Re: [PATCHSET] Refactor barrier=/nobarrier flags from fs to block layer

["Darrick J. Wong" <djwong@us.ibm.com>]

On Wed, Jan 26, 2011 at 10:41:35AM -0600, Eric Sandeen wrote:
> On 1/26/11 5:49 AM, Ric Wheeler wrote:
> > On 01/26/2011 02:12 AM, Darrick J. Wong wrote:
> >> Hello,
> >>
> >>  From what I can tell, most of the filesystems that know how to issue commands
> >> to flush the write cache also have some mechanism for the user to override
> >> whether or not the filesystem actually issues those flushes.  Unfortunately,
> >> the term "barrier" is obsolete having been changed into flushes in 2.6.36, and
> >> many of the filesystems implement the mount options with slightly different
> >> syntaxes (barrier=[0|1|none|flush], nobarrier, etc).
> >>
> >> This patchset adds to the block layer a sysfs knob that an administrator can
> >> use to disable flushes, and removes the mount options from the filesystem code.
> >> As a starting point, I'm removing the mount options and flush toggle from
> >> jbd2/ext4.
> >>
> >> Anyway, I'm looking for some feedback about refactoring the barrier/flush
> >> control knob into the block layer.  It sounds like we want a knob that picks
> >> the safest option (issue flushes when supported) unless the administrator
> >> decides that it is appropriate to do otherwise.  I suspect that there are good
> >> arguments for not having a knob at all, and good arguments for a safe knob.
> >> However, since I don't see the barrier options being removed en masse, I'm
> >> assuming that we still want a knob somewhere.  Do we need the ignore_fua knob
> >> too?  Is this the proper way to deprecate mount options out of filesystems?
> >>
> >> --D
> > 
> > Just to be clear, I strongly object to remove the mount options.
> 
> Agreed, we are just finally, barely starting to win the education battle here.
> Removing or changing the option now will just set us back.  It should at
> LEAST remain as a deprecated option, with the deprecation message pointing
> to crystal-clear documentation.

Ok, how about a second proposal:

1. Put the sysfs knob and the toggle code in the block layer, similar to patch
#1, only make it a per-bdev toggle so each mount can have its own override
parameters.

2. Add some sort of "nocacheflush" option to the VFS layer to adjust the knob.
With this we gain a consistent mount option syntax across all the filesystems,
though what it means for a networked fs is questionable.  I guess you could
reject the mount option if there's no block device under the fs.  Also, any fs
that someday grows an issue-flush feature won't have to add its own option
parsing code.

At umount time, do we undo whatever overrides we set up at mount time?  Seems
sane to me, just wanted to run it by everyone.

3. Change the per-fs option handling code to call the same code as the VFS'
nocacheflush option.  Any fs that wants to deprecate its per-fs option handler
can do so.  Or they can stay forever.

4. Remove all the flush conditionals from the fs code in favor of letting the
block layer handle it.

Hopefully "nocacheflush" is a little more obvious.

--D