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ovl: update documentation w.r.t "xino" feature

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Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
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Amir Goldstein 2018-03-29 16:36:56 +03:00 committed by Miklos Szeredi
parent 795939a93e
commit 16149013f8
1 changed files with 33 additions and 6 deletions
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39
Documentation/filesystems/overlayfs.txt
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@ -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.
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.
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
---------------------------------