[RFC] Btrfs mainline plans

[Chris Mason-4]

Hello everyone,

Thanks to help from David Woodhouse and Linus, I have btrfs source repos
migrated over to git.  My goal is to make the sources easier for people
to review, and to start some discussions around the best time to merge
Btrfs.

The current code has a number of problems (and this is not a complete
list):

* The disk format is not finalized
* ENOSPC can result in BUG()
* blocksize != pagesize does not work
* Error handling is missing in a number of places

But, the code is very actively developed, and I believe the best way to
develop Btrfs from here is to get it into the mainline kernel (with a
large warning label about the disk format) and attract more extensive
review of both the disk format and underlying code.

The Btrfs developers are committed to making the FS work and to working
well within the kernel community.  I think everyone will be happier with
the final result if I am able to attract eyeballs as early as possible.

The plan is to have the disk format finalized by the end of the year.  I
do expect to have a provisional disk format over the next 2 weeks that
should have all the moving pieces btrfs needs to implement the remaining
1.0 features, and I hope to restrict any additional format changes after
that to include backward compatibility.  

The sources:

Kernel module (against rc7):
http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-unstable.git;a=summary


Btrfs progs:

http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-progs-unstable.git;a=summary

General Btrfs information:

http://btrfs.wiki.kernel.org/

Most of the remaining features before 1.0:

http://btrfs.wiki.kernel.org/index.php/Development_timeline

-chris


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[Andrew Morton-9]

On Mon, 29 Sep 2008 15:44:20 -0400 Chris Mason <chris.mason@...> wrote:

> But, the code is very actively developed, and I believe the best way to
> develop Btrfs from here is to get it into the mainline kernel (with a
> large warning label about the disk format) and attract more extensive
> review of both the disk format and underlying code.

For the record...  I have been encouraging Chris to get btrfs into
mainline soon.  Get it into linux-next asap and merge it into 2.6.29.

And do this even though the on-disk format is still changing - we emit a
loud printk at mount time and if someone comes to depend upon some
intermediate format, well, that's their tough luck.

My thinking here is that btrfs probably has a future, and that an early
merge will accelerate its development and will broaden its developer base.
If it ends up failing for some reason, well, we can just delete it
again.


For various reasons this approach often isn't appropriate as a general
policy thing, but I do think that Linux has needed a new local
filesystem for some time, and btrfs might be The One, and hence is
worth a bit of special-case treatment.

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[Jan Engelhardt-3]

On Monday 2008-09-29 15:44, Chris Mason wrote:
>[...]
>But, the code is very actively developed, and I believe the best way to
>develop Btrfs from here is to get it into the mainline kernel (with a
>large warning label about the disk format) and attract more extensive
>review of both the disk format and underlying code.

Definitely. While, due to the changing disk format, long-term storage
is not really an option yet, the filesystem can be really useful in
network-booted hybrid clients that have a local disk in an unionfs
compound mount as a COW rw storage device (because RAM *is* limited
after all). Keep up the work.


Jan
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[Adrian Bunk-3]

On Fri, Oct 03, 2008 at 12:18:59AM -0700, Andrew Morton wrote:

> On Mon, 29 Sep 2008 15:44:20 -0400 Chris Mason <chris.mason@...> wrote:
>
> > But, the code is very actively developed, and I believe the best way to
> > develop Btrfs from here is to get it into the mainline kernel (with a
> > large warning label about the disk format) and attract more extensive
> > review of both the disk format and underlying code.
>
> For the record...  I have been encouraging Chris to get btrfs into
> mainline soon.  Get it into linux-next asap and merge it into 2.6.29.
>
> And do this even though the on-disk format is still changing - we emit a
> loud printk at mount time and if someone comes to depend upon some
> intermediate format, well, that's their tough luck.
>
> My thinking here is that btrfs probably has a future, and that an early
> merge will accelerate its development and will broaden its developer base.
> If it ends up failing for some reason, well, we can just delete it
> again.
>
> For various reasons this approach often isn't appropriate as a general
> policy thing, but I do think that Linux has needed a new local
> filesystem for some time, and btrfs might be The One, and hence is
> worth a bit of special-case treatment.

