Re: raw devices
On Sat, Jul 31, 2004 at 10:25:40PM -0500, Kevin D. Kinsey, DaleCo, S.P. wrote: Matthew Seaman wrote: On Sat, Jul 31, 2004 at 10:30:21PM +0200, Wojciech Puchar wrote: where are raw devices in FreeBSD? do they exist at all? 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 Hmm, now I'm a tad curious --- or confused. ceri@ just committed a revised synopsis I hacked at for the handbook's Vinum chapter which states, among other things: In addition to supporting various cards and controllers for hardware RAID systems, the base FreeBSD system includes the Vinum Volume Manager, a block device driver that implements virtual disk drives. So is there conflicting data here? Might be good to figure out the truth before the next edition handbook goes to the printer (which may be soon...) However, I'd be first to admit a dire lack of knowledge here... help? I think the point is not that a FreeBSD system never communicates with any device in block mode, but that there's no exposure of that interface outside of the kernel. The original BSD distinction between character and block devices let people achieve a degree of optimization in certain circumstances by short circuiting the bufferring etc. involved in using a character device and interacting more directly with the hardware. However, that concept was first developed probably some twenty-odd years ago, and the state of the art in disk and virtual memory technology has come on a long way since then. Nowadays, short circuiting the higher levels of buffer caching just doesn't make sense. Let the VM system choose when to push blocks of data out to the disks or pseudo-disks (ie. RAID arrays, vinum devices etc), or when to read them in. It knows best. Cheers, Matthew -- Dr Matthew J Seaman MA, D.Phil. 26 The Paddocks Savill Way PGP: http://www.infracaninophile.co.uk/pgpkey Marlow Tel: +44 1628 476614 Bucks., SL7 1TH UK pgp0ykp32zI7J.pgp Description: PGP signature
Re: raw devices
On Sat, Jul 31, 2004 at 10:30:21PM +0200, Wojciech Puchar wrote: where are raw devices in FreeBSD? do they exist at all? 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 Cheers, Matthew -- Dr Matthew J Seaman MA, D.Phil. 26 The Paddocks Savill Way PGP: http://www.infracaninophile.co.uk/pgpkey Marlow Tel: +44 1628 476614 Bucks., SL7 1TH UK pgppniBagdrh3.pgp Description: PGP signature
Re: raw devices
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 ___ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: raw devices
Matthew Seaman wrote: On Sat, Jul 31, 2004 at 10:30:21PM +0200, Wojciech Puchar wrote: where are raw devices in FreeBSD? do they exist at all? 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 Cheers, Matthew Hmm, now I'm a tad curious --- or confused. ceri@ just committed a revised synopsis I hacked at for the handbook's Vinum chapter which states, among other things: In addition to supporting various cards and controllers for hardware RAID systems, the base FreeBSD system includes the Vinum Volume Manager, a block device driver that implements virtual disk drives. So is there conflicting data here? Might be good to figure out the truth before the next edition handbook goes to the printer (which may be soon...) However, I'd be first to admit a dire lack of knowledge here... help? Kevin Kinsey DaleCo, S.P. ___ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
