Unbuffered or buffered disk access on UNIX

You can allocate disk space in two ways. You can either use files that are buffered through the operating system, or you can use unbuffered disk access.

Files that are buffered through the operating system are often called cooked files.

Unbuffered disk access is also called raw disk space.

When dbspaces are located on raw disk devices (also called character-special devices), the database server uses unbuffered disk access.

To create a raw device, configure a block device (hard disk) with a raw interface. The storage space that the device provides is called raw disk space. A chunk of raw disk space is physically contiguous.

The name of the chunk is the name of the character-special file in the /dev directory. In many operating systems, you can distinguish the character-special file from the block-special file by the first letter in the file name (typically r). For example, /dev/rsd0f is the character-special device that corresponds to the /dev/sd0f block-special device.

For more information, see Allocating raw disk space on UNIX.

A cooked file is a regular file that the operating system manages. Cooked file chunks and raw disk chunks are equally reliable. Unlike raw disk space, the logically contiguous blocks of a cooked file might not be physically contiguous.

You can more easily allocate cooked files than raw disk space. To allocate a cooked file, you must create the file on any existing partition. The name of the chunk is the complete path name of the file. These steps are described in Allocating cooked file spaces on UNIX.

In a learning environment, where performance is not critical, or for static data, cooked files can be convenient. If you must use cooked UNIX files, store the least frequently accessed data in those files. Store the files in a file system with minimal activity.

For cooked file chunks, the operating system processes all chunk I/O from its own buffer pool and ensures that all writes to chunks are physically written to the disk.

Important: While you must generally use raw disk devices on UNIX to achieve better performance, if you enable the DIRECT_IO configuration parameter, the performance for cooked files can approach the performance of raw devices used for dbspace chunks. This occurs because direct I/O bypasses the use of the file system buffers. If you have an AIX® operating system, you can also enable concurrent I/O for HCL Informix® to use with direct IO when reading and writing to chunks that use cooked files. For more information about using direct IO or concurrent IO, see the HCL Informix Performance Guide.

To determine the best device for performance, perform benchmark testing on the system with both types of devices for the dbspace and table layout.

When using raw disks, you are not required to take any special action to create chunks and files that are larger than two gigabytes. If you want to create large chunks in cooked files, or if you want to use the various database export and import utilities with large files, you must ensure that the files systems that hold the large files are appropriately configured.

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