dracut.cmdline: Update Booting live images section
Add descriptions for the rd.live.overlay.readonly, rd.live.overlay.size, rd.live.overlay.reset, rd.live.squashimg, and rd.live.overlay=none options. Mention the Invalid mark on exhausted overlays. Clarify the rd.writable.fsimg option. Repunctuate the rd.live.overlay.thin option description.master
Frederick Grose
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8e1c4b354c
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33a4198e17
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@ -774,18 +774,21 @@ Booting live images
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Dracut offers multiple options for live booted images:
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=====================
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squashfs with read-only filesystem image::: The system will boot with a read
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only filesystem from the squashfs and apply a writable device mapper snapshot
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SquashFS with read-only filesystem image::: The system will boot with a read
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only filesystem from the SquashFS and apply a writable device-mapper snapshot
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over the read only filesystem. Using this method ensures a relatively fast
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boot and lower RAM usage. Users **must be careful** to avoid writing too many
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blocks to the snapshot volume. Once the blocks of the snapshot are exhaused,
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the live filesystem becomes unusable and requires a reboot.
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blocks to the snapshot volume. Once the blocks of the snapshot overlay are
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exhausted, the root filesystem becomes unusable and requires a reboot. A
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persistent overlay is marked Invalid, and requires a difficult recovery.
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Non-persistent overlays default to 512 MiB in RAM, but the size can be adjusted
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with the **rd.live.overlay.size=** kernel command line option.
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+
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The filesystem structure is expected to be:
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+
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[listing]
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--
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squashfs.img | Squashfs downloaded via network
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squashfs.img | Squashfs from LiveCD .iso downloaded via network
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!(mount)
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/LiveOS
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|- ext3fs.img | Filesystem image to mount read-only
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@ -800,11 +803,34 @@ Dracut uses this method of live booting by default. No additional command line
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options are required other than **root=live:<URL>** to specify the location
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of your squashed filesystem.
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+
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writable filesystem image::: The system will retrieve a compressed filesystem
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image, connect it to a loopback device, and mount it as a writable volume. More
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RAM is required during boot but the live filesystem is easier to manage if it
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becomes full. Users can make a filesystem image of any size and that size will
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be maintained when the system boots.
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- The compressed SquashFS image can be copied during boot to RAM at
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`/run/initramfs/squashed.img` by using the **rd.live.ram=1** option.
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- A device with a persistent overlay can be booted read only by using the
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**rd.live.overlay.readonly** option on the kernel command line. This will
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cause a temporary, writable overlay to be stacked over a read-only snapshot
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of the root filesystem.
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+
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Uncompressed live filesystem image:::
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When the live system was installed with the '--skipcompress' option of the
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__livecd-iso-to-disk__ installation script for Live USB devices, the root
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filesystem image, `ext3fs.img`, is expanded on installation and no SquashFS
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is involved during boot.
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+
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- If **rd.live.ram=1** is used in this situation, the full, uncompressed
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root filesystem is copied during boot to `/run/initramfs/rootfs.img` in the
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`/run` tmpfs.
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+
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- If **rd.live.overlay=none** is provided as a kernel command line option,
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a writable, linear device-mapper target is created on boot with no overlay.
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writable filesystem image:::
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The system will retrieve a compressed filesystem image, extract it to
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`/run/initramfs/fsimg/rootfs.img`, connect it to a loop device, create a
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writable, linear device-mapper target at `/dev/mapper/live-rw`, and mount that
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as a writable volume at `/`. More RAM is required during boot but the live
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filesystem is easier to manage if it becomes full. Users can make a filesystem
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image of any size and that size will be maintained when the system boots. There
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is no persistence of root filesystem changes between boots with this option.
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+
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The filesystem structure is expected to be:
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+
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@ -812,7 +838,7 @@ The filesystem structure is expected to be:
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--
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rootfs.tgz | Compressed tarball containing fileystem image
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!(unpack)
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/rootfs.img | Filesystem image
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/rootfs.img | Filesystem image at /run/initramfs/fsimg/
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!(mount)
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/bin | Live filesystem
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/boot |
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@ -820,11 +846,23 @@ rootfs.tgz | Compressed tarball containing fileystem image
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... |
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--
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+
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To use this boot option, ensure that **rd.writable_fsimg=1** is in your kernel
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To use this boot option, ensure that **rd.writable.fsimg=1** is in your kernel
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command line and add the **root=live:<URL>** to specify the location
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of your compressed filesystem image tarball.
