Changes

The Fedora LiveOS uses the [[wikipedia:Device mapper|Device-mapper]] service of the Linux kernel to manage the file stores on the device.  This is the same service that is used by [[wikipedia:Logical Volume Manager (Linux)|Logical Volume Manager]] to provide disc partition services.

The Fedora LiveOS uses the [[wikipedia:Device mapper|Device-mapper]] service of the Linux kernel to manage the file stores on the device.  This is the same service that is used by [[wikipedia:Logical Volume Manager (Linux)|Logical Volume Manager]] to provide disc partition services.

One limitation, mentioned above, is that the LiveOS persistent overlay is a write-once file space.  This is related to its use of ''device mapper'' snapshots to merge a read-only file system image (copied from the compressed SquashFS.img on the read-only LiveCD or installation .iso file) with a [[wikipedia:Copy-on-write|Copy-on-write]] service that tracks only changed bits of data in the snapshot (overlay) file and then re-referencing file pointers to the updated bits.<ref>http://people.gnome.org/~markmc/code/merge-dm-snapshot.c</ref>  Any changes to the operating system files are stored as differences from the base. As such, "deletions" of files are saved as additional difference references, and the originals are hidden.  With this mechanism, physical storage space is consumed in the write-once file space rather than recovered.

One limitation, mentioned above, is that the LiveOS persistent overlay is a write-once file space.  This is related to its use of ''device mapper'' snapshots to merge a read-only file system image (copied from the compressed SquashFS.img on the read-only LiveCD or installation .iso file) with a [[wikipedia:Copy-on-write|Copy-on-write]] service that tracks only changed bits of data in the snapshot (overlay) file and then re-referencing file pointers to the updated bits.<ref>http://people.gnome.org/~markmc/code/merge-dm-snapshot.c</ref><ref>http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blob;f=drivers/md/dm-snap-persistent.c;hb=HEAD</ref>  Any changes to the operating system files are stored as differences from the base. As such, "deletions" of files are saved as additional difference references, and the originals are hidden.  With this mechanism, physical storage space is consumed in the write-once file space rather than recovered.

Consumption of the space allocated for persistent storage in the snapshot overlay file may be tracked with the ''device mapper'' {{Code|dmsetup status}} report.  '''Sugar Cellar''' is a small, utility script which uses that service to allow for Learner testing and discovery. It will help Learners manage their storage resources and learn ways to economize limited resources.

Consumption of the space allocated for persistent storage in the snapshot overlay file may be tracked with the ''device mapper'' {{Code|dmsetup status}} report.  '''Sugar Cellar''' is a small, utility script which uses that service to allow for Learner testing and discovery. It will help Learners manage their storage resources and learn ways to economize limited resources.

==References==

==References==

<references>http://people.gnome.org/~markmc/code/merge-dm-snapshot.c</references>

<references />

:: See this [http://www.redhat.com/archives/dm-devel/2011-October/msg00148.html dm-devel thread].

:: See this [http://www.redhat.com/archives/dm-devel/2011-October/msg00148.html dm-devel thread].