Notes and Constraints

This section describes the constraints on using EVS.

Specifications

**Table 1** Specifications
Resource Type	Item	Description
Disk capacity	Capacity of a system disk	40 GiB - 1024 GiB
	Capacity of a data disk	10 GiB - 32768 GiB
	Maximum capacity supported by MBR	2 TiB
	Maximum capacity supported by GPT	18 EiB
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 Disk Performance.

Security

**Table 2** Security
Item	Description
Disk encryption	The encryption attribute of a disk cannot be changed after the disk is purchased. 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 an image, the image generated will be an encrypted image. You cannot modify the encryption attribute of the image. 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. If you use an image that does not support lazyloading to create a disk, the disk created will be an encrypted disk. You cannot modify the encryption of the disk. If you use a standard snapshot with Instant Snapshot Restore enabled to create a disk, the disk created will be an encrypted disk. You cannot modify the encryption of the disk.

Quotas

You can sign in to the console to view default quotas. For details, see Querying EVS Resource Quotas. You can submit a service ticket to apply for a larger quota if needed.

**Table 3** Quotas
Resource Type	Default Quota
Tags	20
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	Disk creation from snapshot	For details, see the constraints in Creating a Disk from a Snapshot.
Disk creation	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 only be used to create another disk 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	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.
Disk creation	Device type	The device type of a disk cannot be changed after the disk is created.
Disk creation	Disk sharing	The sharing attribute of a disk cannot be changed after the disk is created.
Disk creation	Disk encryption	The encryption attribute of a disk cannot be changed after the disk is created.
Disk attachment	Constraints on region and AZ	The disk and the 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 Can I Attach Multiple Disks to an ECS? For a BMS, a maximum of 60 EVS disks (1 system disk and 59 data disks) are supported. Only SCSI disks can be attached.
	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 Expand Disk Capacity.
	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 pay-per-use disks Unsubscription of yearly/monthly 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. The disk is not added to any replication pair in the Storage Disaster Recovery Service (SDRS). For any disk already added to a replication pair, you need to first delete the replication pair and then delete the disk. Yearly/Monthly system disks cannot be unsubscribed from separately. They must be unsubscribed from together with their servers. Non-shared, yearly/monthly data disks purchased together with or later added to a yearly/monthly server have the same expiration time as the server. They can be unsubscribed from together with the server or separately when their statuses are In-use, Available, or Error. Yearly/Monthly data disks purchased on the EVS console have different expiration times as the server. They can be unsubscribed from separately.
Snapshot creation	/	For details, see the constraints in Creating an EVS Snapshot.
Use of Instant Snapshot Restore	/	High I/O and Common I/O disks do not support Instant Snapshot Restore. You can only enable Instant Snapshot Restore when creating standard snapshots. It cannot be enabled later. You can enable Instant Snapshot Restore for up to seven snapshots for a disk. When Instant Snapshot Restore is enabled and snapshots are being created, you cannot disable Instant Snapshot Restore. When you delete a disk whose standard snapshots have Instant Snapshot Restore enabled, the snapshots will be not deleted, but Instant Snapshot Restore will be disabled automatically.
Snapshot data rollback to disks	/	Snapshot data can only be rolled back to source EVS disks. Rollback to a different disk is not possible. If the snapshot status is Creating, it cannot be used to roll back disk data. If the standard snapshot status is Available and the snapshot has Instant Snapshot Restore enabled, you can use the snapshot to roll back data even if data upload is in progress. 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 roll back the disk data.
Snapshot deletion	/	For details, see the constraints in Deleting an EVS Snapshot.
Disk type change	Before the change	You can only change the disk type when the disk status is Available or In-use. The disk type cannot be changed when any snapshot of the disk is being deleted. A disk having more than 128 snapshots cannot have its disk type changed. You can delete some snapshots and then perform the change. In rare cases, the disk type may fail to be changed due to a background resource issue. If this happens, submit a service ticket. A disk protected by SDRS cannot have its disk type changed.
	During the change	You can only change the disk type when the disk status is Available or In-use. The disk type cannot be changed when any snapshot of the disk is being deleted. Some operations cannot be performed on the disk. Such operations include creating snapshots, creating backups, expanding the disk capacity, rolling back data from a snapshot, restoring data from a backup, attaching or detaching the disk, deleting the disk, transferring the disk, and creating an image from the ECS. Changing the disk type may take several hours or even longer, and cannot be stopped. The time depends on the throughput, storage space, and original disk type at the time of the change. You can have a maximum of 10 disks with their types being changed at the same time. The OS cannot be changed if you are changing the disk type of a system disk.
	After the change	In rare cases, the disk type may fail to be changed due to a background resource issue. If this happens, try again later. In rare cases, the disk type may fail to be changed after a data rollback from a snapshot. If this happens, submit a service ticket.
Recycle bin management	/	When you delete a disk, regardless of whether the disk will be moved to the recycle bin or not, legacy snapshots of the disk will always be deleted permanently. There are no limits on the capacity and quantity of disks in the recycle bin. Separately deleted disks are stored in the recycle bin for a maximum of seven days by default. During this period, you can recover or permanently delete the disks. After the disks expire, they are permanently deleted and cannot be recovered.