Let's try to learn from the past:

6 days from today ext4 (another new local filesystem for Linux)
celebrates the second birthday of it's inclusion into Linus' tree
as a similar special-case.

You claim "an early merge will accelerate its development and will
broaden its developer base" for Btrfs.

Read the timeline Ted outlined back in June 2006 for ext4 [1].
When comparing with what happened in reality it kinda disproves
your "acceleration" point.

cu
Adrian

[1] http://lkml.org/lkml/2006/6/28/454

--

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        of the darkness. There had been need of rain for many days.
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[Serge E. Hallyn]

Quoting Adrian Bunk (bunk@...):

> On Fri, Oct 03, 2008 at 12:18:59AM -0700, Andrew Morton wrote:
> > On Mon, 29 Sep 2008 15:44:20 -0400 Chris Mason <chris.mason@...> wrote:
> >
> > > But, the code is very actively developed, and I believe the best way to
> > > develop Btrfs from here is to get it into the mainline kernel (with a
> > > large warning label about the disk format) and attract more extensive
> > > review of both the disk format and underlying code.
> >
> > For the record...  I have been encouraging Chris to get btrfs into
> > mainline soon.  Get it into linux-next asap and merge it into 2.6.29.
> >
> > And do this even though the on-disk format is still changing - we emit a
> > loud printk at mount time and if someone comes to depend upon some
> > intermediate format, well, that's their tough luck.
> >
> > My thinking here is that btrfs probably has a future, and that an early
> > merge will accelerate its development and will broaden its developer base.
> > If it ends up failing for some reason, well, we can just delete it
> > again.
> >
> > For various reasons this approach often isn't appropriate as a general
> > policy thing, but I do think that Linux has needed a new local
> > filesystem for some time, and btrfs might be The One, and hence is
> > worth a bit of special-case treatment.
>
> Let's try to learn from the past:
>
> 6 days from today ext4 (another new local filesystem for Linux)
> celebrates the second birthday of it's inclusion into Linus' tree
> as a similar special-case.
>
> You claim "an early merge will accelerate its development and will
> broaden its developer base" for Btrfs.
>
> Read the timeline Ted outlined back in June 2006 for ext4 [1].
> When comparing with what happened in reality it kinda disproves
> your "acceleration" point.

OTOH, maybe it's just me, but I think there is more excitement around
btrfs.  Myself I'm dying for snapshot support, and can't wait to try
btrfs on a separate data/scratch partition (where i don't mind losing
data).  btrfs and nilfs - yay.  Ext4?  <yawn>  That can make all the
difference.

-serge
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[Adrian Bunk-3]

On Sun, Oct 05, 2008 at 09:11:13AM -0500, Serge E. Hallyn wrote:

> Quoting Adrian Bunk (bunk@...):
> > On Fri, Oct 03, 2008 at 12:18:59AM -0700, Andrew Morton wrote:
> > > On Mon, 29 Sep 2008 15:44:20 -0400 Chris Mason <chris.mason@...> wrote:
> > >
> > > > But, the code is very actively developed, and I believe the best way to
> > > > develop Btrfs from here is to get it into the mainline kernel (with a
> > > > large warning label about the disk format) and attract more extensive
> > > > review of both the disk format and underlying code.
> > >
> > > For the record...  I have been encouraging Chris to get btrfs into
> > > mainline soon.  Get it into linux-next asap and merge it into 2.6.29.
> > >
> > > And do this even though the on-disk format is still changing - we emit a
> > > loud printk at mount time and if someone comes to depend upon some
> > > intermediate format, well, that's their tough luck.
> > >
> > > My thinking here is that btrfs probably has a future, and that an early
> > > merge will accelerate its development and will broaden its developer base.
> > > If it ends up failing for some reason, well, we can just delete it
> > > again.
> > >
> > > For various reasons this approach often isn't appropriate as a general
> > > policy thing, but I do think that Linux has needed a new local
> > > filesystem for some time, and btrfs might be The One, and hence is
> > > worth a bit of special-case treatment.
> >
> > Let's try to learn from the past:
> >
> > 6 days from today ext4 (another new local filesystem for Linux)
> > celebrates the second birthday of it's inclusion into Linus' tree
> > as a similar special-case.
> >
> > You claim "an early merge will accelerate its development and will
> > broaden its developer base" for Btrfs.
> >
> > Read the timeline Ted outlined back in June 2006 for ext4 [1].
> > When comparing with what happened in reality it kinda disproves
> > your "acceleration" point.
>
> OTOH, maybe it's just me, but I think there is more excitement around
> btrfs.  Myself I'm dying for snapshot support, and can't wait to try
> btrfs on a separate data/scratch partition (where i don't mind losing
> data).  btrfs and nilfs - yay.  Ext4?  <yawn>  That can make all the
> difference.

