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 log in to the console to view default quotas. For details, see How Do I View My 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 from snapshot	Batch creation is not supported when disks are created from snapshots. Only one disk can be created from a snapshot at a time. Snapshot function in OBT A disk created from a snapshot has the same device type (SCSI or VBD), encryption attribute, AZ, 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. Snapshot function in commercial use If Instant Snapshot Restore is not enabled for a standard snapshot, you can only use it to create disks when its upload progress is complete. If Instant Snapshot Restore is enabled for a standard snapshot, when its upload is in progress, you can use it to create disks but cannot change the device type (SCSI or VBD), encryption attribute, AZ, and type of the new disk. They are kept the same as those of the snapshot's source disk. After a standard snapshot is uploaded, you can change the device type (SCSI or VBD), encryption attribute, AZ, and type of the disks as required.
	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 value varies with ECS types. For details, see Can I Attach Multiple Disks to an ECS?
	Max. number of disks that can be attached to a BMS	60 (1 system disk and 59 data disks) Only SCSI disks can be attached to BMSs.
	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. That is, 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, that is, 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	/	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 function in OBT You can manually create a maximum of seven snapshots for a disk. The enterprise project of a snapshot is the same as that of the snapshot's source disk. Snapshot function in commercial use You can manually create a maximum of 256 standard snapshots for a disk, of which up to seven can have Instant Snapshot Restore enabled. You can create one standard snapshot for a disk at a time. You can only create the next standard snapshot for the same disk after the previous snapshot has been created. Standard snapshots cannot be created for the disks in edge AZs. For details about the differences between edge AZs and general AZs, see the CloudPond User Guide. When standard snapshots are created for Common I/O and High I/O disks, Instant Snapshot Restore cannot be enabled. It usually takes several minutes to create a standard snapshot. The time required varies depending on the amounts of data written to the disk. The larger the data volume, the longer the time required. The initial standard snapshot usually takes more time because data of the entire disk is backed up. Subsequent standard snapshots are quicker, but the time required is still determined by the amounts of changed data compared with each last snapshot. The more the changed data, the longer the time required. If the data on a disk is rolled back from a snapshot, the next standard snapshot created for this disk will be a full snapshot. During the creation of a standard snapshot, any incremental data written to the disk will not be backed up to the snapshot created. During the creation of a standard snapshot, deleting the snapshot's source disk does not affect the creation of the snapshot.
Use of Instant Snapshot Restore	/	Instant Snapshot Restore is supported for the following types of disks: Extreme SSD V2, Extreme SSD, General Purpose SSD V2, General Purpose SSD, and High I/O. 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 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. 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	/	If a snapshot is deleted, disks rolled back or created from this snapshot are not affected. Snapshot function in commercial use Standard snapshots are not deleted even if their source disks are deleted. When you delete a disk whose standard snapshots have Instant Snapshot Restore enabled, the standard snapshots will be not deleted, but Instant Snapshot Restore will be disabled automatically. If you reinstall or change the server OS, standard snapshots will be not deleted, but Instant Snapshot Restore will be disabled automatically if it has been enabled for the standard snapshots of the system disk. Snapshot function in OBT 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. A snapshot whose name starts with autobk_snapshot_vbs_, manualbk_snapshot_vbs_, autobk_snapshot_csbs_, or manualbk_snapshot_csbs_ is automatically generated during backup. You can only check details of such snapshots and cannot delete them.
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. Changing the disk type may affect the disk performance, so change the type during off-peak hours.
	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 backend issue. If this happens, try again later.
Recycle bin management	/	When you delete a disk, regardless of whether the disk will be moved to the recycle bin or not, the disk snapshots will always be deleted permanently. There are no limits on the capacity and quantity of disks in the recycle bin. In the recycle bin, disks can be stored for 7 days. After 7 days, the system will permanently delete the disks, and the disks cannot be recovered.