Integrate the option to use an OverlayFS as the root filesystem
into the 90dmsquash-live module for testing purposes.
The rd.live.overlay.overlayfs option allows one to request an
OverlayFS overlay. If a persistent overlay is detected at the
standard LiveOS path, the overlay & type detected will be used.
Tested primarily with transient, in-RAM overlay boots on vfat-
formatted Live USB devices, with persistent overlay directories
on ext4-formatted Live USB devices, and with embedded, persistent
overlay directories on vfat-formatted devices. (Persistent overlay
directories on a vfat-formatted device must be in an embedded
filesystem that supports the creation of trusted.* extended
attributes, and must provide valid d_type in readdir responses.)
The rd.live.overlay.readonly option, which allows a persistent
overlayfs to be mounted read only through a higher level transient
overlay directory, has been implemented through the multiple lower
layers feature of OverlayFS.
The default transient DM overlay size has been adjusted up to 32 GiB.
This change supports comparison of transient Device-mapper vs.
transient OverlayFS overlay performance. A transient DM overlay
is a sparse file in memory, so this setting does not consume more
RAM for legacy applications. It does permit a user to use all of
the available root filesystem storage, and fails gently when it is
consumed, as the available free root filesystem storage on a typical
LiveOS build is only a few GiB. Thus, when booted on other-
than-small RAM systems, the transient DM overlay should not overflow.
OverlayFS offers the potential to use all of the available free RAM
or all of the available free disc storage (on non-vfat-devices)
in its overlay, even beyond the root filesystem available space,
because the OverlayFS root filesystem is a union of directories on
two different partitions.
This patch also cleans up some message spew at shutdown, shortens
the execution path in a couple of places, and uses persistent
DM targets where required.
Documentation is updated for these changes.
Frederick Grose
ea28824027