_csbs_, or manualbk_snapshot_csbs_ is automatically generated during backup. Such a snapshot can only be viewed. It cannot be used to create new disks.
	Disk creation from backup	Batch creation is not supported. One can create only one disk from a backup at a time. One backup cannot be used for concurrent disk creation operations at the same time. For example, if you are creating disk A from a backup, this backup can be used to create another disk only after disk A has been created. If a disk is created from a backup of a system disk, the new disk can be used as a data disk only.
	Disk creation from image	The device type of the new disk is the same as that of the image's source disk. The encryption attribute of the new disk is the same as that of the image's source disk.
	Device type	The device type of a disk cannot be changed after the disk has been created.
	Disk sharing	The sharing attribute of a disk cannot be changed after the disk has been created.
	Disk encryption	The encryption attribute of a disk cannot be changed after the disk has been created.
Disk attachment	Constraints on region and AZ	The disk and server must be in the same region and AZ.
	Maximum number of servers that a non-shared disk can be attached to	1
	Maximum number of servers that a shared disk can be attached to	16
	Maximum number of disks that can be attached to an ECS	This number varies depending on the ECS type. For details, see section "Can I Attach Multiple Disks to an ECS?" in the Elastic Cloud Server User Guide. For a BMS, a maximum of 60 EVS disks (1 system disk and 59 data disks) are supported.
	Device name	System disk: /dev/vda, /dev/sda, and /dev/xvda Data disk: /dev/vd[b-z], /dev/sd[b-z], and /dev/xvd[b-z]
Disk capacity expansion	Capacity expansion	Disk capacity can be expanded, but cannot be reduced.
	Capacity expansion of non-shared disks	Some server OSs support the capacity expansion of non-shared, In-use disks. For details, see Expanding Capacity for an In-use EVS Disk.
	Capacity expansion of shared disks	A shared disk must be detached from all its servers before expansion. This means that the shared disk status must be Available.
	Expansion increment	1 GiB
Disk detachment	System disk detachment	A system disk can only be detached offline, which means that the server must be in the Stopped state.
Disk detachment	Data disk detachment	A data disk can be detached online or offline, meaning that its server can either be in the Running or Stopped state.
Disk deletion	Deletion of disks	The disk status is Available, Error, Expansion failed, Restoration failed, or Rollback failed. The disk is not locked by any service. The shared disk has been detached from all its servers.
Snapshot creation	/	Snapshots can be created for both system disks and data disks. Snapshots of encrypted disks are stored encrypted, and those of non-encrypted disks are stored non-encrypted.
Snapshot data rollback to disk	/	Snapshot data can only be rolled back to source EVS disks. Rollback to a different disk is not possible. You can only roll back disk data from a snapshot when the source disk status is Available (not attached to any server) or Rollback failed. If the source disk is attached, detach the disk first. If a snapshot is being created, it cannot be used to roll back disk data.
Snapshot deletion	/	If a snapshot is deleted, disks rolled back or created from this snapshot are not affected. If a snapshot's source disk is deleted, all legacy snapshots of this disk are also deleted. If you reinstall or change the server OS, snapshots of the system disk are automatically deleted. Those of the data disks can be used as usual.

Parent topic: Overview

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