# Device Types and Usage Instructions

#### What Device Types Are Available?

There are two EVS device types: Virtual Block Device (VBD) and Small Computer System Interface (SCSI).

- VBD is the default EVS device type. VBD EVS disks support only basic read/write SCSI commands.
- SCSI EVS disks support transparent SCSI command transmission and allow the server OS to directly access the underlying storage media. Besides basic read/write SCSI commands, SCSI disks support advanced SCSI commands.

Device type is configured during creation. It cannot be changed after the disk has been created.

#### Common Application Scenarios and Usage Instructions of SCSI EVS Disks

- BMSs support only SCSI EVS disks.
- Shared SCSI EVS disks: Shared SCSI EVS disks must be used together with a distributed file system or cluster software. Because most cluster applications, such as Windows MSCS, Veritas VCS, and Veritas CFS, require the usage of SCSI reservations, you are advised to use shared EVS disks with SCSI.

  SCSI reservations take effect only when shared SCSI EVS disks are attached to ECSs in the same ECS group. For more information about shared EVS disks, see Shared EVS Disks and Usage Instructions.

#### Do I Need to Install a Driver for SCSI EVS Disks?

To use SCSI EVS disks, you need to install a driver for certain server OSs.

- BMS

  Both the Windows and Linux images for BMSs are pre-installed with the required SDI card driver. Therefore, no driver needs to be installed.

- KVM ECS

  You are advised to use SCSI EVS disks with KVM ECSs. Linux images and Windows images for KVM ECSs already have the required driver. Therefore, no driver needs to be installed for KVM ECSs.

  ECS virtualization types are categorized into KVM and Xen. For details, see ECS Types.

- Xen ECS

  Due to driver limitations, you are advised not to use SCSI EVS disk with Xen ECSs.

  However, a few images support SCSI EVS disks on Xen ECSs. For the supported images, see Table 1.

  After confirming that the OS images of Xen ECSs support SCSI EVS disks, determine whether you need to install the driver:

  - Public Windows images are preinstalled with the Paravirtual SCSI (PVSCSI) driver. Therefore, no driver needs to be installed.
  - Private Windows images are not preinstalled with the PVSCSI driver. You need to download and install it explicitly.

    For details, see (Optional) Optimizing Windows Private Images in the Image Management Service User Guide.

  - Linux images are not preinstalled with the PVSCSI driver. You need to obtain the source code of the open-source Linux driver at https://github.com/UVP-Tools/SAP-HANA-Tools.

    Note that this driver is different from the PVSCSI drivers attached to some Linux distributions.

  Table 1 OSs supporting SCSI EVS disks
  Virtualization Type | OS
  --- | ---
  Xen | Windows | See the Windows images listed on the Public Images page. Log in to the management console, choose Image Management Service, click the Public Images tab, and select ECS image and Windows from the drop-down lists, respectively.

  Linux

  - SUSE Linux Enterprise Server 11 SP4 64bit (The kernel version is 3.0.101-68-default or 3.0.101-80-default.)
  - SUSE Linux Enterprise Server 12 64bit (The kernel version is 3.12.51-52.31-default.)
  - SUSE Linux Enterprise Server 12 SP1 64bit (The kernel version is 3.12.67-60.64.24-default.)
  - SUSE Linux Enterprise Server 12 SP2 64bit (The kernel version is 4.4.74-92.35.1-default.)

Parent topic: Overview