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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.

  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 "Converting non-CDS disks to CDS disks" on page 21.

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" on page 15.

CDS disk groups can be used in the following ways:

• 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.
• Imported as private disk groups, shared disk groups (by CVM), or distributed disk groups (by a volume server).

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

• Volumes of usage type root and swap. 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 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. Device quotas are useful for disk groups which to be transferred 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.

See "Setting the maximum number of devices for CDS disk groups" on page 33.

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 "Displaying the maximum number of devices in a CDS disk group" on page 36.

Minor device numbers

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

  Note   There is a large disparity between the maximum number of devices allowed for devices on the Linux platform with a pre-2.6 kernel, and that for other supported platforms.