Re: [PATCH 10/14] xfs: minimize impact to non-reflink files via reflink per-inode flag

From: Dave Chinner <david@fromorbit.com>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: xfs@oss.sgi.com
Subject: Re: [PATCH 10/14] xfs: minimize impact to non-reflink files via reflink per-inode flag
Date: Thu, 2 Jul 2015 17:07:11 +1000	[thread overview]
Message-ID: <20150702070711.GY22807@dastard> (raw)
In-Reply-To: <20150702023208.GE10043@birch.djwong.org>

On Wed, Jul 01, 2015 at 07:32:08PM -0700, Darrick J. Wong wrote:
> On Wed, Jul 01, 2015 at 11:58:43AM +1000, Dave Chinner wrote:
> > On Thu, Jun 25, 2015 at 04:40:16PM -0700, Darrick J. Wong wrote:
> > > Gate all the reflink functions (which generally involve an expensive
> > > trip to the reflink btree) on an inode flag which is applied to both
> > > inodes at reflink time.  This minimizes reflink's impact on non-CoW
> > > files.
> > 
> > Ah, I see you do this reflink inode flag here. This should be one of
> > the first patches, not the last.  i.e. the patch series should
> > build up all the supporting infrastructure in individual patches
> > before adding any of the actual reflink implementation....
> > 
> > Also, the flag needs to go into the di_flags2 field, as the last
> > flag in the di_flags field is reserved for a "more flags" flag if we
> > ever need to add more flags to a v2 inode in a v4 filesystem...
> 
> It looks to me like di_flags2 only exists in a v3 inode, and v3 inodes
> only exist on v5 filesystems.  I don't really mind using di_flags2 for
> reflink (on the off chance you want to use bit 15 of di_flags for a
> v2 inode) but I'm wondering how is it possible to have di_flags on a v4 fs?

You mean how is it possible to have di_flags2 on a v4 fs?

Internally when inodes are read off disk, they are converted to v3
format in memory. i.e. the struct xfs_icdinode is a v3 format
structure. Hence when reading in v2 inodes, the di_flags2 field is
present in the structure and it gets initialised to zero. When we
format the in-memory inode to disk (in xfs_iflush_int()), we don't
ever write the v3 fields back to the on disk inode structure, and
hence the in-memory value of the di_flags2 field doesn't ever get
written to disk.

So while the various v3 inode fields are always present in the
in-memory inode, if di_version = 2 then the v3 fields will be
initialised to zero on read and will never be written back to
disk...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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