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Rationale and specifications of the libblockdev library

Authors: Vratislav Podzimek <[email protected]>

Scope

libblockdev library is a library for doing operations with block devices replacing and extending the code from the blivet.devicelibs (python-blivet) modules as well as udisks2. It supports the following storage technologies:

  • partitions
  • filesystem operations
  • LVM
  • BTRFS
  • SWAP
  • cryptsetup/LUKS
  • DM (device mapper)
  • loop devices
  • MD RAID
  • multipath
  • DASD
  • NVDIMM namespaces

Of course some additional technologies may be supported in the future.

Architecture

The library itself is only a thin wrapper around a set of plugins, each for a particular technology listed above. The library provides an API consisting of sets of functions provided by its plugins. For example, there is a symbol/function called bd_lvm_lvcreate provided by the library which is dynamically loaded from the LVM plugin when the library's bd_init function will be called. Initially all those functions are no-ops just printing a warning on stderr and doing nothing. This way applications using the library won't crash, the operations just won't be run. Of course, the library has bd_is_plugin_available, which allows applications to check if something is provided/implemented or not.

Although the architecture described in the previous paragraph may look overly complicated, it brings quite nice advantages:

  1. a different implementation of the operations for technologies can be easily used/tested (e.g. LVM plugin using binaries vs. LVM plugin using liblvm), as part of the library initialization, the application may explicitly state it wants to use a particular plugin
  2. if some functionality is missing in a plugin, no crash will appear
  3. plugins may be used per se without the library (if some application needs to do e.g. LVM-only operations)
  4. plugins may be implemented in various languages as long as they provide functions with the right names in a way that those functions can be dynamically loaded

Technologies behind the library

The library is written in C using the GLib library. The GLib library provides a lot of handy utilities that may be used by the library (no need for a new implementation of hash tables, lists, etc.) and moreover, it should be really easy to create bindings for the library via GObject introspection that works even with "not OOP" code. However, instead of returning links to structs (e.g. as a return value of the bd_lvm_vginfo function) it will return references to GObjects with attributes/properties and access methods. The reason is again an easy way to create bindings which we get for free. The implementation gathers knowledge from the blivet, system-storage-manager (SSM), udisks2 projects.

Purpose of the library

The library replaces the blivet.devicelibs code as well as udisks2's low-level code and is available for other new projects.