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author | André Fabian Silva Delgado <emulatorman@parabola.nu> | 2016-04-16 15:30:54 -0300 |
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committer | André Fabian Silva Delgado <emulatorman@parabola.nu> | 2016-04-16 15:30:54 -0300 |
commit | bdcfd44fb5b5fb8fd660e7f93f1095c507481024 (patch) | |
tree | e423b07154d422b711ddfadedb87c43317d3c4f6 /Documentation/filesystems/aufs/design/08shwh.txt | |
parent | 4a327fcef90ba27150a3e8741441b68c605ae248 (diff) |
Linux-libre 4.5.1-gnupck-4.5.1-gnu
Diffstat (limited to 'Documentation/filesystems/aufs/design/08shwh.txt')
-rw-r--r-- | Documentation/filesystems/aufs/design/08shwh.txt | 39 |
1 files changed, 39 insertions, 0 deletions
diff --git a/Documentation/filesystems/aufs/design/08shwh.txt b/Documentation/filesystems/aufs/design/08shwh.txt new file mode 100644 index 000000000..ad58ebe15 --- /dev/null +++ b/Documentation/filesystems/aufs/design/08shwh.txt @@ -0,0 +1,39 @@ + +# Copyright (C) 2005-2016 Junjiro R. Okajima + +Show Whiteout Mode (shwh) +---------------------------------------------------------------------- +Generally aufs hides the name of whiteouts. But in some cases, to show +them is very useful for users. For instance, creating a new middle layer +(branch) by merging existing layers. + +(borrowing aufs1 HOW-TO from a user, Michael Towers) +When you have three branches, +- Bottom: 'system', squashfs (underlying base system), read-only +- Middle: 'mods', squashfs, read-only +- Top: 'overlay', ram (tmpfs), read-write + +The top layer is loaded at boot time and saved at shutdown, to preserve +the changes made to the system during the session. +When larger changes have been made, or smaller changes have accumulated, +the size of the saved top layer data grows. At this point, it would be +nice to be able to merge the two overlay branches ('mods' and 'overlay') +and rewrite the 'mods' squashfs, clearing the top layer and thus +restoring save and load speed. + +This merging is simplified by the use of another aufs mount, of just the +two overlay branches using the 'shwh' option. +# mount -t aufs -o ro,shwh,br:/livesys/overlay=ro+wh:/livesys/mods=rr+wh \ + aufs /livesys/merge_union + +A merged view of these two branches is then available at +/livesys/merge_union, and the new feature is that the whiteouts are +visible! +Note that in 'shwh' mode the aufs mount must be 'ro', which will disable +writing to all branches. Also the default mode for all branches is 'ro'. +It is now possible to save the combined contents of the two overlay +branches to a new squashfs, e.g.: +# mksquashfs /livesys/merge_union /path/to/newmods.squash + +This new squashfs archive can be stored on the boot device and the +initramfs will use it to replace the old one at the next boot. |