[Bug 1611945] Re: /dev/disk/by-path not properly populated for (e)SATA port multiplier disks

[thulle]

I saw the discussion in zfs-discuss and read a bit for fun.

To answer some old questions from what i understood from linked resources:
This bug/deficiency seem to make it impossible to see all drives behind a port multiplier when using links in /dev/disk/by-path/ but not other links, making the drives unadressable by slot.

This can be fixed by patching systemd. Why this haven't been done seems to be summarized here:
https://github.com/systemd/systemd/issues/3943#issuecomment-404996399

The patch submitted here seems to be outdated, later one found here:
https://github.com/sitsofe/systemd/commit/ee26c33ede684138ba9fdc7f286bfa402860aff3

IMHO, the easiest (but not easy) way to add this patch in Ubuntu on your
own would be to create an PPA and rebuild systemd from ubuntu with the
patch added. Note that you'll be responsible for keeping your systemd-
package up to date on your own and that changes can be made upstream
that breaks the patch.

It could be added in the official ubuntu package of systemd, but I'm
guessing they don't want to add a patch that adds a naming scheme for
drives that later might change when the proper fix mentioned last in the
first link is implemented.

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https://bugs.launchpad.net/bugs/1611945

Title:
  /dev/disk/by-path not properly populated for (e)SATA port multiplier
  disks

Status in systemd package in Ubuntu:
  Confirmed

Bug description:
  We have a just-installed Ubuntu 16.04 LTS machine with a number of
  disks behind port-multiplier eSATA ports, all of them driven by a SiI
  3124 controller (sata_sil24 kernel driver). Our machine sees all disks
  on all channels, however under 16.04 only one disk from each channel
  shows up in /dev/disk/by-path/ (all disks show up in /dev/disk/by-id
  and /dev/disk/by-uuid). For our usage this is a severe defect because
  we rotate disks in and out of the external enclosure and rely on
  mounting specific slots in the external enclosure through /dev/disk
  /by-path.

  This did not happen in Ubuntu 12.04 LTS, the release that this machine
  was previously running.

  According to 'udevadm info --export-db' and 'udevadm test-builtin
  path_id' and so on, systemd's udev stuff is assigning all drives
  behind the same port the same disk/by-path data (ID_PATH et al). In
  'udevadm info /sys/block/sdX', the 'P:' and 'E: DEVPATH=' values show
  a difference in the target portion of PCI path, eg:

    P: /devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/ata1/host0/target0:0:0/0:0:0:0/block/sda
    P: /devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/ata1/host0/target0:1:0/0:1:0:0/block/sdb

  However the 'S: disk/by-path', 'E: DEVLINKS=', and 'E: ID_PATH'
  portions do not. For both devices above, we see:

    S: disk/by-path/pci-0000:02:00.0-ata-1
    E: ID_PATH=pci-0000:02:00.0-ata-1

  Naturally only one device can have a /dev/disk/by-
  path/pci-0000:02:00.0-ata-1 symlink, so instead of four disks per
  channel in /dev/disk/by-path we see one.

  Ubuntu release: 16.04

  Package versions from 'apt-cache policy udev systemd':
  udev:
    Installed: 229-4ubuntu7
  systemd:
    Installed: 229-4ubuntu7

  'journalctl -b' reports that during boot systemd does report some
  'appeared twice with different sysfs paths' notes, eg:

  Aug 10 13:34:21 verdandi systemd[1]: dev-disk-by\x2dpath-
  pci\x2d0000:02:00.0\x2data\x2d1\x2dpart1.device: Dev dev-disk-by
  \x2dpath-pci\x2d0000:02:00.0\x2data\x2d1\x2dpart1.device appeared
  twice with different sysfs paths
  /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/ata1/host0/target0:3:0/0:3:0:0/block/sdd/sdd1
  and
  /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/ata1/host0/target0:0:0/0:0:0:0/block/sda/sda1

  However it doesn't seem to be reporting this for all port-multiplier
  drives and their partitions.

  If it would be useful I can attach full 'udevadm info --export-db'
  output or the like.

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