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SubjectRe: [PATCH] vfs: introduce FS_IOC_SYNCFS to sync a single super
On Thu, 26 Aug 2010, Andrew Morton wrote:
> On Mon, 9 Aug 2010 07:33:57 -0700 (PDT)
> Sage Weil <sage@newdream.net> wrote:
>
> > Currently the only way to sync a single super_block (and not all of them
> > via sync(2)) is via the BLKFLSBUF ioctl on the block device. That also
> > invalidates the bdev mapping, which isn't usually desireable
>
> Actually you can do
>
> mount -o remount /dev/whatever
>
> and it will sync the fs and retain caches.
>
> > and it
> > doesn't work for non-block file systems.
>
> And I guess remount will do that also.

Good to know.

> > The ability to sync a single
> > mount can be useful for both applications and administrators (e.g., when
> > other mounts on the system are hung).
> >
> > Introduce a simple ioctl to sync the super associated with an open file.
> > Pass any error returned by sync_filesystem() back to the user.
> >
>
> The changelog forgot to tell us why this is a useful thing to add.
> What is the use-case?

Two use cases:

* An admin who wants to sync only one mount (e.g., 'sync /mnt/foo').
I tend to need this on boxes with lots of NFS mounts where something gets
hung up, I want to reboot, but want to make sure my local fs is synced
first. The remount trick handles this, although I doubt many are aware of
that side-effect, and I'm not sure we should suggest they rely on it.

* My use case is the Ceph storage daemon, which writes gobs of stuff to a
single super and periodically wants to make sure it's synced so that it's
application-level journal can be trimmed. fsync() on individual files
isn't practical (leads to bad IO patterns, ). Ideally, this should
be usable by a non-privileged user (just like sync(2)).

> > ---
> > fs/ioctl.c | 9 +++++++++
> > include/linux/fs.h | 1 +
> > 2 files changed, 10 insertions(+), 0 deletions(-)
> >
> > diff --git a/fs/ioctl.c b/fs/ioctl.c
> > index 2d140a7..2aabb19 100644
> > --- a/fs/ioctl.c
> > +++ b/fs/ioctl.c
> > @@ -593,6 +593,15 @@ int do_vfs_ioctl(struct file *filp, unsigned int fd, unsigned int cmd,
> > case FS_IOC_FIEMAP:
> > return ioctl_fiemap(filp, arg);
> >
> > + case FS_IOC_SYNCFS:
> > + {
> > + struct super_block *sb = filp->f_dentry->d_sb;
> > + down_read(&sb->s_umount);
> > + error = sync_filesystem(sb);
> > + up_read(&sb->s_umount);
> > + break;
> > + }
> > +
>
> `mount -o remount' is surely a Linux-specific side-effect and there's
> really no guarantee that Linux will always retain that side-effect.
> OTOH FS_IOC_SYNCFS is linux-specific.

The key difference I see is that mount -o remount is root-only, whereas
sync(2) and FS_IOC_SYNCFS are not. Also, it seems like a bad idea for
applications to rely on the current remount side effect, particularly for
something as important as data integrity.

> If we're going to add something like this then it will need to be
> documented in manpages. Supposedly, a cc to linux-api@vger.kernel.org
> will help make all that happen, but I'm not sure who if anyone is
> answering the phone over there?

Where would this go in manpages? ioctl_list(2)? I'm happy to prepare a
patch for that as well.

Thanks!
sage


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