On Sun, 20 Sep 2009 07:34:40 +0200 Michael Kerrisk <mtk.manpages@xxxxxxxxxxxxxx> wrote: > Hi Christoph, > > Thanks for this patch. Since Andrew was the original author of this > syscall, I'd like him to take a look at these changes, to get some > review. > > Andrew? > > Thanks > > Michael > > On Thu, Aug 27, 2009 at 8:01 PM, Christoph Hellwig <hch@xxxxxx> wrote: > > This system call is by design completely unsuitable for any data > > integrity operations. __Make that very clear in the manpage. > > > > > > Signed-off-by: Christoph Hellwig <hch@xxxxxx> > > > > Index: man-pages/man2/sync_file_range.2 > > =================================================================== > > --- man-pages.orig/man2/sync_file_range.2 __ __ __ 2009-08-27 14:51:51.373360594 -0300 > > +++ man-pages/man2/sync_file_range.2 __ __2009-08-27 14:57:35.213854927 -0300 > > @@ -80,11 +80,22 @@ after performing any write. > > __Specifying > > __.I flags > > __as 0 is permitted, as a no-op. > > -.SS Some details > > -None of these operations write out the file's metadata. > > +.SS WARNING > > +This system call is extremly dangerous and should not be used in portable > > +programs. __None of these operations write out the file's metadata. > > __Therefore, unless the application is strictly performing overwrites of > > -already-instantiated disk blocks, > > -there are no guarantees that the data will be available after a crash. > > +already-instantiated disk blocks, there are no guarantees that the data will > > +be available after a crash. __There is no user interface to know if a > > +write is purely an overwrite. On filesystem using copy on write semantics > > +like > > +.IR btrfs > > +an over write of existing allocated blocks is impossible. __Writing into > > +pre-allocated space many filesystems also require calls into the block > > +allocator which this system call does not sync out to disk. > > +This system call does not flush disk write caches and thus does not provide > > +any data integrity on systems with volatile disk write caches. > > + > > +.SS Some details > > Shrug. If you want. The syscall is all about dirty memory management and userspace control over IO timing/volume. Trying to use it for data integrity purposes is a bit perverse. -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
