Re: raw devices

From: Dan Strick (strick_at_covad.net)
Date: 08/01/04

  • Next message: Wojciech Puchar: "Re: bandwidth question" 
    Date: Sat, 31 Jul 2004 16:12:30 -0700 (PDT)
To: freebsd-questions@freebsd.org

    On Sat, Jul 31, 2004 at 10:30:21PM +0200, Wojciech Puchar wrote:
    >
    > where are raw devices in FreeBSD? do they exist at all?
    >

    and on Sat, 31 Jul 2004 21:45:17 +0100, Matthew Seaman responded:
    >
    > Actually, all devices under FreeBSD are raw or character devices.
    > Block devices on the other hand disappeared a long time ago. It's all
    > to do with having an advance VM system, apparently:
    >
    > http://www.freebsd.org/doc/en_US.ISO8859-1/books/arch-handbook/driverbasics-block.html
    >

    I checked out the referenced page, which began with something like:

                            13.5 Block Devices (Are Gone)

       Other UNIX systems may support a second type of disk device known as
       block devices. Block devices are disk devices for which the kernel
       provides caching. This caching makes block-devices almost unusable,
       or at least dangerously unreliable. The caching will reorder the
       sequence of write operations, depriving the application of the ability
       to know the exact disk contents at any one instant in time. This makes
       predictable and reliable crash ...

    I knew that the block devices were gone and that the block device names
    now referred to character devices, but I had not examined the reasons for
    this or considered the consequences. Perhaps this explains why old SCSI
    disks are such incredibly bad performers under modern FreeBD. I had just
    assumed that the drivers for the old SCSI host adapters had been botched
    when rehacked for the new FreeBSD SCSI system and nobody cared because
    they were all using modern SCSI host adapters. The performance of my
    old SCSI hardware is so egregiously abysmally atrociously abominably
    inexcusably perversely bad that if I had to use it for my primary disk
    storage I would now be running Linux instead of FreeBSD. (Modern ATA
    disks seem to work quite well under FreeBSD if you can somehow manage
    to avoid ATA controller and cable misconfigurations that drive I/O rates
    way down.)

    Does anyone know if there are online records of discussions of such
    issues?

    Dan Strick
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