At 01:12 PM 6/28/2001 -0400, Nasta wrote:
>Let me give you an example. Suppose someone implemented SCSI on the Q40.
>Then, you could connect an IDE and a SCSI hard disc to it. Boh would use
>the QWA file system, but here is the problem: each device would have it's
>own copy of the QWA handling code. If there was a CD connected to either
>(and here we would already be circumventing another driver that works on
>the same hardware), with a QXL.WIN on it, then the CD driver would also
>need it's own copy of the QWA handling code. And, there would be no way to
>make one be win1_ the other win2_ and the QXL.WIN on the CD be a win3_,
>although they logically should be. It could be done, if they were all set
>to _USE another name, then some sort of DEV_USE would be used to make them
>dev1,2,3, and then a DEV_USE win would make them win 1,2,3. Total number of
>QWA copies: 3. Total number of directory devices used: 4. If you think
>about it, only one copy of the QWA handler code and only one device driver
>would really be needed.
Now, I'm not an expert in XFS (the IRIX file system), but I spent enough
time listening to the experts talking about it so that I know a little
about it.
With XFS, the file system is separated from the physical device by a
virtual file system (vfs). In other words, an XFS inode, points to a vfs
inode, which points to a physical device sector. This way XFS can work
across hard drives and CD's. This also allows for a volume manager (XLV)
to come into play, and step in between XFS and the VFS. XLV allows
striping and disk mirroring.
With SCSI pretty much being the only way to hook up disks and tapes, the
SCSI controller one the one point where every call has to go through and
the controller could handle multiple conversations (sort of timeshared as
the SCSI protocol only allows one conversation at a time).
Something like this could be implemented in SMSQ/E, but it would take a
concerted effort. The XFS team had about 5-8 people at any one time. With
SMSQ/E we just have TT.
Tim Swenson
