lvconvert --cachedevice doesn't support --chunksize

[optix2000]

Background

Running on Debian Bullseye.

% sudo lvm version
  LVM version:     2.03.10(2) (2020-08-09)
  Library version: 1.02.173 (2020-08-09)
  Driver version:  4.39.0
  Configuration:   ./configure --build=x86_64-linux-gnu --prefix=/usr --includedir=${prefix}/include --mandir=${prefix}/share/man --infodir=${prefix}/share/info --sysconfdir=/etc --localstatedir=/var --disable-option-checking --disable-silent-rules --libdir=${prefix}/lib/x86_64-linux-gnu --runstatedir=/run --disable-maintainer-mode --disable-dependency-tracking --libdir=/lib/x86_64-linux-gnu --sbindir=/sbin --with-usrlibdir=/usr/lib/x86_64-linux-gnu --with-optimisation=-O2 --with-cache=internal --with-device-uid=0 --with-device-gid=6 --with-device-mode=0660 --with-default-pid-dir=/run --with-default-run-dir=/run/lvm --with-default-locking-dir=/run/lock/lvm --with-thin=internal --with-thin-check=/usr/sbin/thin_check --with-thin-dump=/usr/sbin/thin_dump --with-thin-repair=/usr/sbin/thin_repair --with-udev-prefix=/ --enable-applib --enable-blkid_wiping --enable-cmdlib --enable-dmeventd --enable-lvmlockd-dlm --enable-lvmlockd-sanlock --enable-lvmpolld --enable-notify-dbus --enable-pkgconfig --enable-readline --enable-udev_rules --enable-udev_sync

Problem

When using lvconvert with --cachedevice with a large cache drive (~2TB), lvconvert will throw an error saying there are too many chunks and recommends using --chunksize to reduce the number.

% sudo lvconvert --type cache --cachedevice /dev/sdb vg/lv
Use all 1.63 TiB from /dev/sdb for cache? [y/n]: y
  Creating cachevol LV lv with size 1.63 TiB.
  Logical volume "lv_cache" created.
  Cache data blocks 3506397184 and chunk size 128 exceed max chunks 1000000.
  Use smaller cache, larger --chunksize or increase max chunks setting.

However, if you try to pass --chunksize to lvconvert it refuses to use it, making it impossible to proceed without increasing max chunks (which isn't desirable in this case).

% sudo lvconvert --type cache --chunksize 2M --cachedevice /dev/sdb vg/lv
  Command does not accept option: --chunksize 2M.

Ideally, lvconvert should support --chunksize when combined with --cachedevice, or support automatic sizing like when using --cachepool

Unfortunately using --cachepool doesn't produce the same results as --cachepool expects free extents, which don't exist, as all of them have been allocated to either the origin LV or the cache.

% sudo lvconvert --type cache --cachepool lv_cache vg/lv
  Using 2.00 MiB chunk size instead of default 64.00 KiB, so cache pool has less than 1000000 chunks.
  WARNING: Converting vg/lv_cache to cache pool's data volume with metadata wiping.
  THIS WILL DESTROY CONTENT OF LOGICAL VOLUME (filesystem etc.)
Do you really want to convert vg/lv_cache? [y/n]: y
  Volume group "vg" has insufficient free space (0 extents): 10 required.
  LV vg/lv_cache could not be converted to a cache pool.

[zkabelac]

For now use simple syntax with cachepools - as this slightly faster use with dm-cache - here is example syntax

lvcreate --cache -Lsizeofcache -c 2M vg/LV_ORIGIN /dev/sdb

you can also use conversion if you create cachepool upfront.