[Bug 1861941] Re: bcache by-uuid links disappear after mounting bcache0

Tue Aug 11 18:07:15 UTC 2020

This bug was fixed in the package bcache-tools - 1.0.8-3ubuntu0.1

---------------
bcache-tools (1.0.8-3ubuntu0.1) focal; urgency=medium

  [ Ryan Harper ]
  * Add helper script to read bcache devs superblock (LP: #1861941)

 -- Rafael David Tinoco <rafaeldtinoco at ubuntu.com>  Thu, 23 Jul 2020
00:33:50 +0000

** Changed in: bcache-tools (Ubuntu Focal)
       Status: Fix Committed => Fix Released

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Title:
  bcache by-uuid links disappear after mounting bcache0

Status in bcache-tools package in Ubuntu:
  Fix Released
Status in systemd package in Ubuntu:
  Fix Released
Status in bcache-tools source package in Bionic:
  In Progress
Status in systemd source package in Bionic:
  Won't Fix
Status in bcache-tools source package in Focal:
  Fix Released
Status in systemd source package in Focal:
  In Progress

Bug description:
  SRU TEAM: The last 2 commits show a summary for the merges/changes
  I added some specific (to Bionic) notes in the template.
  Thanks!

  [Impact]

   * bcache-tools udev created symlinks might disappear when other udev
  events are processed for the same devices.

   * after mkfs.XXX in /dev/bcacheY you might face a condition where
  /dev/bcache/by-{uuid,label}/zzz symlinks are gone.

   * /dev/bcache/by-{uuid,label}/ symlinks are important so bcache
  devices can be addressed by their UUIDs and not the ordering they were
  assembled (MAAS depends on this feature, for example).

   * it was also discussed in this bug that systemd-udev should *not*
  populate /dev/disk/by-uuid/ with symlinks of disks that were bcache
  backing devices. this was turned into a discussion whether blkid
  should report those or not, and this discussion "died" after sometime.
  This last item is what the systemd update is all about: to disallow
  /dev/disk/by-XXX/yyyy creation for bcache backing devices (a simple
  change that will reduce end users confusion).

  [Test Case]

   * The reproducer script is here:

     https://paste.ubuntu.com/p/37KGy2Smnp/

   * Bionic can't reproduce the issue with the 18.04 kernel, nor with
  the HWE kernel. Nevertheless, it is preferable that Bionic also do the
  same thing: to read bcache superblock and feed environment for
  /dev/bcache/by-{uuid,label} symlinks creation.

  [Regression Potential]

   * We are not depending on bcache device udev events any more when
  creating the /dev/bcache/by-{uuid,label}/ symlinks. Instead, we are
  depending on a wrapper script that heads bcache device superblock. If
  there is a bug in this wrapper the symlinks wouldn't work.

   * Previously we were thinking in asking the kernel team to remove the
  bcache udev event delta script they've done for previous case (LP:
  #1729145). It creates the udev events that were being read and filling
  the symlinks. We decided not to remove those (just from Groovy and on)
  so we don't need to worry on Breaks/Conflicts in between the kernel
  package and bcache-tools (and udev)).

   * Long story short: kernel will continue to emit bcache udev events
  as it did previously but now those events won't be used by anything -
  after installing this SRU - because udev wrapper script is doing this
  job (and doing it properly)

  [Other Info]

  - Original Description:

  1.
  root at ubuntu:~# lsb_release -rd
  Description:	Ubuntu Focal Fossa (development branch)
  Release:	20.04

  2.
  root at ubuntu:~# lsb_release -rd
  Description:	Ubuntu Focal Fossa (development branch)
  Release:	20.04
  root at ubuntu:~# apt-cache policy linux-image-virtual
  linux-image-virtual:
    Installed: 5.4.0.12.15
    Candidate: 5.4.0.12.15
    Version table:
   *** 5.4.0.12.15 500
          500 http://archive.ubuntu.com/ubuntu focal/main amd64 Packages
          100 /var/lib/dpkg/status
  root at ubuntu:~# apt-cache policy linux-image-5.4.0-12-generic
  linux-image-5.4.0-12-generic:
    Installed: 5.4.0-12.15
    Candidate: 5.4.0-12.15
    Version table:
   *** 5.4.0-12.15 500
          500 http://archive.ubuntu.com/ubuntu focal/main amd64 Packages
          100 /var/lib/dpkg/status

  3. mount /dev/bcache0 && ls -al /dev/bcache/by-uuid/
  + ls -al /dev/bcache/by-uuid/
  total 0
  drwxr-xr-x 2 root root 60 Feb  4 23:31 .
  drwxr-xr-x 3 root root 60 Feb  4 23:31 ..
  lrwxrwxrwx 1 root root 13 Feb  4 23:31 abdfd1f6-44ce-4266-91db-24667b9ae51a -> ../../bcache0

  4.
  root at ubuntu:~# ls -al /dev/bcache/by-uuid
  ls: cannot access '/dev/bcache/by-uuid': No such file or directory

  ProblemType: Bug
  DistroRelease: Ubuntu 20.04
  Package: linux-image-5.4.0-12-generic 5.4.0-12.15
  ProcVersionSignature: Ubuntu 5.4.0-12.15-generic 5.4.8
  Uname: Linux 5.4.0-12-generic x86_64
  ApportVersion: 2.20.11-0ubuntu16
  Architecture: amd64
  Date: Tue Feb  4 23:31:52 2020
  ProcEnviron:
   TERM=xterm-256color
   PATH=(custom, no user)
   LANG=C.UTF-8
   SHELL=/bin/bash
  SourcePackage: linux-signed-5.4
  UpgradeStatus: No upgrade log present (probably fresh install)

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