Re: What still uses the block layer?

2007/10/15, Rob Landley <rob@xxxxxxxxxxx>:
On Sunday 14 October 2007 8:45:03 pm Theodore Tso wrote:
On Sun, Oct 14, 2007 at 06:45:44PM -0500, Rob Landley wrote:
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.)

That's because modern USB, ATAPI (what was once known as IDE), SATA
really *all* using the SCSI command protocols at the low level,

Ok, I'll bite. If it's all "real" scsi, why does ioctl(SG_EMULATED_HOST)
exist? exist if it's all "real" scsi?

How do you define real SCSI ? The definition of SCSI in the kernel is
"a device that accept the SCSI command set" (more precisely "a
suitably large subset a the SCSI command set". It looks as if you
definition of SCSI is "a device that is sold with written SCSI on the
box and that attaches to a card with SCSI written on the box"; is it
correct ?

The host is the expansion card that connects the device to the
motherboard. If it is emulated this means that it is not a native
SCSI host. In case of USB drives/keys this is probably the case.

just as Ethernet and PPP interfaces really are fundamentally the
same thing.

They're the same thing?

Do you mean that on a system with both, going:
ifconfig eth1 66.92.53.140
ifconfig ppp 192.168.0.42

Would be functionally equivalent to:
ifconfig eth1 192.168.0.42
ifconfig ppp 66.92.53.140

So if on one boot the addresses are assigned the first way, and upon reboot
they're assigned in the second way by exact the same set of commands... well
that's not IMPORTANT, is it? (Or is it that everyone everywhere should use
dhcp for everything, and static addressing is obsolete and no longer
supported?

You are really looking like you are out for a fight.

Apparently dhcp addresses should be delivered by machines with
only one network interface of any type...)

I don't understand this one.

This is my objection. Even when enumerating multiple devices of the same type
is tricky, enumerating multiple devices of _different_ types should not be.
There's a great big type indicator that is being _deliberately_ ignored, and
large classes of devices (millions of laptops) where you know there's only
going to be _one_ instance of a given type.

Your objection is interesting. It is lost in the middle of e-mails which,
to the untrained eye, look like you are trying to fight everyone and
everybody.

By the way, ethernet cards contain a unique MAC address. Hard drives do not
seem to, or if they do it's not being consistently exposed in a way I can
find. This is sad. (No, reading data from the device to determine this gets
us back to the "spinning up the external USB drive to find my root partition"
gripe mentioned earlier.)

As far as I can tell the hard drives do not have serial numbers easily
readable by the kernel (I think it's only printed on the label). However
(feverishly plugging his USB key in the laptop), you can tell how a drive
is attached to the motherboard:

Laptop's SATA drive:
cognac $ readlink /sys/block/sda/device
../../devices/pci0000:00/0000:00:12.0/host0/target0:0:0/0:0:0:0

USB key:
coghac $ readlink /sys/block/sdb/device
../../devices/pci0000:00/0000:00:13.5/usb6/6-3/6-3:1.0/host4/target4:0:0/4:0:0:0

By the way, did you look in /dev/disk/by-id (udev magic) ? It's probably
not very difficult to reconfigure udevd to not read the UUIDs of the
partitions and not spin up your holy external disk at each reboot. I think
the one that is spinning up your holy external hard drive is udevd. By
the way, how many time do you reboot instead of resuming from
suspend-to-disk ? Have you given a try to TuxOnIce ?

If you had asked your first question in a way similar to this one:

"I have my laptop hard drive that shows as different devices depending
whether there are USB drives plugged in or not, what should I do ?
Shouldn't SATA/USB drives/PATA/iSCSI drives be enumerated in different
queues ?"

You would probably have received more interesting answers and less
insults.

You can rail against it, but that's the mark of someone who
refuses to accept reality.

Let me clarify: I'm talking about device enumeration.

I've never had trouble enumerating a device that was _not_ routed through the
scsi layer, largely because the systems I work with don't usually have more
than one device of the same type. (There are millions of laptop and desktop
devices out there where this is the common case. As I said, I may have four
USB ports and the ability to plug hubs into them, but you can't add another
SATA hard drive to my laptop without a soldering iron.)

However, as soon as a device _is_ routed through the scsi layer (as PATA was a
few versions back), it gets conflated with numerous other devices. This
creates problems. SATA isn't hard to enmerate in my laptop, USB potentially
is. Dumping all the SATA devices into the same bucket with the USB devices
makes both harder to enumerate.

Indeed. Propose a solution. Remember that it is indispensable that all
goes through the SCSI layer(s) because all those devices respond to the
same command set thus you do not want several implementations of
the routines.

