Signed-off-by: Erez Zadok <ezk@xxxxxxxxxxxxx> --- Documentation/filesystems/unionfs/usage.txt | 17 +++++++++++++++++ 1 files changed, 17 insertions(+), 0 deletions(-) diff --git a/Documentation/filesystems/unionfs/usage.txt b/Documentation/filesystems/unionfs/usage.txt index d8c15de..a6b1aca 100644 --- a/Documentation/filesystems/unionfs/usage.txt +++ b/Documentation/filesystems/unionfs/usage.txt @@ -74,6 +74,23 @@ to read-write, and change /bar from read-write to read-only: # mount -t unionfs -o remount,mode=/foo=rw,mode=/bar=ro none MOUNTPOINT +Note: in Unionfs 2.x, you cannot set the leftmost branch to readonly because +then Unionfs won't have any writable place for copyups to take place. +Moreover, the VFS can get confused when it tries to modify something in a +file system mounted read-write, but isn't permitted to write to it. +Instead, you should set the whole union as readonly, as described above. +If, however, you must set the leftmost branch as readonly, perhaps so you +can get a snapshot of it at a point in time, then you should insert a new +writable top-level branch, and mark the one you want as readonly. This can +be accomplished as follows, assuming that /foo is your current leftmost +branch: + +# mount -t tmpfs -o size=NNN /new +# mount -t unionfs -o remount,add=/new,mode=/foo=ro none MOUNTPOINT +<do what you want safely in /foo> +# mount -t unionfs -o remount,del=/new,mode=/foo=rw none MOUNTPOINT +<check if there's anything in /new you want to preserve> +# umount /new CACHE CONSISTENCY ================= -- 1.5.2.2 - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
