[SOLVED] UDEV - sata HDD always as sda

displace · 09-17-2013, 06:56 AM

Hello,

I'm having some trouble with device names on Cubieboard - the device has a SATA port and two USB 2.0 host ports. When a HDD is connected via the SATA port it appears as /dev/sda. When I connect an extra USB flash key it appears as /dev/sdb. But when cubieboard boots with both devices attached (it boots from a sd-card), the device names switch - usb is now sda, and the disk is sdb.

Is there a way to make the SATA disk always appear as /dev/sda (with a catch*)? I read this can be done with appropriate udev rules. I'm writing my own udev rules for the first time, so I'm having some trouble understanding them, and I don't want the rule to interfere with other devices. There is only one sata port and therefore only one scsi disk. Unfortunately, there's a catch* - the HDD is going to be replaced regularly, so no unique disk identifiers are allowed. All other devices should start at sdb, sdc, sdd, etc even when the SATA HDD is not connected.

The disk symlink also appears:
/dev/disk/by-path/platform-sw_ahci.0-scsi-0:0:0:0
/dev/disk/by-path/platform-sw_ahci.0-scsi-0:0:0:0-part1

And the usb key:
/dev/disk/by-path/platform-sw-ehci.1-usb-0:1:1.0-scsi-0:0:0:0
/dev/disk/by-path/platform-sw-ehci.1-usb-0:1:1.0-scsi-0:0:0:0-part1

Here's the udev info when there's only SATA disk attached.

Code:

root@cubie:/lib/udev/rules.d# udevadm info -a -n /dev/sda

Udevadm info starts with the device specified by the devpath and then
walks up the chain of parent devices. It prints for every device
found, all possible attributes in the udev rules key format.
A rule to match, can be composed by the attributes of the device
and the attributes from one single parent device.

  looking at device '/devices/platform/sw_ahci.0/ata1/host0/target0:0:0/0:0:0:0/block/sda':
    KERNEL=="sda"
    SUBSYSTEM=="block"
    DRIVER==""
    ATTR{ro}=="0"
    ATTR{size}=="312581808"
    ATTR{stat}=="     336        3     2778     1270        5        1       48       60        0      830     1330"
    ATTR{range}=="16"
    ATTR{discard_alignment}=="0"
    ATTR{events}==""
    ATTR{ext_range}=="256"
    ATTR{events_poll_msecs}=="-1"
    ATTR{alignment_offset}=="0"
    ATTR{inflight}=="       0        0"
    ATTR{removable}=="0"
    ATTR{capability}=="50"
    ATTR{events_async}==""

  looking at parent device '/devices/platform/sw_ahci.0/ata1/host0/target0:0:0/0:0:0:0':
    KERNELS=="0:0:0:0"
    SUBSYSTEMS=="scsi"
    DRIVERS=="sd"
    ATTRS{rev}=="04.0"
    ATTRS{type}=="0"
    ATTRS{scsi_level}=="6"
    ATTRS{model}=="WDC WD1600BEVS-2"
    ATTRS{state}=="running"
    ATTRS{unload_heads}=="0"
    ATTRS{queue_type}=="simple"
    ATTRS{iodone_cnt}=="0x17f"
    ATTRS{iorequest_cnt}=="0x17f"
    ATTRS{queue_ramp_up_period}=="120000"
    ATTRS{timeout}=="30"
    ATTRS{evt_media_change}=="0"
    ATTRS{ioerr_cnt}=="0x1"
    ATTRS{queue_depth}=="31"
    ATTRS{vendor}=="ATA     "
    ATTRS{device_blocked}=="0"
    ATTRS{iocounterbits}=="32"

  looking at parent device '/devices/platform/sw_ahci.0/ata1/host0/target0:0:0':
    KERNELS=="target0:0:0"
    SUBSYSTEMS=="scsi"
    DRIVERS==""

  looking at parent device '/devices/platform/sw_ahci.0/ata1/host0':
    KERNELS=="host0"
    SUBSYSTEMS=="scsi"
    DRIVERS==""

  looking at parent device '/devices/platform/sw_ahci.0/ata1':
    KERNELS=="ata1"
    SUBSYSTEMS==""
    DRIVERS==""

  looking at parent device '/devices/platform/sw_ahci.0':
    KERNELS=="sw_ahci.0"
    SUBSYSTEMS=="platform"
    DRIVERS=="sw_ahci"

  looking at parent device '/devices/platform':
    KERNELS=="platform"
    SUBSYSTEMS==""
    DRIVERS==""

And here it is again when there is another HDD attached to USB. The SATA disk gets renamed to sdb:

Code:

root@cubie:/var/log# udevadm info -a -n /dev/sdb

Udevadm info starts with the device specified by the devpath and then
walks up the chain of parent devices. It prints for every device
found, all possible attributes in the udev rules key format.
A rule to match, can be composed by the attributes of the device
and the attributes from one single parent device.

