diff options
Diffstat (limited to 'Documentation/filesystems/overlayfs.txt')
| -rw-r--r-- | Documentation/filesystems/overlayfs.txt | 39 |
1 files changed, 33 insertions, 6 deletions
diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt index 6ea1e64d1464..961b287ef323 100644 --- a/Documentation/filesystems/overlayfs.txt +++ b/Documentation/filesystems/overlayfs.txt @@ -14,9 +14,13 @@ 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. -This approach is 'hybrid' because the objects that appear in the -filesystem do not all appear to belong to that filesystem. In many -cases an object accessed in the union will be indistinguishable + +Overlay objects +--------------- + +The overlay filesystem approach is 'hybrid', because the objects that +appear in the filesystem do not always appear to belong to that filesystem. +In many cases, an object accessed in the union will be indistinguishable from accessing the corresponding object from the original filesystem. This is most obvious from the 'st_dev' field returned by stat(2). @@ -34,6 +38,19 @@ make the overlay mount more compliant with filesystem scanners and overlay objects will be distinguishable from the corresponding objects in the original filesystem. +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 bit. + + Upper and Lower --------------- @@ -290,10 +307,19 @@ Non-standard behavior --------------------- The copy_up operation essentially creates a new, identical file and -moves it over to the old name. The new file may be on a different -filesystem, so both st_dev and st_ino of the file may change. +moves it over to the old name. Any open files referring to this inode +will access the old data. + +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. -Any open files referring to this inode will access the old data. +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. 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 @@ -302,6 +328,7 @@ propagated to other names referring to the same inode. Unless "redirect_dir" feature is enabled, rename(2) on a lower or merged directory will fail with EXDEV. + Changes to underlying filesystems --------------------------------- |