Date: Sat, 31 Jul 2004 16:12:30 -0700 (PDT) From: Dan Strick <strick@covad.net> To: freebsd-questions@freebsd.org Subject: Re: raw devices Message-ID: <200407312312.i6VNCUCe000467@mist.nodomain>
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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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