Re: [Ext2-devel] [RFC 0/13] extents and 48bit ext3

[Nicholas Miell]

On Fri, 2006-06-09 at 20:09 -0600, Andreas Dilger wrote:
> On Jun 09, 2006  21:21 -0400, Valdis.Kletnieks@xxxxxx wrote:
> > On Fri, 09 Jun 2006 17:21:08 MDT, Andreas Dilger said:
> > > You mount with the new kernel without "-o extents", and find files with
> > > extents "lsattr -R /mnt/tmp | awk '/----e / print { $2 }'", copy those
> > > files, mv over old files, unmount.
> > 
> > How do you "copy those files" when you don't have extent support at that
> > point?  Remember - the whole problem here is that if you don't have
> > extent support, you can't read the file, it's backward-incompatible.
> > (If you *are* able to read the file even without extents, then this whole
> > thread is total BS).
> 
> The "-o extents" mount option only affects new files that are created
> while that option is enabled.  It doesn't affect existing files (even if
> they are modified while "-o extents" is set).  It also doesn't affect any
> new files after "-o extents" is removed.  Also, directories will not
> be extent-mapped, because their allocation pattern doesn't mix well with
> extent-mapped files (i.e. they are mostly single-block allocations).
> 
> Files that are created with "-o extents" are of course only readable with
> a kernel that supports it.  To be safe, the whole filesystem is marked
> with an EXT3_FEATURE_INCOMPAT_EXTENTS flag when the first extent file
> is created so that users don't inadvertently get strange errors while
> accessing the inodes marked with EXT3_EXTENT_FL with an old kernel.
> New kernels that understand INCOMPAT_EXTENTS of course can access extent
> and non-extent files equally well.
> 
> In an emergency it would also be possible to remove the INCOMPAT_EXTENTS
> filesystem flag and access all of the non-extent files, but this would
> risk filesystem corruption if any of the extent files were modified or
> unlinked, as that is the only indication older kernels have of this change.
> 
> So, to answer your question, if you _really_ want to get rid of extents
> on a filesystem, you mount the filesystem with INCOMPAT_EXTENTS on a new
> kernel that supports extents, but without -o extents so new files will
> use the old block-map layout, so if "orig-file" is an extent-mapped file:
> 
> 	cp /mnt/tmp/orig-file /mnt/tmp/temp-block-mapped-file
> 	mv /mnt/tmp/temp-block-mapped-file /mnt/tmp/orig-file
> 
> and now /mnt/tmp/orig-file is no longer extent-mapped.  Do this for all
> the extent-mapped files, unmount, use "debugfs -w -R 'feature ^extents' {dev}"
> and your filesystem is mountable with any old kernel.
> 
> No, it's not quite as easy as ext3 journal recovery->ext2 mounting,
> but then again "-o extents" isn't something that happens automatically
> (at least not for a couple of years, and hopefully distros will be smart
> enough never to do this for filesystems like /boot or / that are critical
> for mounting on a wide variety of kernels.  Besides which, we don't want
> to have to teach GRUB about extent-mapped files.  Concievably, if this
> becomes an issue then it should be possible to add a flag to inodes and
> parent directories to add a "no extents" flag that is inherited by new
> files that should never be extent mapped.
> 
> Cheers, Andreas
> --
> Andreas Dilger
> Principal Software Engineer
> Cluster File Systems, Inc.


I think changing all of this mess to:

[root@localhost root]# tune2fs -O extents /dev/whatever
WARNING: Enabling extents on /dev/whatever will make this filesystem
unreadable in Linux kernels versions before 2.6.19!
Are you sure you want to do this? <y/n>

[root@localhost root]# tune2fs -O ^extents /dev/whatever
WARNING: Disabling extents on /dev/whatever requires you to run e2fsck
on this filesystem before it can be used again!
Are you sure you want to do this? <y/n>

might assuage many of the fears presented in this thread.
-- 
Nicholas Miell <nmiell@xxxxxxxxxxx>

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