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.)

You're showing your ignorance here.

I have buckets of ignorance. It's why I ask questions.

Once again. You are so aggressive in your asking that it does not
lead to an interesting discussion.

In fact in the past few years,
ATA and SCSI has been converging significantly,

And down far enough all these devices are powered by electricity. Are we
going to wind up with /dev/electric[1-999]?

Out for a fight ?

SATA != PATA != USB. But /dev/sda can be PATA, /dev/sdb SATA, and /dev/sdc
USB. And they can move relative to each other. This didn't used to be the
case. Why is it considered an improvement?

Each device is different from each other (they do not share their atoms).
Where do you want to put the line between using a single driver for them
or not ?

In that case the sd driver registers all disk-like devices that respond to
(a suitably large subset of) the SCSI command set. It is an improvment
to have all devices share a driver because if you improve it, you improve
it for all the devices; if you debug it, you debug it for all your
devices; you
use less memory.

The enumeration of the devices is not the nightmare you are trying to
imply. If it is hard in your particular case, many people are likely to want
to help you. Just try to ask politely, without shouting, without saying that
block layer is useless, without saying that device enumeration in the
SCSI layer (or sd device driver) is braindead. If you want to propose a
change, propose it: "we could also do it that other way". "The way the
SCSI disk is attached should show in the name of the device". I suspect
this will be refused anyway because that would mean that you need
a (series of) major block number(s) for each type of SCSI attachment
(if the device has not a different major block number, there is nothing
short of udev that can give it a different name).

with the ATAPI
specification has essentially incorporating the SCSI protocol by
reference and by value --- with the point that SAS was developed by
the SCSI Trade Association, and SAS is effectively a superset of SATA,
to the point where with care, you can actually mix SAS and SATA drives
on the same in enclosure (SAS and SATA are physically compatible on
the connector level).

I'm aware of this, and under the impression they're both modified gigabit
ethernet at the PHY level. Should the hard drive become eth2?

Out for a fight ?

More to the point, with SATA, hot plugging has been designed in, so
probing order is not going to be well defined,

The spec may define the capability to hotplug, but your average
laptop doesn't not offer the capability to hotplug anything into its
SATA controllers.

How long before eSATA enabled laptops (with eSATA enumerated
before SATA obviously) ?

The hard drive is screwed in (due to the portability part of laptopness),
all the controllers wired onto the motherboard are accounted for, none
are exposed externally. What _is_ exposed externally is USB, and if you
want to add an extra hard drive you can buy a cheap USB one at Fry's.

In such a case, which is common, the first SATA hard drive is reliably the
disk containing the root partition, and there's no need to stick a UUID
in /etc/fstab.

The problem is, "the first SATA hard drive" is not a stable identifier in a
system where SATA and USB devices are dumped in the same bucket
and given big stir. Dumping SATA and USB devices into the same
bucket (because they smell a bit like SCSI) is what I am objecting to.

You should have told it in the first place -- with cooler tone.

just as with USB
devices. And there are already relatively common situations where the
same disk can show up via multiple different interfaces.

It was also possible to buy a hotplug PATA ide enclosure. So what? The vast
majority of traditional IDE users happily ignored this, and went on with
their lives.

For example, if you have a modern Thinkpad with an secondary SATA hard
drive in an Ultrabay, and you plug it into the Ultrabay in your T60,
it will show up as a SATA drive.

I remember the config option about enumerating onboard IDE controllers first.
It didn't really matter what order they were enumerated in as long as it was
controllable.

Presumably if the primary SATA hard drive was /dev/sata and the slot
with "secondary" in its name got /dev/satb, life would be good. And the
presence or absence of /dev/satb wouldn't affect USB devices and such if they
weren't in the same namespace.

However, if you plug it into the
Advanced dock, it shows up as a USB device.

You plug it in somewhere else, it shows up somewhere else. This sounds
familiar to old IDE users. :)

How is it harder for udev to make a stable symlink for this drive that
sometimes points to /dev/satb and sometimes to /dev/usb1? (Harder than a
symlink that sometimes points to /dev/sdb and sometimes to /dev/sdd? You
don't have persistent naming _now_, so the objection seems to be that
maintaining the distinction between device types would not be a perfect
solution in all cases. I agree. So?)

And with iSCSI not only
can you encapsulate a SCSI command stream over USB, you can do so over
IP as well.

Yup. And you've been able to make a network block device for years. They
showed up as /dev/nd0, a distinct type of block device which you (and your
scripts) could find. Now yet another way of doing the same thing is mixed
into the same scsi bucket and given a stir...