"accelerate its development and will broaden its developer base" is not
about users/testers but about people doing code development.

For people wanting to try WIP code you don't need it in mainline.

Stable kernels will anyway usually contain months old code of the
WIP filesystem that is not usable for testing, so for any meaningful
testing you will still have to follow the btrfs tree and not mainline.

This is not meant as a statement on the quality of ext4 or btrfs, or any
comparison of the development times of ext4 and btrfs, but for ext4 the
advantages Andrew thinks would happen with an early btrfs merge do not
seem to have happened.

I just realize that I forgot to add Ted and the ext4 mailing list into
the Cc of my first email. Adding them to the Cc, so if I'm talking
nonsense about the experiences with ext4 they can correct me.

> -serge

cu
Adrian

--

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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[Chris Mason-4]

On Sun, 2008-10-05 at 18:09 +0300, Adrian Bunk wrote:
> On Sun, Oct 05, 2008 at 09:11:13AM -0500, Serge E. Hallyn wrote:
> > Quoting Adrian Bunk (bunk@...):
> >

[ when to merge btrfs ]

> > > Let's try to learn from the past:
> > >
> > > 6 days from today ext4 (another new local filesystem for Linux)
> > > celebrates the second birthday of it's inclusion into Linus' tree
> > > as a similar special-case.
> > >
> > > You claim "an early merge will accelerate its development and will
> > > broaden its developer base" for Btrfs.
> > >
> > > Read the timeline Ted outlined back in June 2006 for ext4 [1].
> > > When comparing with what happened in reality it kinda disproves
> > > your "acceleration" point.
> >

The btrfs timelines have always been aggressive, and as btrfs gets
closer to feature complete, the testing matrix grows dramatically.  I
can't promise my crazy timelines won't slip, but I've been hacking away
in the basement for almost 18 months now and it's time for me to get off
the pot and make it stable.

Ext4 has always had to deal with the ghost of ext3.  Both from a
compatibility point of view and everyone's expectations of stability.  I
believe that most of us underestimated how difficult it would be to move
ext4 forward.

Btrfs is different for lots of reasons, and being in mainline will
definitely increase the pressure on the btrfs developers to finish, and
the resources available for us to finish with.

> > OTOH, maybe it's just me, but I think there is more excitement around
> > btrfs.  Myself I'm dying for snapshot support, and can't wait to try
> > btrfs on a separate data/scratch partition (where i don't mind losing
> > data).  btrfs and nilfs - yay.  Ext4?  <yawn>  That can make all the
> > difference.
>
> "accelerate its development and will broaden its developer base" is not
> about users/testers but about people doing code development.
>

People want btrfs for different reasons.  I want btrfs in the kernel
because when you're in the kernel more people look at it, and when
people look at it they send me email with the mistakes they found.

For example, see the streaming write patches I sent to fsdevel last
week.  I wouldn't test against ext4 as often if I had to hunt down
external repos just to get something consistent with the current
development kernels.  ext4 in mainline makes it much easier for me to
kick the tires.

> For people wanting to try WIP code you don't need it in mainline.
>
> Stable kernels will anyway usually contain months old code of the
> WIP filesystem that is not usable for testing, so for any meaningful
> testing you will still have to follow the btrfs tree and not mainline.

For ext4 at least, the mainline code is very usable.  I hope to have
btrfs in shape for that by the 2.6.29 merge cycle.

