Virtual Disk Backend

The virtual disk backend is the location where data of a virtual disk are stored. The backend can be a disk, a disk slice, a file, or a volume, such as ZFS, SVM, or VxVM. A backend appears in a guest domain either as a full disk or as single slice disk, depending on whether the slice option is set when the backend is exported from the service domain. By default, a virtual disk backend is exported non-exclusively as a readable-writable full disk.

Physical Disk or Disk LUN

A physical disk or disk LUN is always exported as a full disk. In that case, virtual disk drivers (vds and vdc) forward I/O from the virtual disk and act as a pass-through to the physical disk or disk LUN.

A physical disk or disk LUN is exported from a service domain by exporting the device that corresponds to the slice 2 (s2) of that disk without setting the slice option. If you export the slice 2 of a disk with the slice option, only this slice is exported and not the entire disk.

Export a Physical Disk as a Virtual Disk

1. Export a physical disk as a virtual disk.

   For example, to export the physical disk c1t48d0 as a virtual disk, you must export slice 2 of that disk (c1t48d0s2).

   primary# ldm add-vdsdev /dev/dsk/c1t48d0s2 c1t48d0@primary-vds0

2. Assign the disk to a guest domain.

   For example, assign the disk (pdisk) to guest domain ldg1.

   primary# ldm add-vdisk pdisk c1t48d0@primary-vds0 ldg1

3. After the guest domain is started and running the Solaris OS, verify that the disk is accessible and is a full disk.

   A full disk is a regular disk that has eight (8) slices.

   For example, the disk being checked is c0d1.

   ldg1# ls -1 /dev/dsk/c0d1s* /dev/dsk/c0d1s0 /dev/dsk/c0d1s1 /dev/dsk/c0d1s2 /dev/dsk/c0d1s3 /dev/dsk/c0d1s4 /dev/dsk/c0d1s5 /dev/dsk/c0d1s6 /dev/dsk/c0d1s7

Physical Disk Slice

A physical disk slice is always exported as a single slice disk. In that case, virtual disk drivers (vds and vdc) forward I/O from the virtual disk and act as a pass-through to the physical disk slice.

A physical disk slice is exported from a service domain by exporting the corresponding slice device. If the device is different from slice 2 then it is automatically exported as a single slice disk whether or not you specify the slice option. If the device is the slice 2 of the disk, you must set the slice option to export only slice 2 as a single slice disk; otherwise, the entire disk is exported as full disk.

Export a Physical Disk Slice as a Virtual Disk

1. Export a slice of a physical disk as a virtual disk.

   For example, to export slice 0 of the physical disk c1t57d0 as a virtual disk, you must export the device that corresponds to that slice (c1t57d0s0) as follows.

   primary# ldm add-vdsdev /dev/dsk/c1t57d0s0 c1t57d0s0@primary-vds0

   You do not need to specify the slice option, because a slice is always exported as a single slice disk.

2. Assign the disk to a guest domain.

   For example, assign the disk (pslice) to guest domain ldg1.

   primary# ldm add-vdisk pslice c1t57d0s0@primary-vds0 ldg1

3. After the guest domain is started and running the Solaris OS, you can list the disk (c0d13, for example) and see that the disk is accessible.

   ldg1# ls -1 /dev/dsk/c0d13s* /dev/dsk/c0d13s0 /dev/dsk/c0d13s1 /dev/dsk/c0d13s2 /dev/dsk/c0d13s3 /dev/dsk/c0d13s4 /dev/dsk/c0d13s5 /dev/dsk/c0d13s6 /dev/dsk/c0d13s7

   Although there are 8 devices, because the disk is a single slice disk, only the first slice (s0) is usable.

Export Slice 2

1. To export slice 2 (disk c1t57d0s2, for example) you must specify the slice option; otherwise, the entire disk is exported.

   # ldm add-vdsdev options=slice /dev/dsk/c1t57d0s2 c1t57d0s2@primary-vds0

File and Volume

A file or volume (for example from ZFS or SVM) is exported either as a full disk or as single slice disk depending on whether or not the slice option is set.

File or Volume Exported as a Full Disk

If you do not set the slice option, a file or volume is exported as a full disk. In that case, virtual disk drivers (vds and vdc) forward I/O from the virtual disk and manage the partitioning of the virtual disk. The file or volume eventually becomes a disk image containing data from all slices of the virtual disk and the metadata used to manage the partitioning and disk structure.

When a blank file or volume is exported as full disk, it appears in the guest domain as an unformatted disk; that is, a disk with no partition. Then you need to run the format(1M) command in the guest domain to define usable partitions and to write a valid disk label. Any I/O to the virtual disk fails while the disk is unformatted.

Prior to the Solaris 10 5/08 OS release, when a blank file was exported as a virtual disk, the system wrote a default disk label and created default partitioning. This is no longer the case with the Solaris 10 5/08 OS release, and you must run format(1M) in the guest domain to create partitions.

