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Re: [RFC, PATCH 0/2] fiemap: filesystem free space mapping

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [RFC, PATCH 0/2] fiemap: filesystem free space mapping
From: Chris Mason <chris.mason@xxxxxxxxxxxx>
Date: Wed, 24 Oct 2012 07:47:17 -0400
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, "linux-fsdevel@xxxxxxxxxxxxxxx" <linux-fsdevel@xxxxxxxxxxxxxxx>, "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
In-reply-to: <20121023215313.GQ4291@dastard>
Mail-followup-to: Chris Mason <chris.mason@xxxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, "linux-fsdevel@xxxxxxxxxxxxxxx" <linux-fsdevel@xxxxxxxxxxxxxxx>, "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
References: <1350537079-16246-1-git-send-email-david@xxxxxxxxxxxxx> <20121023123044.GG7341@xxxxxxxxxxxxx> <20121023215313.GQ4291@dastard>
User-agent: Mutt/1.5.21 (2011-07-01)
On Tue, Oct 23, 2012 at 03:53:13PM -0600, Dave Chinner wrote:
> On Tue, Oct 23, 2012 at 08:30:44AM -0400, Christoph Hellwig wrote:
> > On Thu, Oct 18, 2012 at 04:11:17PM +1100, Dave Chinner wrote:
> > > So, I was bored a few days ago, and I was sick of having to run
> > > xfs_db incorrectly report free space extents when the filesytem is
> > > mounted, so I decided to extend fiemap to export freespace mappings
> > > to userspace so I could get the information coherently through the
> > > mounted filesystem.
> > > 
> > > Yes, this could probably be considered interface abuse but, well, it
> > > was simple to do because extent mapping is exactly what fiemap is
> > > designed to do. Hence I didn't have to write new walkers/formatters
> > > and I was using code I knew worked correctly.
> > 
> > I think the right way to handle this is to introduce a new ioctl which
> > uses the same structures.  That way we have a reasonable interface,
> > without issue like which file does it need to be called on because the
> > VFS glue can turn it into a superblock op.
> 
> A VFS level ioctl or an XFS ioctl?
> 
> I thought about a new ioctl, but then what's the point of having an
> extensible fiemap interface if we create new ioctls with an
> identical interface for doing something that the existing ioctl is
> perfectly capable of doing?  I'd still need special flags to control
> the ioctl behaviour even though it uses struct fiemap and plumbing,
> so it seemed pointless to introduce a new ioctl....

This brings us one step close to the norton disk doctor defrag display.
I'm all for it in the main fiemap call, it makes much more sense for the
users I think.

-chris

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