Re: vfs: Add MS_FLUSHONFSYNC mount flag

[Fernando Luis Vazquez Cao]

On Fri, 2009-02-13 at 23:20 +1100, Dave Chinner wrote:
> On Fri, Feb 13, 2009 at 12:20:17AM -0600, Eric Sandeen wrote:
> > I'm just a little leery of the "dangerous" mount option proliferation, I
> > guess.
> 
> You're not the only one, Eric. It's bad enough having to explain to
> users what barriers do once they have lost data after a power loss,
> let alone confusing them further by adding more mount options they
> will get wrong by accident....

That is precisely the reason why we should use sensible defaults, which
in this case means enabling barriers and flushing disk caches on
fsync()/fdatasync() by default.

Adding either a new mount option (as you yourself suggest below) or a
sysfs tunable is desirable for those cases when we really do not need to
flush the disk write cache to guarantee integrity (battery-backed block
devices come to mind), or we want to be fast at the cost of potentially
losing some data.

> Quite frankly, the VFS should do stuff that is slow and safe
> and filesystems can choose to ignore the VFS (via filesystem
> specific mount options) if they want to be fast and potentially
> unsafe.

To avoid unnecessary flushes and allow for filesystem-specific
optimizations I was considering the following approach:

1- Add flushonfsync mount option (as an aside, I am of the opinion that
it should be set by default).
2- Modify file_fsync() so that it checks whether FLUSHONFSYNC is set and
flushes the underlying device accordingly. With this we would cover all
filesystems that use the vfs-provided file_fsync() as their fsync method
(commonly used filesystems such as fat fall in this group).
3- Advanced filesystems (ext3/4, XFS, btrfs, etc) which provide their
own fsync implementations are allowed to perform filesystem-specific
optimizations there to minimize the number of flushes and maximize
throughput.

In this patch set I implemented (1) and (3) for ext3/4 to have some code
to comment on.

Does this approach make sense? Thoughts?

- Fernando

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