> > It was also stated that in the next kernel development of 2.7 that raw > > device > > support could be removed. Now I suppose that might be changed by popular > > demand as well. > > I believe that the reason for not implementing raw devices on Linux was the > fact > that Linux was designed for x86 and the way Linux did implement DMA to > devices > would have made a raw device slower than a buffered device.
I believe that your belief is entirely wrong. I've used the (2.2 and 2.4) /dev/raw/* version of Linux raw devices for several years now, to create a high-speed disk duplication system. It runs on commodity X86 hardware, using off-the-shelf PCI-bus UDMA IDE controllers. It can copy 1-disk-in, 4-disks-out, DMA direct to/from memory, at very high speeds. In fact, it appears to me (based on the timing tests I've done) that its upper speed limit is not the CPU - it's the total amount of PCI bus bandwidth to/from memory available. It's quite a bit faster than doing a buffered-device copy. I don't know the reasons behind the apparent decision to switch from the /dev/raw/ approach, to an O_DIRECT approach, which has apparently been made in the 2.6 kernel. I suspect that it has to do with the "administrative" issues - it eliminates the need to have some sort of centralized coordination or registry to "manage" the decision of what underlying devices are bound to which /dev/raw/* pseudodevices. It may also allow for slightly better security. With /dev/raw/* devices of the Linux sort, you have to be root (or be able to run a suid-root program) to bind the underlying device or partition to a /dev/raw/* pseudodevice. With O_DIRECT, you just have to have read/write access to the device. The O_DIRECT approach would allow database partitions (for example) to be made read/write to the database daemon or program, and accessed directly, with no root-level administrative access required. -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
