RAW device and block device

[Mridul Dutta]

HI all

What is the difference between RAW device and Block device in HP-UX?

MRIDUL

[Pete Randall]

The block device is cached. I/O to the device is read into memory, referred to as buffer cache, in large blocks. A raw device is more typically used with a data base application which provides the necessary buffering to achieve efficient I/O.


Pete

[Christine Hartman · Accepted solution]

Raw devices bypass the buffer cache and use DMA directly to/from the program's I/O buffers; they are normally restricted to full-sector transfers. Using raw devices generally will give you faster performance. Raw devices are used when making new filesystems, when checking unmounted filesystems, or for copying quiescent filesystems. The block devices are used to mount filesystems. Each disk has a block device interface where the system makes the device byte addressable and you can write a single byte in the middle of the disk.

[HGN]

Hi

Here are a few threads which talks about the difference
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=878764
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=792810
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=90852
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=575829

Hope this helps

Rgds

HGN

[Ivan Ferreira]

RAW devices can be faster for certain applications like databases because they does not contain a file system, and they don't use cache for the same reason. You don't mount a RAW device.

The problem is the management, you cannot manage a raw device using O.S. tools, for example, check capacity, backup and restore data (dd could be used but is not efficient), find files, etc.