  looking at device '/devices/platform/sw_ahci.0/ata1/host1/target1:0:0/1:0:0:0/block/sdb':
    KERNEL=="sdb"
    SUBSYSTEM=="block"
    DRIVER==""
    ATTR{ro}=="0"
    ATTR{size}=="312581808"
    ATTR{stat}=="     336        3     2778     1140        8       36      352       60        0      910     1200"
    ATTR{range}=="16"
    ATTR{discard_alignment}=="0"
    ATTR{events}==""
    ATTR{ext_range}=="256"
    ATTR{events_poll_msecs}=="-1"
    ATTR{alignment_offset}=="0"
    ATTR{inflight}=="       0        0"
    ATTR{removable}=="0"
    ATTR{capability}=="50"
    ATTR{events_async}==""

  looking at parent device '/devices/platform/sw_ahci.0/ata1/host1/target1:0:0/1:0:0:0':
    KERNELS=="1:0:0:0"
    SUBSYSTEMS=="scsi"
    DRIVERS=="sd"
    ATTRS{rev}=="04.0"
    ATTRS{type}=="0"
    ATTRS{scsi_level}=="6"
    ATTRS{model}=="WDC WD1600BEVS-2"
    ATTRS{state}=="running"
    ATTRS{unload_heads}=="0"
    ATTRS{queue_type}=="simple"
    ATTRS{iodone_cnt}=="0x182"
    ATTRS{iorequest_cnt}=="0x182"
    ATTRS{queue_ramp_up_period}=="120000"
    ATTRS{timeout}=="30"
    ATTRS{evt_media_change}=="0"
    ATTRS{ioerr_cnt}=="0x1"
    ATTRS{queue_depth}=="31"
    ATTRS{vendor}=="ATA     "
    ATTRS{device_blocked}=="0"
    ATTRS{iocounterbits}=="32"

  looking at parent device '/devices/platform/sw_ahci.0/ata1/host1/target1:0:0':
    KERNELS=="target1:0:0"
    SUBSYSTEMS=="scsi"
    DRIVERS==""

  looking at parent device '/devices/platform/sw_ahci.0/ata1/host1':
    KERNELS=="host1"
    SUBSYSTEMS=="scsi"
    DRIVERS==""

  looking at parent device '/devices/platform/sw_ahci.0/ata1':
    KERNELS=="ata1"
    SUBSYSTEMS==""
    DRIVERS==""

  looking at parent device '/devices/platform/sw_ahci.0':
    KERNELS=="sw_ahci.0"
    SUBSYSTEMS=="platform"
    DRIVERS=="sw_ahci"

  looking at parent device '/devices/platform':
    KERNELS=="platform"
    SUBSYSTEMS==""
    DRIVERS==""

How to create a rule match for the sata port? I've checked the dmesg, and it shows that the USB driver loads first. Afterwards the sata driver loads. I'm guessing this is the reason that the USB disk gets placed as /dev/sda. I tried a rule like this, but it doesn't work because the kernel subsystem changes too.

Code:

KERNEL=="sd[a-z]", SUBSYSTEM=="block", KERNELS=="0:0:0:0", SUBSYSTEMS=="scsi", NAME="sda"

smallpond · 09-17-2013, 03:22 PM

The real question is why you care what the name is. Any time you have more than one disk you should be mounting by UUID in fstab.

zeebra · 09-17-2013, 03:47 PM

Quote:

Originally Posted by smallpond

The real question is why you care what the name is. Any time you have more than one disk you should be mounting by UUID in fstab.

I second that. Use UUID!

Perhaps the name has something to do with the boot order in his BIOS?

displace · 09-17-2013, 04:11 PM

Because the disks change, UUID's change too, so no unique identifiers. Also, my device has no BIOS. It would seem I will have to settle for symlinks instead. All I needed was a way to identify the port where the disk was connected to, not the disk itself. Looks like the /dev/disk/by-path/platform-sw_ahci.0-scsi-0:0:0:0 symlink will have to do. I'll need to adapt some scripts for this purpose.

I was hoping I could somehow reserve the sda device name for a specific port by using udev rules, but it would seem that what I was trying to do is impossible.

As Kay Sievers just explained to me:

Quote:

It is not possible at all, and unlikely that it will be in the future.

Udev can only add symlinks with names not clashing with the kernel
names, it can never rename the kernel device names.

Stable names are only provided by symlinks, never by the plain kernel names.

zeebra · 09-17-2013, 04:27 PM

Quote:

Originally Posted by displace

Because the disks change, UUID's change too, so no unique identifiers. Also, my device has no BIOS. It would seem I will have to settle for symlinks instead. All I needed was a way to identify the port where the disk was connected to, not the disk itself. Looks like the /dev/disk/by-path/platform-sw_ahci.0-scsi-0:0:0:0 symlink will have to do. I'll need to adapt some scripts for this purpose.

I was hoping I could somehow reserve the sda device name for a specific port by using udev rules, but it would seem that what I was trying to do is impossible.

As Kay Sievers just explained to me:

I wouldn't think it should. That is the whole purpose of UUID, isn't it?

What about disk labels? Can that help?

displace · 09-18-2013, 12:45 AM

Afaik disk labels are derived from partition names. There is no guarantee that different disks will use the same partition layout and names (labels). As I said before, the solution was to mount the disks by port (sata, usb).