Re: [PATCH v2] Documenting the crash-recovery guarantees of Linux file systems

Filipe Manana
2019-03-13T17:13:14Z

On Tue, Mar 12, 2019 at 7:27 PM Jayashree <jaya@cs.utexas.edu> wrote:
>
> In this file, we document the crash-recovery guarantees
> provided by four Linux file systems - xfs, ext4, F2FS and btrfs. We also
> present Dave Chinner's proposal of Strictly-Ordered Metadata Consistency
> (SOMC), which is provided by xfs. It is not clear to us if other file systems
> provide SOMC.
>
> Signed-off-by: Jayashree Mohan <jaya@cs.utexas.edu>
> Reviewed-by: Amir Goldstein <amir73il@gmail.com>

Mostly for the Btrfs part:

Reviewed-by: Filipe Manana <fdmanana@suse.com>

Nit: you can have lines up to 80 characters, many of them are
significantly and unnecessarily smaller than that.

Thanks for writing this document.

> ---
>
> We would be happy to modify the document if file-system
> developers claim that their system provides (or aims to provide) SOMC.
>
> Changes since v1:
> * Addressed few nits identified in the review
> * Added the fsync guarantees for F2FS and its SOMC compliance
> ---
> .../filesystems/crash-recovery-guarantees.txt | 193 +++++++++++++++++++++
> 1 file changed, 193 insertions(+)
> create mode 100644 Documentation/filesystems/crash-recovery-guarantees.txt
>
> diff --git a/Documentation/filesystems/crash-recovery-guarantees.txt b/Documentation/filesystems/crash-recovery-guarantees.txt
> new file mode 100644
> index 0000000..be84964
> --- /dev/null
> +++ b/Documentation/filesystems/crash-recovery-guarantees.txt
> @@ -0,0 +1,193 @@
> +=====================================================================
> +File System Crash-Recovery Guarantees
> +=====================================================================
> +Linux file systems provide certain guarantees to user-space
> +applications about what happens to their data if the system crashes
> +(due to power loss or kernel panic). These are termed crash-recovery
> +guarantees.
> +
> +Crash-recovery guarantees only pertain to data or metadata that has
> +been explicitly persisted to storage with fsync(), fdatasync(), or
> +sync() system calls. By default, write(), mkdir(), and other
> +file-system related system calls only affect the in-memory state of
> +the file system.
> +
> +The crash-recovery guarantees provided by most Linux file systems are
> +significantly stronger than what is required by POSIX. POSIX is vague,
> +even allowing fsync() to do nothing (Mac OSX takes advantage of
> +this). However, the guarantees provided by file systems are not
> +documented, and vary between file systems. This document seeks to
> +describe the current crash-recovery guarantees provided by major Linux
> +file systems.
> +
> +What does the fsync() operation guarantee?
> +----------------------------------------------------
> +fsync() operation is meant to force the physical write of data
> +corresponding to a file from the buffer cache, along with the file
> +metadata. Note that the guarantees mentioned for each file system below
> +are in addition to the ones provided by POSIX.
> +
> +POSIX
> +-----
> +fsync(file) : Flushes the data and metadata associated with the
> +file. However, if the directory entry for the file has not been
> +previously persisted, or has been modified, it is not guaranteed to be
> +persisted by the fsync of the file [1]. What this means is, if a file
> +is newly created, you will have to fsync(parent directory) in addition
> +to fsync(file) in order to ensure that the file's directory entry has
> +safely reached the disk.
> +
> +fsync(dir) : Flushes directory data and directory entries. However if
> +you created a new file within the directory and wrote data to the
> +file, then the file data is not guaranteed to be persisted, unless an
> +explicit fsync() is issued on the file.
> +
> +ext4
> +-----
> +fsync(file) : Ensures that a newly created file's directory entry is
> +persisted (no need to explicitly persist the parent directory). However,
> +if you create multiple names of the file (hard links), then their directory
> +entries are not guaranteed to persist unless each one of the parent
> +directory entries are persisted [2].
> +
> +fsync(dir) : All file names within the persisted directory will exist,
> +but does not guarantee file data.
> +
> +xfs
> +----
> +fsync(file) : Ensures that a newly created file's directory entry is
> +persisted. Additionally, all the previous dependent modifications to
> +this file are also persisted. If any file shares an object
> +modification dependency with the fsync-ed file, then that file's
> +directory entry is also persisted.
> +
> +fsync(dir) : All file names within the persisted directory will exist,
> +but does not guarantee file data. As with files, fsync(dir) also persists
> +previous dependent metadata operations.
> +
> +btrfs
> +------
> +fsync(file) : Ensures that a newly created file's directory entry
> +is persisted, along with the directory entries of all its hard links.
> +You do not need to explicitly fsync individual hard links to the file.
> +
> +fsync(dir) : All the file names within the directory will persist. All the
> +rename and unlink operations within the directory are persisted. Due
> +to the design choices made by btrfs, fsync of a directory could lead
> +to an iterative fsync on sub-directories, thereby requiring a full
