Linux: How to create RAW device to be used with Sybase?

When using Sybase on Unix, than the recommended way of saving the databases is directly on disk devices and not on files (what is possible as well). There are two types of disk devices on Linux.

One is called a block device and applications use that device using block access. This means that data is read into cache and given to application in blocks. Example of a block device can be: /dev/sda2. If you issue command ll than in the result you can find, that the type of the device is "b" - block:

[root@SAP01 /]# ll /dev/sda2
brw-rw----. 1 root disk 8, 2 Jul  1 08:11 /dev/sda2

However block devices are not recommended for databases. Database should avoid the system cache. The system cache is good for normal files and in case of database, that is the database engine who should manage the caching of data. 

Unfortunately, on Linux (Fedora) the raw devices are not created automatically. So let's review a couple of commands that may help working with pure raw devices:

raw -qa

displays information regarding existing raw devices. If you did not create them, than initailly the result will be empty.

[root@SAP01 /]# raw /dev/raw/raw2 /dev/sda2
/dev/raw/raw2:  bound to major 8, minor 2

This command creates a raw device bound to the block disk device /dev/sda2. Note the arguments:

• the first one is the name of device that will be created and it must point to /dev/raw/ and must begin with raw and must end with a number
• the second is the existing block device

After issuing the command:

[root@SAP01 /]# raw -qa
/dev/raw/raw2: bound to major 8, minor 2

you will see information that now a new raw device is available. This device could be used by Sybase. Please note, that if a block device and raw device would be used simultaneously, than the data may be/ will be corrupted. Use only one type of a physical disk device a time.

More info regarding raw command:
http://www.tldp.org/HOWTO/SCSI-2.4-HOWTO/rawdev.html