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of your compressed filesystem image tarball or SquashFS image.
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=====================
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**rd.writable.fsimg=**1::
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Enables writable filesystem support. The system will boot with a fully
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writable (but non-persistent) filesystem without snapshots __(see notes above
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about available live boot options)__. You can use the **rootflags** option to
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set mount options for the live filesystem as well __(see documentation about
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rootflags in the **Standard** section above)__.
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This implies that the whole image is copied to RAM before the boot continues.
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+
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NOTE: There must be enough free RAM available to hold the complete image.
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+
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This method is very suitable for diskless boots.
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**root=**live:__<url>__::
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Boots a live image retrieved from __<url>__. Valid handlers: __http, https, ftp, torrent, tftp__.
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+
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@ -843,15 +881,22 @@ Enables debug output from the live boot process.
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Specifies the directory within the squashfs where the ext3fs.img or rootfs.img
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can be found. By default, this is __LiveOS__.
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**rd.live.squashimg=**__<filename of SquashFS image>__::
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Specifies the filename for a SquashFS image of the root filesystem.
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By default, this is __squashfs.img__.
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**rd.live.ram=**1::
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Copy the complete image to RAM and use this for booting. This is useful
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when the image resides on i.e. a DVD which needs to be ejected later on.
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when the image resides on, i.e., a DVD which needs to be ejected later on.
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**rd.live.overlay=**_<devspec>_:_(<pathspec>|auto)__
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**rd.live.overlay=**__<devspec>__:__(<pathspec>|auto)__|__none__::
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Allow the usage of a permanent overlay.
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_<devspec>_ specifies the path to a device with a mountable filesystem.
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_<pathspec>_ is the path to a file within that filesystem, which shall be used to
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persist the changes made to the device specified by **root=live:__<url>__** option.
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- _<devspec>_ specifies the path to a device with a mountable filesystem.
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- _<pathspec>_ is the path to a file within that filesystem, which shall be
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used to persist the changes made to the device specified by the
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**root=live:__<url>__** option.
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- _none_ specifies no overlay when an uncompressed live root filesystem is
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available.
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+
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[listing]
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.Example
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@ -859,24 +904,24 @@ persist the changes made to the device specified by **root=live:__<url>__** opti
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rd.live.overlay=/dev/sdb1:persistent-overlay.img
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--
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**rd.live.overlay.size=**__<size_MiB>__::
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Specifies a non-persistent overlay size in MiB. The default is _512_.
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**rd.live.overlay.readonly=**1::
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Specifies a non-persistent, writable snapshot overlay to be stacked over a
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read-only snapshot of the root filesystem, `/dev/mapper/live-ro`.
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**rd.live.overlay.reset=**1::
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Specifies that a persistent overlay should be reset on boot. All root
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filesystem changes are vacated by this action.
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**rd.live.overlay.thin=**1::
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Enables the usage of thin snapshots instead of classic dm snapshots.
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The advantage of thin snapshots is, that they support discards, and will free
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blocks which are not claimed by the filesystem. In this use case this means,
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that memory is given back to the kernel, when the filesystem does not claim it
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The advantage of thin snapshots is that they support discards, and will free
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blocks that are not claimed by the filesystem. In this use case, this means
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that memory is given back to the kernel when the filesystem does not claim it
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anymore.
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**rd.writable.fsimg=**1::
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Enables writable filesystem support. The system will boot with a fully
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writable filesystem without snapshots __(see notes above about available live boot options)__.
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You can use the **rootflags** option to set mount options for the live
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filesystem as well __(see documentation about rootflags in the **Standard** section above)__.
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This implies that the whole image is copied to RAM before the boot continues.
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+
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NOTE: There must be enough free RAM available to hold the complete image.
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+
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This method is very suitable for diskless boots.
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Plymouth Boot Splash
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~~~~~~~~~~~~~~~~~~~~
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