Re: Re: Reviewing ext3 improvement patches (delalloc, mballoc, extents)

[Theodore Ts'o <tytso@mit.edu>]

On Thu, Mar 03, 2005 at 02:40:21AM -0700, Andreas Dilger wrote:
> 
> Well, for a starter, the extents format changes are not forced on
> users, only if they mount with "-o extents" and write files will
> it mark the superblock incompatible and start allocating files
> this way.  I believe (though I have never tested) that even if
> extents are enabled, writes to a block-mapped file will continue
> to work and that file will not be converted to an extent file.
> 

I was about to start a new thread discussing this, but you started
commenting about it here, so I'll address it here.

The way most of the other ext3 extensions that involve feature changes
work is that you enable them by using tune2fs (for example, "tune2fs
-O dir_index /dev/hdaXX"), instead of using a mount option which then
causes the kernel to automatically flip on an feature incompatble
flag.  It would be good if the extents were changed to follow this
convention for the following reasons:

	1) Consistency is less confusing for users, and features like
	   dir_index, has_journal work this way already.

	2) Long term, you don't want users to have to specify -o
	   extents as a mount option, or specify "extents" in
	   /etc/fstab in order to make the filesystem work correctly.
	   From an initial examination of the extents code won't be
	   initialized properly if you don't specify -o extents.  So
	   I'm pretty sure that if you try to mount a filesystem that
	   has extents, but forget to specify -o extents, things will
	   break in an entertaining fashion.  I haven't yet tried it
	   yet, though.

	3) It means that users who are fooling around with the patch
	   will have at least some kind of patched userspace first (or
	   understand how to use debugfs to set the feature manually).
	   This makes it less likely that they will apply the patch,
	   or get it applied for free when they use the -mm tree (once
	   the patches get accepted by Andrew), and then when they
	   specify -o extents, all of sudden the filesystem becomes
	   incompatible and no longer be mountable using standard
	   tools.

This is a bit of a pet peeve of mine, since I am still getting
personal e-mail from people who were using Red Hat 7, and decided to
just "try out" Fedora Core 3, only to find that any filesystems
mounted by FC3 could no longer be mountable on their RH7 or RH8
systems.  That's why I prefer users to have to do something that
obviously acknowledges that they are making a change to their
filesystem's compatibility prospects --- and that's something which is
a lot more obvious with a "tune2fs -O " command, as compared to using
a magic mount option. 

						- Ted


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