General Concepts

CDS provides you with a foundation for moving data between different systems within a heterogeneous environment. The machines may be running HP-UX, AIX, Linux or the Solaris^TM operating system (OS), and they may all have direct access to physical devices holding data. CDS allows VERITAS products and applications to access data storage independently of the operating system platform, enabling them to work transparently in heterogeneous environments.

The following levels in the device hierarchy, from disk through file system, must provide support for CDS to be used:

• End-user applications --- application level
• VERITAS File System (VxFS) --- file system level
• VERITAS Volume Manager (VxVM) --- volume level
• Operating system --- device level

Although CDS allows data volumes and their contents to be easily migrated between heterogeneous systems, it does not enable concurrent access from different types of platform unless this is supported at all levels that are required.

CDS utilizes a new disk type (auto:cdsdisk). To effect data sharing, VxVM supports a new disk group attribute (cds) and also supports different OS block sizes.

CDS is a license-enabled feature that is supported at the disk group level by VxVM and at the file system level by VxFS.

Sharing Data Across Platforms

It must be emphasized that, while volumes can be exported across platforms, the data on the volumes can be shared only if data sharing is supported at the application level. That is, to make data sharing across platforms possible, it must be supported throughout the entire software stack.

For example, if a VxFS file system on a VxVM volume contains files comprising a database, then:

• Disks can be recognized (as cds disks) across platforms.
• Disk groups can be imported across platforms
• The file system can be mounted on different platforms

However, it is very likely that, because of the inherent characteristics of databases, you may not be able to start up and use the database on a platform different from the one on which it was created.

Another example is where an executable file, compiled on one platform, can be accessed across platforms (using CDS), but may not be executable on a different platform.

Sector Size

Sector size is an attribute of a disk drive (or SCSI LUN for an array-type device), which is set when the drive is formatted. Sectors are the smallest addressable unit of storage on the drive, and are the units in which the device performs I/O. The sector size is significant because it defines the atomic I/O size at the device level. Any multi-sector writes which VxVM submits to the device driver are not guaranteed to be atomic (by the SCSI subsystem) in the case of system failure.

Block Size

The block size is a platform-dependent value that is greater than or equal to the sector size. Each platform accesses the disk on block boundaries and in quantities that are multiples of the block size. Data that is created on one platform, and then accessed by a platform of a different block size, can suffer from the following problems:

• Addressing issues
• The data may not have been created on a block boundary compatible with that used by the accessing platform.
• The accessing platform cannot address the start of the data.
• Bleed-over issues

The size of the data written may not be an exact multiple of the block size used by the accessing platform. Therefore the accessing platform cannot constrain its I/O within the boundaries of the data on disk.

Operating System Data

Some Operating Systems (OS) require OS-specific data on disks in order to recognize and control access to the disk.