Hi Jayashree, Sorry for the delay. On 2019-3-8 2:51, Jayashree Mohan wrote: > [cc : f2fs-dev] > Thanks for the suggestions! Will incorporate these changes and send out a v2. > > We would also like to update the document to correctly reflect whether each file > system is SOMC compliant. As of now, we only know for sure that xfs provides > SOMC. Could developers of ext4, btrfs and F2FS comment whether your file system > is SOMC complaint (or aims to be complaint)? @Theodore Ts'o > <mailto:tytso@xxxxxxx> , @Chao Yu <mailto:chao@xxxxxxxxxx> , @Filipe Manana > <mailto:fdmanana@xxxxxxxxx> > > @Chao Yu <mailto:chao@xxxxxxxxxx> We are also unsure about the fsync behaviour > of F2FS. Is it just POSIX in the default mode, and SOMC if mounted with fsync_mode= > strict? Yes, that's the rule f2fs tries to keep. :) Thanks, > > Thanks, > Jayashree Mohan > > > > On Wed, Mar 6, 2019 at 3:14 AM Amir Goldstein <amir73il@xxxxxxxxx > <mailto:amir73il@xxxxxxxxx>> wrote: > > On Wed, Mar 6, 2019 at 4:59 AM Jayashree <jaya@xxxxxxxxxxxxx > <mailto:jaya@xxxxxxxxxxxxx>> 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 > > Nice work. > You may add > Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx <mailto:amir73il@xxxxxxxxx>> > > Few nits below. > > > ; we would be happy to modify the document if file-system > > developers claim that their system provides (or aims to provide) SOMC. > > This part belongs after the --- line > IOW, it does not belong in the commit message. > > > > > Signed-off-by: Jayashree Mohan <jaya@xxxxxxxxxxxxx > <mailto: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. > > No. In order to ensure that the file's *directory entry* will persist. > Throughout the doc, if you just say "file will persist" the meaning > is ambiguous. "file data will persist" "file metadata will persist" > and "file directory entry will persist" are three distinguished > outcomes. > > > + > > +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 > > newly created file directory entry is persisted > > > +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]. > > "...then the hard linked directory entries are not guarantied to persist > unless each one of the parent directories are persisted." > > > + > > +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. > > Rephrase to disambiguate > > > + > > +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 > > >
