diff options
Diffstat (limited to 'Documentation/filesystems')
| -rw-r--r-- | Documentation/filesystems/overlayfs.rst | 26 |
1 files changed, 12 insertions, 14 deletions
diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst index 78240e29b0bb..455ca86eb4fc 100644 --- a/Documentation/filesystems/overlayfs.rst +++ b/Documentation/filesystems/overlayfs.rst @@ -40,17 +40,17 @@ On 64bit systems, even if all overlay layers are not on the same underlying filesystem, the same compliant behavior could be achieved with the "xino" feature. The "xino" feature composes a unique object identifier from the real object st_ino and an underlying fsid index. - -If all underlying filesystems support NFS file handles and export file -handles with 32bit inode number encoding (e.g. ext4), overlay filesystem -will use the high inode number bits for fsid. Even when the underlying -filesystem uses 64bit inode numbers, users can still enable the "xino" -feature with the "-o xino=on" overlay mount option. That is useful for the -case of underlying filesystems like xfs and tmpfs, which use 64bit inode -numbers, but are very unlikely to use the high inode number bits. In case +The "xino" feature uses the high inode number bits for fsid, because the +underlying filesystems rarely use the high inode number bits. In case the underlying inode number does overflow into the high xino bits, overlay filesystem will fall back to the non xino behavior for that inode. +The "xino" feature can be enabled with the "-o xino=on" overlay mount option. +If all underlying filesystems support NFS file handles, the value of st_ino +for overlay filesystem objects is not only unique, but also persistent over +the lifetime of the filesystem. The "-o xino=auto" overlay mount option +enables the "xino" feature only if the persistent st_ino requirement is met. + The following table summarizes what can be expected in different overlay configurations. @@ -66,14 +66,13 @@ Inode properties | All layers | Y | Y | Y | Y | Y | Y | Y | Y | | on same fs | | | | | | | | | +--------------+-----+------+-----+------+--------+--------+--------+-------+ -| Layers not | N | Y | Y | N | N | Y | N | Y | +| Layers not | N | N | Y | N | N | Y | N | Y | | on same fs, | | | | | | | | | | xino=off | | | | | | | | | +--------------+-----+------+-----+------+--------+--------+--------+-------+ | xino=on/auto | Y | Y | Y | Y | Y | Y | Y | Y | -| | | | | | | | | | +--------------+-----+------+-----+------+--------+--------+--------+-------+ -| xino=on/auto,| N | Y | Y | N | N | Y | N | Y | +| xino=on/auto,| N | N | Y | N | N | Y | N | Y | | ino overflow | | | | | | | | | +--------------+-----+------+-----+------+--------+--------+--------+-------+ @@ -81,7 +80,6 @@ Inode properties /proc files, such as /proc/locks and /proc/self/fdinfo/<fd> of an inotify file descriptor. - Upper and Lower --------------- @@ -461,7 +459,7 @@ 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 +overlay filesystem and the value of st_ino for filesystem objects may not be persistent and could change even while the overlay filesystem is mounted, as summarized in the `Inode properties`_ table above. @@ -476,7 +474,7 @@ a crash or deadlock. Offline changes, when the overlay is not mounted, are allowed to the upper tree. Offline changes to the lower tree are only allowed if the -"metadata only copy up", "inode index", and "redirect_dir" features +"metadata only copy up", "inode index", "xino" and "redirect_dir" features have not been used. If the lower tree is modified and any of these features has been used, the behavior of the overlay is undefined, though it will not result in a crash or deadlock. |