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SubjectRe: What still uses the block layer?
On Sunday 14 October 2007 5:24:32 pm James Bottomley wrote:
> On Sat, 2007-10-13 at 16:05 -0600, Matthew Wilcox wrote:
> > On Thu, Oct 11, 2007 at 08:11:21PM -0500, Rob Landley wrote:
> > > My impression from asking questions on the linux-scsi mailing list is
> > > that the scsi upper/middle/lower layers doesn't use the block layer
> > > described in Documentation/block/*.
> >
> > Entirely incorrect.
> OK, right ... could we please get a sense of decorum back on this list.

Did I reply to the insult?

> Rob, if you didn't ask your alleged questions in such a pejorative
> manner, we'd get a lot further

I'm not attempting to be pejorative.

I admit a certain amount of personal annoyance that once the SCSI layer
consumes a category of device (USB, SATA, PATA), they can often _only_ be
used by going through the SCSI midlayer. (This strikes me as analogous to
TCP/IP claiming ethernet and PPP devices so thoroughly that you can no longer
address them as eth1 or /dev/ttyS0.)

This has the annoying effect of bundling together different types of devices
and making device enumeration unnecessarily difficult: my laptop only has one
SATA hard drive and can't gain another without a soldering iron, but that
drive could move from /dev/sda to /dev/sdb if I reboot the system with a USB
key plugged in. This seems like a regrettable loss of orthogonality to me.
I remember back when /dev/usb0 and /dev/hda were separate devices that showed
up in /dev, but these days "it's SCSI" seems to trump "it's USB", "it's ATA",
or "it's SATA". (Even though none of those are actually SCSI hardware, they
just send a similar packet protocol across the wire.)

The fact that udev can theoretically unwind this hairball is not an excuse for
conflating different categories of devices in the first place. Avoiding an
unnecessary problem seems superior to trying to get udev to solve it. Note
that Ubuntu 7.04 solves it by sticking a UUID on every _partition_, and then
spinning up my external USB hard drive trying to find the root partition on a
reboot. Tell me how this can be considered progress:

> # /etc/fstab: static file system information.
> #
> # <file system> <mount point> <type> <options> <dump> <pass>
> proc /proc proc defaults 0 0
> # /dev/sda1
> UUID=04d1b984-bd65-46f1-9a77-c158cf4bed1b / ext3
defaults,errors=remount-ro,noatime 0 1
> # /dev/sda5
> UUID=cdf0936d-9f19-42c6-b131-9fefcf1321ef none swap sw
0 0
> /dev/scd0 /media/cdrom0 udf,iso9660 user,noauto 0 0
> UUID=86bbb512-ab7e-4a12-8618-1190f032c082 /boot ext3 defaults 0 0

Conflating categories of hardware that cannot easily be enumerated (USB) with
categories that can (the SATA hard drive in my laptop, of which there can be
only one) strikes me as a bad thing. Putting them in a common "scsi device
pool" within which they do not enumerate consistently is not something I
enjoy dealing with.

However, the response to my attempts to express this dissatisfaction on the
SCSI list a few months ago came too close to a flamewar for me to consider
continuing it productive. I'd still love to update the "2.4 scsi howto" and
corresponding sg howto, but lack the expertise. The SCSI layer really isn't
my area, and I was much happier back when I could avoid using it at all.

The question I was trying to ask _here_ was about the block layer. I seem not
to have asked it very well. Sorry 'bout that.

"One of my most productive days was throwing away 1000 lines of code."
- Ken Thompson.
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