Question about scsi device names

Question about scsi device names

[walt]

Hi list,

Here is some background for my question:

I have a machine with two sata disks, one connected to the
onboard sata controller, the other to an add-on ESATA board
plugged into the PCIX slot.

My puzzle:

When I boot a kernel from the 2.6.28 series, the onboard
controller's disk gets dubbed /dev/sdb, and the ESATA disk
is /dev/sda.

When I boot the same machine with a more recent kernel like
Linus's 2.6.30 series, the device names are reversed.

Is this change in behavior expected based on recent kernel
patches?

Thanks.

Re: Question about scsi device names

[Billy Crook]

It's been long enough someone probably already answered you off list,
but just in case you're still wondering, the letter naming of block
devices in /dev/ is arbitrary and should be presumed unreliable.  It
is dependant on the order in which the devices were detected.  In your
case, it is likely that modules are being loaded in another order than
before.  This could happen for a variety of reasons including module
renames, removals, or additions.

To reliably reference block devices, use LABEL=, or UUID= in fstab, or
/dev/disk/by-*/* anywhere else.

This is particularly useful in SANs or switched storage like FC,
iSCSI, or SAS, where the order individual disks are detected can
realistically change from second to second.

On Thu, Jun 11, 2009 at 19:36, walt<w41ter@gmail.com> wrote:
> Hi list,
>
> Here is some background for my question:
>
> I have a machine with two sata disks, one connected to the
> onboard sata controller, the other to an add-on ESATA board
> plugged into the PCIX slot.
>
> My puzzle:
>
> When I boot a kernel from the 2.6.28 series, the onboard
> controller's disk gets dubbed /dev/sdb, and the ESATA disk
> is /dev/sda.
>
> When I boot the same machine with a more recent kernel like
> Linus's 2.6.30 series, the device names are reversed.
>
> Is this change in behavior expected based on recent kernel
> patches?
>
> Thanks.
>
>
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Re: Question about scsi device names

[walt]

Billy Crook wrote:
> ... the letter naming of block
> devices in /dev/ is arbitrary and should be presumed unreliable.  It
> is dependant on the order in which the devices were detected.  In your
> case, it is likely that modules are being loaded in another order than
> before.  This could happen for a variety of reasons including module
> renames, removals, or additions.
>
> To reliably reference block devices, use LABEL=, or UUID= in fstab, or
> /dev/disk/by-*/* anywhere else...

Thanks, that's helpful info.  The reason this came up is that the sata
disk on the onboard controller is the boot disk, and it's annoying to
have the boot fail because the ESATA disk is powered on or off, as the
case may be.

The kernel chooses the root partition based on its own value of rdev,
I believe, and that's what I'm trying to set properly.  I've been trying
various different combos of the label and /dev/disk/foo/bar but in the
end rdev just winds up being /dev/sda or /dev/sdb, regardless of what I
type.

Do you know of any way to get around this problem?

Thanks!


> On Thu, Jun 11, 2009 at 19:36, walt<w41ter@gmail.com>  wrote:

>> I have a machine with two sata disks, one connected to the
>> onboard sata controller, the other to an add-on ESATA board
>> plugged into the PCIX slot.
>>
>> When I boot a kernel from the 2.6.28 series, the onboard
>> controller's disk gets dubbed /dev/sdb, and the ESATA disk
>> is /dev/sda.
>>
>> When I boot the same machine with a more recent kernel like
>> Linus's 2.6.30 series, the device names are reversed...

Re: Question about scsi device names

[Konrad Rzeszutek]

On Tue, Jun 16, 2009 at 04:26:58PM -0700, walt wrote:
> Billy Crook wrote:
>> ... the letter naming of block
>> devices in /dev/ is arbitrary and should be presumed unreliable.  It
>> is dependant on the order in which the devices were detected.  In your
>> case, it is likely that modules are being loaded in another order than
>> before.  This could happen for a variety of reasons including module
>> renames, removals, or additions.
>>
>> To reliably reference block devices, use LABEL=, or UUID= in fstab, or
>> /dev/disk/by-*/* anywhere else...
>
> Thanks, that's helpful info.  The reason this came up is that the sata
> disk on the onboard controller is the boot disk, and it's annoying to
> have the boot fail because the ESATA disk is powered on or off, as the
> case may be.
>
> The kernel chooses the root partition based on its own value of rdev,

Which on most distros is the initrd image (/dev/ram0)?

You _need_ to have an initrd image. Then rdev points to (1,0 -
/dev/ram0) and executes the /init process which takes care of scaning
the disks for the labels and pivots in a root filesystem depending on the label
(or the LVM if that is there).

Re: Question about scsi device names

[walt]

Konrad Rzeszutek wrote:
> On Tue, Jun 16, 2009 at 04:26:58PM -0700, walt wrote:

>> The kernel chooses the root partition based on its own value of rdev,
>
> Which on most distros is the initrd image (/dev/ram0)?
>
> You _need_ to have an initrd image. Then rdev points to (1,0 -
> /dev/ram0) and executes the /init process which takes care of scaning
> the disks for the labels and pivots in a root filesystem depending on the label
> (or the LVM if that is there).

Excellent, thanks very much.  This is the first time in many years I've
had to worry about a vaguely defined root partition, so I forgot about
initrd.