Bug#852384: lvm2: unable to reset settings set with --cachesettings, making it impossible to switch cachepolicy

[Marc Lehmann]

Package: lvm2
Version: 2.02.168-1
Severity: normal

Dear Maintainer,

the --cachesettings swotch (to lvchange) apparently accepts
space-separated key-value pairs (this is undocumented, but not the issue
here). For example:

  # lvchange --cachesettings 'read_promote_adjustment=8' vg_cerebro/bp

However, different cache policies accept different key-value pairs. For
example, smq does not support any key-value pairs, therefore switching the
cachepolicy to it fails:

  # lvchange --cachepolicy smq vg_cerebro/bp
  device-mapper: reload ioctl on (252:1) failed: Invalid argument

Unfortunately, --cachesettings, while enbaling changes to cachesettings,
does not allow clearing the key-value pairs:

  # lvchange --cachesettings '' vg_cerebro/bp
  Command failed with status code 5.
  # lvchange --cachepolicy smq --cachesettings '' vg_cerebro/bp
  Command failed with status code 5.

The fix, IMHO, would be to allow one to clear cachesettings.

-- System Information:
Debian Release: 8.5
  APT prefers stable
  APT policy: (990, 'stable'), (500, 'unstable'), (500, 'testing'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.4.41-040441-generic (SMP w/8 CPU cores)
Locale: LANG=C, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash
Init: systemd (via /run/systemd/system)

Versions of packages lvm2 depends on:
ii  dmeventd                  2:1.02.90-2.2+deb8u1
ii  dmsetup                   2:1.02.137-1
ii  init-system-helpers       1.22
ii  libblkid1                 2.25.2-6
ii  libc6                     2.24-8
ii  libdevmapper-event1.02.1  2:1.02.90-2.2+deb8u1
ii  libdevmapper1.02.1        2:1.02.137-1
ii  liblvm2app2.2             2.02.168-1
ii  libreadline5              5.2+dfsg-2
ii  libudev1                  230-7
ii  lsb-base                  4.1+Debian13+nmu1

lvm2 recommends no packages.

Versions of packages lvm2 suggests:
ii  thin-provisioning-tools  0.3.2-1

-- Configuration Files:
/etc/lvm/lvm.conf changed [not included]

-- debconf information excluded