> +file system commit. So btrfs does not advocate fsync of directories
> +[2].
> +
> +F2FS
> +----
> +fsync(file) or fsync(dir) : In the default mode (fsync-mode=posix),
> +F2FS only guarantees POSIX behaviour. However, it provides xfs-like
> +guarantees if mounted with fsync-mode=strict option.
> +
> +fsync(symlink)
> +-------------
> +A symlink inode cannot be directly opened for IO, which means there is
> +no such thing as fsync of a symlink [3]. You could be tricked by the
> +fact that open and fsync of a symlink succeeds without returning a
> +error, but what happens in reality is as follows.
> +
> +Suppose we have a symlink “foo”, which points to the file “A/bar”
> +
> +fd = open(“foo”, O_CREAT | O_RDWR)
> +fsync(fd)
> +
> +Both the above operations succeed, but if you crash after fsync, the
> +symlink could be still missing.
> +
> +When you try to open the symlink “foo”, you are actually trying to
> +open the file that the symlink resolves to, which in this case is
> +“A/bar”. When you fsync the inode returned by the open system call, you
> +are actually persisting the file “A/bar” and not the symlink. Note
> +that if the file “A/bar” does not exist and you try the open the
> +symlink “foo” without the O_CREAT flag, then file open will fail. To
> +obtain the file descriptor associated with the symlink inode, you
> +could open the symlink using “O_PATH | O_NOFOLLOW” flags. However, the
> +file descriptor obtained this way can be only used to indicate a
> +location in the file-system tree and to perform operations that act
> +purely at the file descriptor level. Operations like read(), write(),
> +fsync() etc cannot be performed on such file descriptors.
> +
> +Bottomline : You cannot fsync() a symlink.
> +
> +fsync(special files)
> +--------------------
> +Special files in Linux include block and character device files
> +(created using mknod), FIFO (created using mkfifo) etc. Just like the
> +behavior of fsync on symlinks described above, these special files do
> +not have an fsync function defined. Similar to symlinks, you
> +cannot fsync a special file [4].
> +
> +
> +Strictly Ordered Metadata Consistency
> +-------------------------------------
> +With each file system providing varying levels of persistence
> +guarantees, a consensus in this regard, will benefit application
> +developers to work with certain fixed assumptions about file system
> +guarantees. Dave Chinner proposed a unified model called the
> +Strictly Ordered Metadata Consistency (SOMC) [5].
> +
> +Under this scheme, the file system guarantees to persist all previous
> +dependent modifications to the object upon fsync(). If you fsync() an
> +inode, it will persist all the changes required to reference the inode
> +and its data. SOMC can be defined as follows [6]:
> +
> +If op1 precedes op2 in program order (in-memory execution order), and
> +op1 and op2 share a dependency, then op2 must not be observed by a
> +user after recovery without also observing op1.
> +
> +Unfortunately, SOMC's definition depends upon whether two operations
> +share a dependency, which could be file-system specific. It might
> +require a developer to understand file-system internals to know if
> +SOMC would order one operation before another. It is worth noting
> +that a file system can be crash-consistent (according to POSIX),
> +without providing SOMC [7].
> +
> +As an example, consider the following test case from xfstest
> +generic/342 [8]
> +-------
> +touch A/foo
> +echo “hello” > A/foo
> +sync
> +
> +mv A/foo A/bar
> +echo “world” > A/foo
> +fsync A/foo
> +CRASH
> +
> +What would you expect on recovery, if the file system crashed after
> +the final fsync returned successfully?
> +
> +Non-SOMC file systems will not persist the file
> +A/bar because it was not explicitly fsync-ed. But this means, you will
> +find only the file A/foo with data “world” after crash, thereby losing
> +the previously persisted file with data “hello”. You will need to
> +explicitly fsync the directory A to ensure the rename operation is
> +safely persisted on disk.
> +
> +Under SOMC, to correctly reference the new inode via A/foo,
> +the previous rename operation must persist as well. Therefore,
> +fsync() of A/foo will persist the renamed file A/bar as well.
> +On recovery you will find both A/bar (with data “hello”)
> +and A/foo (with data “world”).
> +
> +It is noteworthy that xfs, ext4, F2FS (when mounted with fsync_mode=strict)
> +and btrfs provide SOMC-like behaviour in this particular example.
> +However, in writing, only XFS claims to provide SOMC. F2FS aims to provide
> +SOMC when mounted with fsync_mode=strict. It is not clear if ext4 and
> +btrfs provide strictly ordered metadata consistency.
> +
> +--------------------------------------------------------
> +[1] http://man7.org/linux/man-pages/man2/fdatasync.2.html
> +[2] https://www.spinics.net/lists/linux-btrfs/msg77340.html
> +[3] https://www.spinics.net/lists/fstests/msg09370.html
> +[4] https://bugzilla.kernel.org/show_bug.cgi?id=202485
> +[5] https://marc.info/?l=fstests&m=155010885626284&w=2
> +[6] https://marc.info/?l=fstests&m=155011123126916&w=2
> +[7] https://www.spinics.net/lists/fstests/msg09379.html
> +[8] https://patchwork.kernel.org/patch/10132305/
> +
> --
> 2.7.4
>

--
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”