On Mon, Oct 15, 2007 at 03:04:00AM -0500, Rob Landley wrote:
SG_EMULATED_HOST was added before Linux 2.4, at least six or seven
years ago. Back then the migration of ATA devices through the various
versions the ATAPI specification and then into SATA was very early in
its evolution, and back then, yes there were people who considered
anything that didn't use the honking huge parallel SCSI cables not
"real" SCSI. Over time, that distinction at both the physical
connector level and logical level has declined to the point of almost
non-existence. It's note quite at the point where SAS exists only to
justify massive prices differences between commodity and "data-center
grade" disks to the benefit of hard drive manufacturers, but it's
darned close. (There are differences such as voltage levels so that
the max cable differences for SAS are larger, etc., but those could
have been optional additions to the SATA spec, and allegedly SAS
drives are supposedly manufactered to be more robust --- although some
recent papers published at FAST have raised some interesting questions
about how true those marketing claims really are in practice.)
No, of course not. But we don't have separate IP stacks for ethernet
and ppp devices. And how we connect to a host via ssh makes no
difference whether we accessed it via Ethernet or PPP. And I would
argue that how we address a filesystem should also make no difference
depending on the path to hard drive.
You can pull a Model and Serial number via hdparm -i, but it's not as
easy to manipulate as a fixed-length MAC address. That's why people
tend to use filesystem UUID's.
That may be true for laptops today, but Linux doesn't run just on
servers. You can easily get home servers with hot-swap SATA bays. My
home fileserver, which is a white box I purchased on my own nickel,
NOT IBM big iron, has 3TB of raw storage for less than $10,000 a year
ago. Today, that amount of home storage with hot-swap SATA drives and
a battery-backed hardware RAID controller could probably be purchased
for about half that price.
And even for laptops, if you need the performance, you can get Cardbus
cards that will allow you to connect eSATA drives to your laptop at
Fry's.
So even if you ignore "big data center" interconnects like FC, this
problem exists even for commodity grade SATA devices.
I agree at the moment we have an issue where if the root device isn't
guaranteed, it forces people to use initrd's, and the quality and
debuggability of initrd's between distro's is highly variable and not
standardized. In practice though the /dev/sda is actually pretty
stable on laptops, especially if you end up compiling ehci and uhci
support as modules (which is a good idea from a power savings point of
view anyway). The reason why Ubuntu and other distributions are using
UUID-based labels is not just because of the root device, but also for
all of the other disks that might be mounted on the system, including
some that might be using USB devices that don't have stable /dev
names.
As another example, it's easy to see a home media server running Linux
which doesn't have any expansion bays for additional hard drive --- so
the only way a user could expand their storage is by using one or more
permanently connected USB disks. So we do need to solve the general
device enumeration problem in the general case; it's not just the case
of IBM "big iron" as you seem to think.
News flash! The kernel wasn't built just for you, and over time, more
and more people will have multiple disk drives of the same type, so we
will need to solve the device naming problem sooner or later. Why not
solve it sooner, especially given that a number of companies (not just
IBM) are funding the organization that is paying *your* salary are
interested in solving the general case?
Furthermore, I've already pointed a number of situations where the
home user might have multiple USB devices on their system today, and
that is probably going to go up over time, not down. Have you seen
how cheap 500GB USB disks are at Costco? And for a typical
unsophisticated user, plugging in another 500G USB disk when they need
more storage is a lot easier than cracking open the computer case and
futzing with screws and disk cables and power connectors.
- Ted
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