Mailinglist Archive: opensuse (3349 mails)

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Re: Why /dev/sda [WAS: can no longer see usb drive]
  • From: Greg Freemyer <greg.freemyer@xxxxxxxxx>
  • Date: Mon, 2 May 2005 09:39:41 -0400
  • Message-id: <87f94c3705050206395a3b21bf@xxxxxxxxxxxxxx>
On 5/1/05, Colin Carter <colincarter@xxxxxxxxxxxxxx> wrote:
> On Sunday 01 May 2005 23:59, Greg Freemyer wrote:
> > On 4/30/05, Colin Carter <colincarter@xxxxxxxxxxxxxx> wrote:
> > > On Sunday 01 May 2005 01:35, Greg Freemyer wrote:
> > >
> > > Off topic a little, but can you explain the philosophy of SuSE emulating
> > > SCSI connection? (I assume that that is what they are doing above.)
> > > This emulation actually prevented me from installing SuSE 9.0 as it
> > > was reading my DVD drive as IDE until it started to install and then
> > > it switched to emulating SCSI and then said it couldn't read the same
> > > drive which it had been reading. Ahhhhh!
> > > I had to go out and buy 9.1 which (quote SuSE) "fixed that issue".
> > > I didn't have an issue with the IDE drive being an IDE drive; why
> > > do SuSE make it an issue? I have been told that SCSI is faster, but
> > > surely that statement only applies when it is true SCSI and not just
> > > and *extra* emulation filter/process?
> > > snip
> >
> > It is not a SuSE issue, it is a Linux Kernel issue, so all the distros
> > should behave similarily.
> >
> > As I understand it, the difference between /dev/hdx and /dev/sdx is
> > which basic block driver set you use.
> >
> > The /dev/hdx series is only used by traditional Parrallel ATA (PATA)
> > drives as far as I know.
> >
> > In addition to SCSI, all the newer drive technologies (SATA / USB /
> > Firewire / FC / SAS / iSCSI) are being handled exclusively by the
> > /dev/sdx block driver.
> >
> > The sata kernel library (libata) is slowly being enhanced to support
> > PATA drives, so in a year or two I would expect /dev/hdx to simply go
> > away for new 2.6 kernels.
> >
> > Greg
> Hi Greg,
> Thanks very much for this reply/info. Much appreciated.
> I understand what you wrote - very interesting.
> But why are most current hard drives ATA-100 ?
> I believe that SCSI are generally more expensive.
> Is Linux just betting on SCSI being the future?
I don't think it is a bet of ATA drives vs. SCSI drives.

My impression is that the SCSI code was better modulorized in the
kernel than the ATA / IDE code. Indeed one of the major complaints
about the 2.5 kernels (R&D kernels only - pre 2.6) was the "IDE driver
mess" (as I recall).

In the 2.6 kernel SCSI has at least two layers, the "scsi midlayer"
and the LLDD (low level device driver). There is an ongoing effort to
put most of the common code in the midlayer and restrict the LLDD to
device specific requirements only.

With the IDE drivers, I think there is far less common code.

So when new types of drive technolgies needed drivers, the kernel
developers decided to leverage the existing scsi midlayer code by
extending it to handle the new device types.

Thus the "scsi midlayer" is evolving into a "storage medium midlayer"
or something like that.

As I said, the SATA and USB disk drivers were developed around the
scsi midlayer. I have seen discussions of the SATA driver (libata)
being extended to handle PATA drives.

If/When that happens and the current IDE driver is dropped, I think
all storage sub-systems (SCSI / PATA / SATA / USB / Firewire / FC /
SAS / iSCSI) will be using the "scsi midlayer" and thus will be
/dev/sdx devices.

Greg Freemyer
The Norcross Group
Forensics for the 21st Century

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