On Tue, Oct 24, 2023 at 01:30:03PM +0100, John Garry wrote: > On 09/10/2023 18:44, Darrick J. Wong wrote: > > On Fri, Sep 29, 2023 at 09:37:15AM +0000, John Garry wrote: > > > From: Himanshu Madhani <himanshu.madhani@xxxxxxxxxx> > > > > > > Add RWF_ATOMIC flag description for pwritev2(). > > > > > > Signed-off-by: Himanshu Madhani <himanshu.madhani@xxxxxxxxxx> > > > #jpg: complete rewrite > > > Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx> > > > --- > > > man2/readv.2 | 45 +++++++++++++++++++++++++++++++++++++++++++++ > > > 1 file changed, 45 insertions(+) > > > > > > diff --git a/man2/readv.2 b/man2/readv.2 > > > index fa9b0e4e44a2..ff09f3bc9792 100644 > > > --- a/man2/readv.2 > > > +++ b/man2/readv.2 > > > @@ -193,6 +193,51 @@ which provides lower latency, but may use additional resources. > > > .B O_DIRECT > > > flag.) > > > .TP > > > +.BR RWF_ATOMIC " (since Linux 6.7)" > > > +Allows block-based filesystems to indicate that write operations will be issued > > > > "Require regular file write operations to be issued with torn write > > protection." > > ok > > > > > > +with torn-write protection. Torn-write protection means that for a power or any > > > +other hardware failure, all or none of the data from the write will be stored, > > > +but never a mix of old and new data. This flag is meaningful only for > > > +.BR pwritev2 (), > > > +and its effect applies only to the data range written by the system call. > > > +The total write length must be power-of-2 and must be sized between > > > +stx_atomic_write_unit_min and stx_atomic_write_unit_max, both inclusive. The > > > +write must be at a natural offset within the file with respect to the total > > > > What is a "natural" offset? > > I really meant naturally-aligned offset > > > That should be defined with more > > specificity. Does that mean that the position of a XX-KiB write must > > also be aligned to XX-KiB? > > Yes > > > e.g. a 32K untorn write can only start at a > > multiple of 32K? > > Correct > > > What if the device supports untorn writes between 4K > > and 64K, does that mean I /cannot/ issue a 32K untorn write at offset > > 48K? > > Correct > > Do you think that an example would help? Yes. > > > > > +write length. Torn-write protection only works with > > > +.B O_DIRECT > > > +flag, i.e. buffered writes are not supported. To guarantee consistency from > > > +the write between a file's in-core state with the storage device, > > > +.BR fdatasync (2) > > > +or > > > +.BR fsync (2) > > > +or > > > +.BR open (2) > > > +and > > > +.B O_SYNC > > > +or > > > +.B O_DSYNC > > > +or > > > +.B pwritev2 () > > > +flag > > > +.B RWF_SYNC > > > +or > > > +.B RWF_DSYNC > > > +is required. > > > > I'm starting to think that this manpage shouldn't be restating > > durability information here. > > > > "Application programs with data or file integrity completion > > requirements must configure synchronous writes with the DSYNC > > or SYNC flags, as explained above." > > ok > > > > > > +For when regular files are opened with > > > +.BR open (2) > > > +but without > > > +.B O_SYNC > > > +or > > > +.B O_DSYNC > > > +and the > > > +.BR pwritev2() > > > +call is made without > > > +.B RWF_SYNC > > > +or > > > +.BR RWF_DSYNC > > > +set, the range metadata must already be flushed to storage and the data range > > > +must not be in unwritten state, shared, a preallocation, or a hole. > > > > I think that we can drop all of these flags requirements, since the > > contiguous small space allocation requirement means that the fs can > > provide all-or-nothing writes even if metadata updates are needed: > > > > If the file range is allocated and marked unwritten (i.e. a > > preallocation), the ioend will clear the unwritten bit from the file > > mapping atomically. After a crash, the application sees either zeroes > > or all the data that was written. > > > > If the file range is shared, the ioend will map the COW staging extent > > into the file atomically. After a crash, the application sees either > > the old contents from the old blocks, or the new contents from the new > > blocks. > > > > If the file range is a sparse hole, the directio setup will allocate > > space and create an unwritten mapping before issuing the write bio. The > > rest of the process works the same as preallocations and has the same > > behaviors. > > > > If the file range is allocated and was previously written, the write is > > issued and that's all that's needed from the fs. After a crash, reads > > of the storage device produce the old contents or the new contents. > > > > Summarizing: > > > > An (ATOMIC|SYNC) request provides the strongest guarantees (data > > will not be torn, and all file metadata updates are persisted before > > the write is returned to userspace. Programs see either the old data or > > the new data, even if there's a crash. > > > > (ATOMIC|DSYNC) is less strong -- data will not be torn, and any file > > updates for just that region are persisted before the write is returned. > > > > (ATOMIC) is the least strong -- data will not be torn. Neither the > > filesystem nor the device make guarantees that anything ended up on > > stable storage, but if it does, programs see either the old data or the > > new data. > > > > > Will respond to later mail in thread. Ok, thank you! --D > > Maybe we should rename the whole UAPI s/atomic/untorn/... > > > --D > > > > > +.TP > > > .BR RWF_SYNC " (since Linux 4.7)" > > > .\" commit e864f39569f4092c2b2bc72c773b6e486c7e3bd9 > > > Provide a per-write equivalent of the > > > -- > > > 2.31.1 > > > > > Thanks, > John >
