Adaptive Server 12.5 allows you to create and mount its database devices on raw bound devices for raw disk I/O. Raw disk I/O has performance advantages since it enables direct memory access from user address space to the physical sectors on the disk, omitting needless memory copy operations from the user address space to the kernel buffers.
Raw disk I/O also assumes that logical and physical I/O are simultaneous, and writes are guaranteed to flush to the disk when the system write() call returns.
For Red Hat 6.2 distributions, you must use a distribution with kernel 2.2.18, at minimum. Correct kernel errata packages can be downloaded from the Red Hat Web sitehttp://www.redhat.com.
When preparing a raw partition device, follow these guidelines:
Do not initialize a database device on the partition that contains your Sybase installation software. Doing so destroys all existing files on that partition.
A raw partition designated for use by Sybase cannot be mounted for use by the operating system for any other purpose, such as for file systems or swap space.
After a Sybase configuration utility or the disk init command has initialized a portion of a partition as a database device, the entire partition cannot be used for any other purpose. Any space left on the partition beyond the size specified for the device becomes inaccessible, unless you drop and re-create the device.
For best performance, place the Sybase software and all disk devices, including the master device, on the same machine.
To avoid any possibility of using a partition that contains the partition map, do not use cylinder 0.
Place the database device on a character device, because the Adaptive Server recovery system needs unbuffered system I/O.
To determine whether a device is a block device or a character device, run this command on the /dev/raw directory:
ls -l
“b” (block) or “c” (character) appears in the left-most column.
Choosing a raw partition
Determine which raw partitions are available.
Determine the sizes of the raw partitions.
From the list of available raw partitions, select a raw partition for each device, based on the size recommendations in Table 2-13.
Verify with the operating system administrator that the partition you have chosen is available.
Make sure the “sybase” user has read and write privileges to the raw partition.
For more information on choosing a raw partition, see
your operating system documentation.
