device-mapper crypt



Description:this option specifies the mode in which cryptsetup works. It can be luks or plain. It kicks in only if the --dmcrypt option is passed to ceph-disk. See also cryptsetup document for more details.


Description:the size of the random string in bytes used as the LUKS key. The string is read from /dev/urandom and then encoded using base64. It will be stored with the key of dm-crypt/osd/$uuid/luks using config-key.
Default:1024 if osd_dmcrypt_type is luks, 256 otherwise.


ceph-disk supports dmcrypt (device-mapper crypt). If dmcrypt is enabled, the partitions will be encrypted using this machinary. For each OSD device, a lockbox is introduced for holding the information regarding how the dmcrypt key is stored. To prepare a lockbox, ceph-disk

  1. creates a dedicated lockbox partition on device, and
  2. populates it with a tiny filesystem, then
  3. automounts it at /var/lib/ceph/osd-lockbox/$uuid, read-only. where the uuid is the lockbox’s uuid.

under which, settings are stored using plain files:

  • key-management-mode: ceph-mon v1
  • osd-uuid: the OSD’s uuid
  • ceph_fsid: the fsid of the cluster
  • keyring: the lockbox’s allowing one to fetch the LUKS key
  • block_uuid: the partition uuid for the block device
  • journal_uuid: the partition uuid for the journal device
  • block.db_uuid: the partition uuid for the block.db device
  • block.wal_uuid: the partition uuid for the block.wal device
  • magic: a magic string indicating that this partition is a lockbox. It’s not used currently.
  • ${space_uuid}: symbolic links named after the uuid of space partitions pointing to /var/lib/ceph/osd-lockbox/$uuid. in the case of FileStore, the space partitions are data and journal partitions, for BlueStore, they are data, block.db and block.wal.

Currently, ceph-mon v1 is the only supported key-management-mode. In that case, the LUKS key is stored using the config-key in the monitor store with the key of dm-crypt/osd/$uuid/luks.


ceph-disk creates partitions for preparing a device for OSD deployment. Their partition numbers are hardcoded. For instance, data partition’s partition number is always 1 :

  1. data partition
  2. journal partition, if co-located with data
  3. block.db for BlueStore, if co-located with data
  4. block.wal for BlueStore, if co-located with data
  5. lockbox

prepare class hierarchy

The ceph-disk prepare class hierarchy can be challenging to read and this guide is designed to explain how it is structured.

The Prepare class roughly replaces the prepare_main function but also handles the prepare subcommand argument parsing. It creates the data and journal objects and delegate the actual work to them via the prepare() method.

The Prepare class assumes that preparing an OSD consists of the following phases:

  • optionally prepare auxiliary devices, such as the journal
  • prepare a data directory or device
  • populate the data directory with fsid etc. and optionally symbolic links to the auxiliary devices

The PrepareFilestore class is derived from Prepare and implements the current model where there only is one auxiliary device, the journal. It utilizes PrepareJournal and PrepareFilestoredata for preparing its journal and its data directory respectively. The latter is a derived class of PrepareData.

The PrepareJournal class implements the journal functions and is based on a generic class PrepareSpace, which handles the allocation of an auxiliary device. The only journal specific feature is left to the PrepareJournal class: querying the OSD to figure out if a journal is wanted or not.

The OSD data directory is prepared via the PrepareData class. It creates a file system if necessary (i.e. if a device) and populates the data directory. Further preparation is then delegated to the auxiliary devices (i.e. adding a symlink to the device for a journal).

There was some code paths related dmcrypt / multipath devices in the prepare functions, although it is orthogonal. A class tree for Devices was created to isolate that.

Although that was the primary reason for adding a new class tree, two other aspects have also been moved there: ptypes (i.e. partition types) and partition creation. The ptypes are organized into a data structure with a few helpers in the hope that it will be easier to maintain. All references to the *_UUID variables have been updated.

The creation of a partition is delegated to sgdisk and a wrapper helps reduce the code redundancy.

The ptype of a given partition depends on the type of the device (is it dmcrypt’ed or a multipath device ?). It is best implemented by derivation so the prepare function does not need to be concerned about how the ptype of a partition is determined.

Many functions could be refactored into a Device class and its derivatives, but that was not done to minimize the size of the refactor.

  • Device knows how to create a partition and figure out the ptype tobe
  • DevicePartition a regular device partition
  • DevicePartitionMultipath a partition of a multipath device
  • DevicePartitionCrypt base class for luks/plain dmcrypt, can map/unmap
  • DevicePartitionCryptPlain knows how to setup dmcrypt plain
  • DevicePartitionCryptLuks knows how to setup dmcrypt luks

The CryptHelpers class is introduced to factorize the code snippets that were duplicated in various places but that do not really belong because they are convenience wrappers to figure out:

  • if dcmrypt should be used
  • the keysize
  • the dmcrypt type (plain or luks)

state transition of partitions