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; we would be happy to modify the document if file-system developers claim that their system provides (or aims to provide) SOMC. Signed-off-by: Jayashree Mohan <jaya@xxxxxxxxxxxxx> --- .../filesystems/crash-recovery-guarantees.txt | 173 +++++++++++++++++++++ 1 file changed, 173 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..4d1a9c6b --- /dev/null +++ b/Documentation/filesystems/crash-recovery-guarantees.txt @@ -0,0 +1,173 @@ +===================================================================== +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 data 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 is persisted (no need +to explicitly persist the parent directory). However, if you create +multiple names of the file (hard links), then they are not guaranteed +to persist unless each one of the hard links are persisted [2]. + +fsync(dir) : All file names within the persisted directory will exist, +but does not guarantee file data. + +btrfs +------ +fsync(file) : Ensures that the newly created file is persisted, along +with all its hard links. You do not need to persist individual hard +links to the file. + +fsync(dir) : All the file names within the directory 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 persisting directories +[2]. + +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 a 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 is file-system specific. A developer would +need 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]. + +Example +------- +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” [8]. You will need to +explicitly persist 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, on document, only XFS claims to provide SOMC. +It is not clear if ext4, F2FS 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
