diff options
Diffstat (limited to 'Documentation/filesystems')
| -rw-r--r-- | Documentation/filesystems/overlayfs.txt | 51 |
1 files changed, 30 insertions, 21 deletions
diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt index 72615a2c0752..c2c71be75e3d 100644 --- a/Documentation/filesystems/overlayfs.txt +++ b/Documentation/filesystems/overlayfs.txt @@ -10,10 +10,6 @@ union-filesystems). An overlay-filesystem tries to present a filesystem which is the result over overlaying one filesystem on top of the other. -The result will inevitably fail to look exactly like a normal -filesystem for various technical reasons. The expectation is that -many use cases will be able to ignore these differences. - Overlay objects --------------- @@ -306,27 +302,40 @@ the copied layers will fail the verification of the lower root file handle. Non-standard behavior --------------------- -The copy_up operation essentially creates a new, identical file and -moves it over to the old name. Any open files referring to this inode -will access the old data. +Overlayfs can now act as a POSIX compliant filesystem with the following +features turned on: + +1) "redirect_dir" + +Enabled with the mount option or module option: "redirect_dir=on" or with +the kernel config option CONFIG_OVERLAY_FS_REDIRECT_DIR=y. + +If this feature is disabled, then rename(2) on a lower or merged directory +will fail with EXDEV ("Invalid cross-device link"). + +2) "inode index" + +Enabled with the mount option or module option "index=on" or with the +kernel config option CONFIG_OVERLAY_FS_INDEX=y. -The new file may be on a different filesystem, so both st_dev and st_ino -of the real file may change. The values of st_dev and st_ino returned by -stat(2) on an overlay object are often not the same as the real file -stat(2) values to prevent the values from changing on copy_up. +If this feature is disabled and a file with multiple hard links is copied +up, then this will "break" the link. Changes will not be propagated to +other names referring to the same inode. -Unless "xino" feature is enabled, when overlay layers are not all on the -same underlying filesystem, the value of st_dev may be different for two -non-directory objects in the same overlay filesystem and the value of -st_ino for directory objects may be non persistent and could change even -while the overlay filesystem is still mounted. +3) "xino" -Unless "inode index" feature is enabled, if a file with multiple hard -links is copied up, then this will "break" the link. Changes will not be -propagated to other names referring to the same inode. +Enabled with the mount option "xino=auto" or "xino=on", with the module +option "xino_auto=on" or with the kernel config option +CONFIG_OVERLAY_FS_XINO_AUTO=y. Also implicitly enabled by using the same +underlying filesystem for all layers making up the overlay. -Unless "redirect_dir" feature is enabled, rename(2) on a lower or merged -directory will fail with EXDEV. +If this feature is disabled or the underlying filesystem doesn't have +enough free bits in the inode number, then overlayfs will not be able to +guarantee that the values of st_ino and st_dev returned by stat(2) and the +value of d_ino returned by readdir(3) will act like on a normal filesystem. +E.g. the value of st_dev may be different for two objects in the same +overlay filesystem and the value of st_ino for directory objects may not be +persistent and could change even while the overlay filesystem is mounted. Changes to underlying filesystems |