DASDs and SCSI disks

DASDs and FC-attached SCSI disks can be virtualized as virtio block devices.

On the host, you manage various types of disk devices and their configuration topology. For production systems, DASDs and FC-attached SCSI disks are typically set up with multipathing to boost availability through path redundancy.

From the virtual server point of view, these are virtual block devices that are attached by one virtual channel path. There is no difference whether a virtual block device is implemented as a DASD, a SCSI disk, or an image file on the host.

QEMU uses the current libvirt-internal configuration to assign the virtual devices of a virtual server to the underlying host devices.

To provide DASDs and FC-attached SCSI disks as virtual block devices for a virtual server:

Set up the DASDs and FC-attached SCSI disks.
Prepare multipathing, because virtual block devices cannot be multipathed on the virtual server.
It is also important that you provide unique device nodes that are persistent across host reboots. Unique device nodes ensure that your configuration remains valid after a host reboot. In addition, device nodes that are unique for a disk device on different hosts allow the live migration of a virtual server to a different host, or the migration of a disk to a different storage server or storage controller.
See Preparing DASDs and Preparing SCSI disks.
Configure the DASDs and FC-attached SCSI disks as virtual block devices.
You configure devices that are to be defined with the virtual server in its domain configuration-XML file. You can also define devices in a separate device configuration-XML file. Such devices can be attached to an already defined virtual server.
See Configuring devices and Configuring virtual block devices.

DASD and SCSI disk configuration topology

Figure 1 shows how multipathed DASD and SCSI disks are configured as virtual block devices.

This graphic shows multipathed DASD and SCSI disks that are configured as virtual block devices. — Figure 1. Multipathed DASD and SCSI disks configured as virtual block devices

Disk device identification

There are multiple ways to identify a disk device on the host or on the virtual server.

Device bus-ID and device number of an FCP device

On the host, a SCSI device is connected to an FCP device, which has a device bus-ID of the form:

0.m.dddd

Where:


`0`	is the channel subsystem-ID.
`m`	is the subchannel set-ID.
`dddd`	is the device number of the FCP device.


Example:
`0.0.1700`	device bus-ID of the FCP device.
`1700`	device number of the FCP device.

Device bus-ID and device number of a DASD

On the host, a DASD is attached to a FICON® channel. It has a device bus-ID of the form:

0.m.dddd


Example:
`0.0.e717`	device bus-ID of the DASD.
`e717`	device number of the DASD.

Unique ID (UID) of a DASD

PAV and HyperPAV provide means to create unique IDs to identify DASDs.


Example:
`IBM.75000000010671.5600.00`

Device bus-ID and device number of a virtual block device

On the virtual server, all virtual block devices are accessed through a single virtual channel subsystem. The virtual server directly identifies a virtual block device through its device bus-ID, which is of the form:

0.m.dddd

Where:


`0`	is the channel subsystem-ID.
`m`	is the subchannel set-ID.
`dddd`	is the device number of the virtual block device.


Example:
`0.0.1a12`	device bus-ID of the virtual device.
`1a12`	device number of the virtual device.

Standard device name

Standard device names are of the form:


`dasd<x>`	for DASDs on the host.
`sd<x>`	for SCSI disks on the host.
`vd<x>`	for virtual block devices on the virtual server.

Where <x> can be one or more letters.

They are assigned in the order in which the devices are detected and thus can change across reboots.


Example:
`dasda`	on the host.
`sda`	on the host.
`vda`	on the virtual server.

If there is only one attached SCSI disk, you can be sure that host device sda is mapped to virtual server device vda.

Standard device node

User space programs access devices through device nodes. Standard device nodes are of the form:

/dev/<standard-device-name>


Example:
`/dev/sda`	for SCSI disks on the host.
`/dev/dasda`	for DASDs on the host.
`/dev/vda`	for virtual block devices on the virtual server.

udev-created device node

If udev is available with your product or distribution, it creates device nodes which are based on unique properties of a device and so identify a particular device. udev creates various device nodes for a device which are based on the following information:

Hardware / storage server (by-uid device node)
Device bus-ID (by-path device node)
SCSI identifier for SCSI disks or disk label (VOLSER) for DASDs (by-ID device node)
File system information (by-uuid device node)


Example for DASDs on the host:
`/dev/disk/by-path/ccw-0.0.1607`
`/dev/disk/by-path/ccw-0.0.1607-part1`
	where:
	`0.0.1607`	is the device bus-ID of the DASD.
	`part1`	denotes the first partition of the DASD.
`/dev/disk/by-id/ccw-IBM.750000000R0021.1600.07`
`/dev/disk/by-id/ccw-IBM.750000000R0021.1600.07-part1`
	where:
	`IBM.750000000R0021.1600.07`	is the UID of the DASD.
	`part1`	denotes the first partition of the DASD.
`/dev/disk/by-uuid/a6563ff0-9a0f-4ed3-b382-c56ad4653637`
	where:
	`a6563ff0-9a0f-4ed3-b382-c56ad4653637`
		is the universally unique identifier (UUID) of a file system.

Example for SCSI devices on the host:
`/dev/disk/by-path/ccw-0.0.3c40-zfcp-0x500507630300c562:0x401040ea00000000`
	where:
	`0.0.3c40`	is the device bus-ID of the FCP device.
	`0x500507630300c562`	is the worldwide port name (WWPN) of the storage controller port.
	`0x401040ea00000000`	is the FCP LUN.
`/dev/disk/by-id/scsi-36005076303ffc56200000000000010ea`
	where:
	`scsi-36005076303ffc56200000000000010ea`
		is the SCSI identifier.
`/dev/disk/by-uuid/7eaf9c95-55ac-4e5e-8f18-065b313e63ca`
	where:
	`7eaf9c95-55ac-4e5e-8f18-065b313e63ca`
		is the universally unique identifier (UUID) of a file system.

Since device-specific information is hidden from the virtual server, udev creates by-path device nodes on the virtual server. They are derived from the device number of the virtual block device, which you can specify in the domain configuration-XML or in the device configuration-XML.

The udev rules to derive by-path device nodes depend on your product or distribution.

Tip: Prepare a strategy for specifying device numbers for the virtio block devices, which you provide for virtual servers. This strategy makes it easy to identify the virtualized disk from the device bus-ID or device number of the virtual block device.


Virtual server example:
`/dev/disk/by-path/ccw-0.0.1a12`
`/dev/disk/by-path/ccw-0.0.1a12-part1`
	where:
	`0.0.1a12`	is the device bus-ID.
	`part1`	denotes the first partition of the device.

Device mapper-created device node

The multipath device mapper support assigns a unique device mapper-created device node to a SCSI disk. The device mapper-created device node can be used on different hosts to access the same SCSI disk.


Example:
`/dev/mapper/36005076305ffc1ae00000000000021d5`
`/dev/mapper/36005076305ffc1ae00000000000021d5p1`
	where
	`p1`	denotes the first partition of the device.

Tip: Use device mapper-created device nodes for SCSI disks and udev-created device nodes for DASDs in your configuration-XML files to support a smooth live migration of virtual servers to a different host.