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Diffstat (limited to 'Documentation/filesystems')
| -rw-r--r-- | Documentation/filesystems/overlayfs.rst | 50 |
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diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst index af5a69f87da4..eb846518e6ac 100644 --- a/Documentation/filesystems/overlayfs.rst +++ b/Documentation/filesystems/overlayfs.rst @@ -783,6 +783,56 @@ controlled by the "uuid" mount option, which supports these values: mounted with "uuid=on". +Durability and copy up +---------------------- + +The fsync(2) system call ensures that the data and metadata of a file +are safely written to the backing storage, which is expected to +guarantee the existence of the information post system crash. + +Without an fsync(2) call, there is no guarantee that the observed +data after a system crash will be either the old or the new data, but +in practice, the observed data after crash is often the old or new data +or a mix of both. + +When an overlayfs file is modified for the first time, copy up will +create a copy of the lower file and its parent directories in the upper +layer. Since the Linux filesystem API does not enforce any particular +ordering on storing changes without explicit fsync(2) calls, in case +of a system crash, the upper file could end up with no data at all +(i.e. zeros), which would be an unusual outcome. To avoid this +experience, overlayfs calls fsync(2) on the upper file before completing +data copy up with rename(2) or link(2) to make the copy up "atomic". + +By default, overlayfs does not explicitly call fsync(2) on copied up +directories or on metadata-only copy up, so it provides no guarantee to +persist the user's modification unless the user calls fsync(2). +The fsync during copy up only guarantees that if a copy up is observed +after a crash, the observed data is not zeroes or intermediate values +from the copy up staging area. + +On traditional local filesystems with a single journal (e.g. ext4, xfs), +fsync on a file also persists the parent directory changes, because they +are usually modified in the same transaction, so metadata durability during +data copy up effectively comes for free. Overlayfs further limits risk by +disallowing network filesystems as upper layer. + +Overlayfs can be tuned to prefer performance or durability when storing +to the underlying upper layer. This is controlled by the "fsync" mount +option, which supports these values: + +- "auto": (default) + Call fsync(2) on upper file before completion of data copy up. + No explicit fsync(2) on directory or metadata-only copy up. +- "strict": + Call fsync(2) on upper file and directories before completion of any + copy up. +- "volatile": [*] + Prefer performance over durability (see `Volatile mount`_) + +[*] The mount option "volatile" is an alias to "fsync=volatile". + + Volatile mount -------------- |