Export a File as a Full Disk

1. From the service domain, create a file (fdisk0 for example) to use as the virtual disk.

   service# mkfile 100m /ldoms/domain/test/fdisk0

   The size of the file defines the size of the virtual disk. This example creates a 100- megabyte blank file to get a 100-megabyte virtual disk.

2. From the control domain, export the file as a virtual disk.

   primary# ldm add-vdsdev /ldoms/domain/test/fdisk0 fdisk0@primary-vds0

   In this example, the slice option is not set, so the file is exported as a full disk.

3. From the control domain, assign the disk to a guest domain.

   For example, assign the disk (fdisk) to guest domain ldg1.

   primary# ldm add-vdisk fdisk fdisk0@primary-vds0 ldg1

4. After the guest domain is started and running the Solaris OS, verify that the disk is accessible and is a full disk.

   A full disk is a regular disk with 8 slices.

   The following example shows how to list the disk, c0d5, and verify that it is accessible and is a full disk.

   ldg1# ls -1 /dev/dsk/c0d5s* /dev/dsk/c0d5s0 /dev/dsk/c0d5s1 /dev/dsk/c0d5s2 /dev/dsk/c0d5s3 /dev/dsk/c0d5s4 /dev/dsk/c0d5s5 /dev/dsk/c0d5s6 /dev/dsk/c0d5s7

File or Volume Exported as a Single Slice Disk

If the slice option is set, then the file or volume is exported as a single slice disk. In that case, the virtual disk has only one partition (s0), which is directly mapped to the file or volume backend. The file or volume only contains data written to the virtual disk with no extra data like partitioning information or disk structure.

When a file or volume is exported as a single slice disk, the system simulates a fake disk partitioning which makes that file or volume appear as a disk slice. Because the disk partitioning is simulated, you do not create partitioning for that disk.

Export a ZFS Volume as a Single Slice Disk

1. Create a ZFS volume to use as a single slice disk.

   The following example shows how to create a ZFS volume, zdisk0, to use as a single slice disk.

   service# zfs create -V 100m ldoms/domain/test/zdisk0

   The size of the volume defines the size of the virtual disk. This example creates a 100-megabyte volume to get a 100-megabyte virtual disk.

2. From the control domain, export the corresponding device to that ZFS volume, and set the slice option so that the volume is exported as a single slice disk.

   primary# ldm add-vdsdev options=slice /dev/zvol/dsk/ldoms/domain/test/zdisk0 \ zdisk0@primary-vds0

3. From the control domain, assign the volume to a guest domain.

   The following shows how to assign the volume, zdisk0, to guest domain ldg1.

   primary# ldm add-vdisk zdisk0 zdisk0@primary-vds0 ldg1

4. After the guest domain is started and running the Solaris OS, you can list the disk (c0d9, for example) and see that the disk is accessible and is a single slice disk (s0).

   ldg1# ls -1 /dev/dsk/c0d9s* /dev/dsk/c0d9s0 /dev/dsk/c0d9s1 /dev/dsk/c0d9s2 /dev/dsk/c0d9s3 /dev/dsk/c0d9s4 /dev/dsk/c0d9s5 /dev/dsk/c0d9s6 /dev/dsk/c0d9s7

Exporting Volumes and Backward Compatibility

Prior to the Solaris 10 5/08 OS release, the slice option did not exist, and volumes were exported as single slice disks. If you have a configuration exporting volumes as virtual disks and if you upgrade the system to the Solaris 10 5/08 OS, volumes are now exported as full disks instead of single slice disks. To preserve the old behavior and to have your volumes exported as single slice disks, you need to do either of the following:

• Use the ldm set-vdsdev command in Logical Domains 1.3 software, and set the slice option for all volumes you want to export as single slice disks. Refer to the ldm(1M) man page for more information about this command.

• Add the following line to the /etc/system file on the service domain.

  set vds:vd_volume_force_slice = 1

  ---

  Note –

  Setting this tunable forces the export of all volumes as single slice disks, and you cannot export any volume as a full disk.

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Summary of How Different Types of Backends Are Exported

Guidelines for Exporting Files and Disk Slices as Virtual Disks

This section includes guidelines for exporting a file and a disk slice as a virtual disk.

Using the Loopback File (lofi) Driver

It is possible to use the loopback file (lofi) driver to export a file as a virtual disk. However, doing this adds an extra driver layer and impacts performance of the virtual disk. Instead, you can directly export a file as a full disk or as a single slice disk. See File and Volume.

Directly or Indirectly Exporting a Disk Slice

To export a slice as a virtual disk either directly or indirectly (for example through a SVM volume), ensure that the slice does not start on the first block (block 0) of the physical disk by using the prtvtoc(1M) command.

If you directly or indirectly export a disk slice which starts on the first block of a physical disk, you might overwrite the partition table of the physical disk and make all partitions of that disk inaccessible.