Messages in this thread | | | Date | Sun, 30 May 1999 09:17:06 +0200 (MET DST) | From | Gerard Roudier <> | Subject | Re: XFS and journalling filesystems |
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On Thu, 27 May 1999, Theodore Y. Ts'o wrote:
> Date: Mon, 24 May 1999 11:24:35 -0600 > From: Larry McVoy <lm@bitmover.com> > > : (And there are certain features of XFS, such as the features that allow > : Irix to tell the disk controller to send disk blocks directly to the > : ethernet controller, which then slaps on the TCP header and calclates > : the TCP checksum without the disk data ever hitting memory > > This is not quite right, in fact, it is a little unfair to IRIX. There is > no interaction between the file system and/or the block device system > and the networking stack. The way it works is this (I know this code > extremely well since I'm the guy that originally made NFS use both the > networking and the file system to go at 94MByte/sec - Ethan Solomita is > the guy who made it go at 640MByte/second over Super HIPPI): > > I stand corrected. I had heard about the zero copy algorithm you > described, but then when I later heard that Irix had bypassed even the > DMA in and out of memory, I was appropriately horrified. I'm glad to > hear it wasn't true, and that you had no part in implementing or > designing such a horrible hack (i.e., direct device-to-device I/O). :-)
You can design I/O Buses on which the CPU uses the same bus protocol as normal I/O devices and that allows any device to access the whole memory and memory mapped IO space. PCI is obviously a good example of that. (Note that it seems that, when only CPU-to-brain-wired-O/S-guys are involved in the design of an interface with a PCI sub-system, they used to allow the CPU some features regarding BUS accesses that normal I/O devices haven't. This is an hack too, in my opinion).
It is possible by design on those Buses for any device acting as a master to directly talk to another device acting as a target and it is this way I/Os involving the CPU are _actually_ working. In order for direct I/O to I/O not looking like a hack, we probably need to have sophisticated I/O controllers that can host some logic that is currently part of the O/S. This let me think that it is possible, at least on paper, to implement such a system that is absolutely not a hack.
Gérard.
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