In any case, regardless of how the physical SATA drive is
attached to the system, you want it to show up as the same device and
be mounted in the same location.

If my laptop's hard drive reliably showed up as /dev/sda every time, and I
could count on that, I wouldn't be complaining about it. The entire problem
is that it isn't guaranteed to do that, and thus /etc/fstab is a nightmare I
can't edit.

You could meet this definition of "the same" by having every block device in
the system show up as /dev/block[a-z] no matter what type it was, and all the
char devices show up as /dev/char[aa-zz], shuffle them all each reboot, and
then have all the programs iterate through all of them any time they wanted
something specific.

I'm rather glad that /dev/ttyS0 and /dev/zero aren't easy to mix up.

That's why identifying filesystem by UUID's or Labels is so critical.
This is not a new concept; we've had the capability to do this for
over a decade, and I always knew it would be necessary for us to do
this sooner or later --- which is why I added the UUID support to ext2
back in 1996.

It's necessary for IBM big iron to do this. It's generally not necessary for
laptops or embedded systems to do this if they distinguish between _types_ of
devices, which is something they until recently did for the types of devices
I was interested in, and something they _stopped_ doing when everything got
merged into the scsi layer, and I consider this a regression.

No, distinguishing between types of devices is not a perfect solution to
device enumeration, but it was sufficient for all my use cases for many
years, and would still be if the kernel still did it, and I'm not alone here.

The fact that udev can theoretically unwind this hairball is not an
excuse for conflating different categories of devices in the first
place.

See the thinkpad Ultrabay drive example above.

Last week I drove my laptop so deep into swap (with a "make -j" on qemu) that
after half an hour trying to repaint my kmail window, it locked solid.
Again. You'd think the oom killer would come to the rescue, but it didn't.
Maybe Ubuntu disabled it. I have _2_gigs_ of ram in this sucker, on a stock
Ubuntu 7.04 install (with the "upgrade all" tab pressed a few times), and yet
I managed to make it swap itself to death one more time.

Virtual memory isn't perfect. I've _always_ been able to come up with
examples where it just doesn't work for me. This doesn't mean VM overcommit
should be abolished, because it's useful more often than not.

So you have a counterexample. Ok. I can't actually see how your
counterexample would be worse off than it is now; just differently worse off.

You address hosts by
IP address; it doesn't matter whether you access them via a PPP
interface, or a wireless interface, or a ethernet interface.

It does when I'm configuring the interfaces.

Similarly, a disk could in theory be accessible over USB, SATA, or
iSCSI, and the Thinkpad example is only one such where the same
filesystem might be accessible over multiple interfaces. And with
multipath fiber channel SAN's (and I hate to break it to you, but FC
also uses SCSI protocols) storage is very much looking more and more
like networking.

And in the networking world I'm able to say that this local machine has a
static IP that is not world-routable. It is separate, manually configured, I
put it _right_here_, and I personally know that it's not going to move
because I'm the one who put it there and I'm the only one who would move it.

Over on the networking side of things I can "ifconfig lo 127.0.0.1" without
first probing all the interfaces to figure out which one's loopback and which
one's the wireless card.

I note that the eth0 and eth1 names are dynamically assigned on a first come
first serve basis (like scsi). This never causes me a problem because the
driver loading order is constant, and once you figure out that eth0 is
gigabit and eth1 is the 80211g it _stays_ that way across reboots, reliably.
Yeah, it's a heuristic. Hands up everybody relying on such a heuristic in
the real world.

It' also easier because you have the MAC address to help.

Possibly one solution here is to document that the SATA drivers load
before any other scsi device, and the driver subsystem _waits_ for
that to finish enumerating before trying any other kind of scsi device,
with a barrier of some kind), and then any SATA devices present at
boot time will reliably get those names in that order (no races, no
variation) and anything after that is a separate problem. (Of course
this would involve making it true if it currently isn't. It's still a mess to
dump all sorts of different devices in the same namespace, but at
least for the common case of a laptop with a SATA root partition this
would let us get the UUID out of /etc/fstab).

There is also the need to ensure that each distribution loads the ahci
driver before any other SCSI or emulated SCSI host. It looks as if Ubuntu
does not do it that way (I've tried to understand the module probing logic
in the initramfs but have miserably failed; too much magic probably).

Remember that udevd starts your holy external hard disk, not the
UUID in /etc/fstab (obiously if you have an UUID in fstab, you need udev
to probe for it, but it'll do it regardless of whether or not UUSID is present
in fstab.

Regards,

Loïc
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