Thursday, November 12, 2009


I think I can say that filesystem probing (identification) code in Linux distributions has been successfully consolidated to the one library. The libblkid.

The library is Ted Tso's baby and was maintained and distributed within e2fsprogs package for many years. Unfortunately, the library didn't match with udev requirements. The udev project requires lightweight probing library without any high-level logic. This was reason why udev developers created a new probing library -- libvolume_id.

And finally and unfortunately there was very old filesystem probing code in the mount(8) utility too. The useless code in mount(8) was ignored by mainstream Linux distributions (for example Suse linked their utils against libvolume_id and RHEL/Fedora against libblkid) .... and this is not end of nightmare.

The other filesystem probing code was in xfsprogs project (libdisk), and the other code is still in the parted (libparted).

And now about consolidation. The libblkid was merged into util-linux-ng (one year ago). The old version in e2fsprogs is still maintained for people who are very conservative. All new features go to libblkid in util-linux-ng project.

Some important functionality from libvolume_id was merged into libblkid including a new low-level API. The blkid(8) command supports new "-p" (low-level probe) and "-o udev" (udev output format) options now.

The libvolume_id was completely removed from udev project a few months ago. The mount(8) command uses only libblkid.

The libblkid has been heavily extended in the latest (incoming) util-linux-ng-2.17. It supports partition tables probing and allows to gather information about blkdev topology. This latest change allows to use libblkid instead of libdisk in mkfs programs (already in mke2fs and mkfs.xfs).

And finally libparted could be linked against libblkid. Currently, it uses libblkid for topology information only, but I hope one day it will use it for filesystems probing too.

The last remaining place where is any filesystem probing code is cfdisk. It will be fixed in some next util-linux-ng release.

So... now is excellent opportunity to say THANKS to Ted (e2fsprogs), Kay (udev), Christoph (xfsprogs) and to others who help me with this task.

[...yeah, this is my first blog post ... and yeah, my English is horrible:-)]


  1. I can't say thank you enough times here...

    I was tearing my hair out trying to figure out the dual maze of partition/volume identification and the licensing of the components that can do it. I'm working on a project where parts of it are open-source and parts of it aren't, and needed to do the exact kind of low-level discovery that the new low-level API offers.

    One thought - how about an API for obtaining those pieces of information that are specific to a particular partitioning scheme (i.e., the "active" flag for MBR, or the "attributes" field for GPT)?

    Lastly, actually, your English isn't any worse than that of many of its native speakers' is... :)

  2. to eleccham:
    that's partially implemented by

    blkid_partition_get_{uuid,type,start, ...}()

    functions. Things like flags or attributes will be available in the next release.