CDS Disk Access and Format

For a disk to be accessible by multiple platforms, the disk must be consistently recognized by the platforms, and all platforms must be capable of performing I/O on the disk. CDS disks contain specific content at specific locations to identify or control access to the disk on different platforms. The same content and location are used on all CDS disks, independent of the platform on which the disks are initialized.

In order for a disk to be initialized as, or converted to a CDS disk, it must satisfy the following requirements:

• Must be a SCSI disk that supports Mode Sense
• Cannot be an EFI disk
• Must be the entire physical disk (LUN)
• Only one volume manager (such as VxVM) can manage a physical disk (LUN)
• There can be no disk partition (slice) which is defined, but which is not configured on the disk
• Cannot contain a volume whose use-type is either root or swap (for example, it cannot be a boot disk)

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  Note    The CDS conversion utility, vxcdsconvert, is provided to convert non-CDS VM disk formats to CDS disks, and disk groups with a version number less than 110 to disk groups that support CDS disks. See Setting up Your System for more details.

Disk groups with version numbers less than 110 are not supported for the Solaris OS on the x64 platform.

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CDS Disk Types

The CDS disk format, cdsdisk, is recognized by all VxVM platforms (including Windows). This is the default disk format for all newly created VM disks unless overridden in a defaults file (see Defaults Files). The vxcdsconvert utility is provided to convert other disk formats and types to CDS.

Private and Public Regions

A VM disk usually has a private and a public region.

The private region is a small area on the disk where VxVM configuration information is stored, such as a disk header label, configuration records for VxVM objects (such as volumes, plexes and subdisks), and an intent log for the configuration database. The default private region size is 2048 blocks (equivalent to 1024 kilobytes on AIX, Linux and the Solaris OS, and 2048 kilobytes on HP-UX), which is large enough to record the details of about 4000 VxVM objects in a disk group.

The public region covers the remainder of the disk, and is used for the allocation of storage space to subdisks.

The private and public regions are aligned and sized in multiples of 8K to permit the operation of CDS. The alignment of VxVM objects within the public region is controlled by the disk group alignment attribute. The value of this attribute must also be 8k to permit the operation of CDS.

Disk Access Type auto

The disk access (DA) disk type auto supports multiple disk formats, including cdsdisk, which is supported across all platforms. It is associated with the DA records created by the VxVM auto-configuration mode. Disk type auto automatically determines which format is on the disk.

Platform Block

The Platform Block resides on disk sector 0, and contains data specific to the Operating System for the platforms. It is necessary for proper interaction with each of those platforms. The platform block allows a disk to perform as if it was initialized by each of the specific platforms.

AIX Coexistence Label

The AIX Coexistence label resides on the disk, and identifies the disk to the AIX volume manager (LVM) as being controlled by VxVM.

HP-UX Coexistence Label

The HP-UX Coexistence label resides on the disk, and identifies the disk to the HP volume manager (LVM) as being controlled by VxVM.

VxVM ID Block

The VxVM ID block resides on the disk, and indicates the disk is under VxVM control. It provides dynamic VxVM private region location and other information.

CDS Disk Groups

A CDS disk group allows cross-platform data sharing of VxVM objects, so that data written on one of the supported platforms may be accessed on any other supported platform. A CDS disk group is composed only of CDS disks (VM disks with the disk format cdsdisk), and is only available for disk group version 110 and greater.

All VxVM objects in a CDS disk group are aligned and sized so that any system can access the object using its own representation of an I/O block. The CDS disk group uses a platform-independent alignment value to support system block sizes of up to 8K. See Disk Group Alignment for details.

CDS disk groups can be:

• Initialized on one system and then used "as-is" by VxVM on a system employing a different type of platform.
• Imported (in a serial fashion) by Linux, Solaris, AIX, and HP-UX systems.
• Private, shared, or distributed (SAN VM).
• Shared by CVM.

You cannot include the following disks or volumes in a CDS disk group:

• Volumes of usage type root and swap. (This means that you cannot use CDS to share boot devices.)
• Encapsulated disks.

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  Note    On Solaris and Linux systems, the process of disk encapsulation places the slices or partitions on a disk (which may contain data or file systems) under VxVM control. On AIX and HP-UX systems, LVM volumes may be similarly be converted to VxVM volumes.

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Device Quotas

Device quotas limit the number of objects in the disk group which create associated device nodes in the file system. (This is useful for disk groups which share serially between Linux with a pre-2.6 kernel and other supported platforms. Prior to the 2.6 kernel, Linux supported only 256 minor devices per major device.)

You can limit the number of devices that can be created in a given CDS disk group by setting the device quota (refer to Setting Device Quotas for CDS Disk Groups).

When you create a device, an error is returned if the number of devices would exceed the device quota. You then either need to increase the quota, or remove some objects using device numbers, before the device can be created.

See also Displaying Device Quotas for CDS Disk Groups for instructions on displaying the device quota value.

Minor Device Numbers

Importing a disk group will fail if it will exceed the maximum devices for that platform.