-chris


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[Adrian Bunk-3]

On Mon, Oct 06, 2008 at 09:40:03AM -0400, Chris Mason wrote:

> On Sun, 2008-10-05 at 18:09 +0300, Adrian Bunk wrote:
> > On Sun, Oct 05, 2008 at 09:11:13AM -0500, Serge E. Hallyn wrote:
> > > Quoting Adrian Bunk (bunk@...):
> > >
>
> [ when to merge btrfs ]
>
> > > > Let's try to learn from the past:
> > > >
> > > > 6 days from today ext4 (another new local filesystem for Linux)
> > > > celebrates the second birthday of it's inclusion into Linus' tree
> > > > as a similar special-case.
> > > >
> > > > You claim "an early merge will accelerate its development and will
> > > > broaden its developer base" for Btrfs.
> > > >
> > > > Read the timeline Ted outlined back in June 2006 for ext4 [1].
> > > > When comparing with what happened in reality it kinda disproves
> > > > your "acceleration" point.
> > >
>
> The btrfs timelines have always been aggressive, and as btrfs gets
> closer to feature complete, the testing matrix grows dramatically.  I
> can't promise my crazy timelines won't slip, but I've been hacking away
> in the basement for almost 18 months now and it's time for me to get off
> the pot and make it stable.
>
> Ext4 has always had to deal with the ghost of ext3.  Both from a
> compatibility point of view and everyone's expectations of stability.  I
> believe that most of us underestimated how difficult it would be to move
> ext4 forward.
>
> Btrfs is different for lots of reasons, and being in mainline will
> definitely increase the pressure on the btrfs developers to finish, and
> the resources available for us to finish with.

Your last sentence does not make sense:

According to your timeline btrfs 1.0 will be released in Q408 [1] - and
the merge window for 2.6.29 will be in Q109.

>...
> > For people wanting to try WIP code you don't need it in mainline.
> >
> > Stable kernels will anyway usually contain months old code of the
> > WIP filesystem that is not usable for testing, so for any meaningful
> > testing you will still have to follow the btrfs tree and not mainline.
>
> For ext4 at least, the mainline code is very usable.  I hope to have
> btrfs in shape for that by the 2.6.29 merge cycle.

One risk you should be aware of is that when btrfs is in 2.6.29 part of
the Linux press might pick it up and stress test and benchmark this new
filesystem.

JFS still suffers from from not being that good when it was
initially merged.

> -chris

cu
Adrian

[1] http://btrfs.wiki.kernel.org/index.php/Development_timeline

--

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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[Chris Mason-4]

On Tue, 2008-10-07 at 18:27 +0300, Adrian Bunk wrote:

> On Mon, Oct 06, 2008 at 09:40:03AM -0400, Chris Mason wrote:
>  
> >
> > The btrfs timelines have always been aggressive, and as btrfs gets
> > closer to feature complete, the testing matrix grows dramatically.  I
> > can't promise my crazy timelines won't slip, but I've been hacking away
> > in the basement for almost 18 months now and it's time for me to get off
> > the pot and make it stable.
> >
> > Ext4 has always had to deal with the ghost of ext3.  Both from a
> > compatibility point of view and everyone's expectations of stability.  I
> > believe that most of us underestimated how difficult it would be to move
> > ext4 forward.
> >
> > Btrfs is different for lots of reasons, and being in mainline will
> > definitely increase the pressure on the btrfs developers to finish, and
> > the resources available for us to finish with.
>
> Your last sentence does not make sense:
>
> According to your timeline btrfs 1.0 will be released in Q408 [1] - and
> the merge window for 2.6.29 will be in Q109.
>

Planning for mainline inclusion is always a guessing game.  Cutting 1.0
is different from being in mainline, and the dates don't have to be the
same.

> >...
> > > For people wanting to try WIP code you don't need it in mainline.
> > >
> > > Stable kernels will anyway usually contain months old code of the
> > > WIP filesystem that is not usable for testing, so for any meaningful
> > > testing you will still have to follow the btrfs tree and not mainline.
> >
> > For ext4 at least, the mainline code is very usable.  I hope to have
> > btrfs in shape for that by the 2.6.29 merge cycle.
>
> One risk you should be aware of is that when btrfs is in 2.6.29 part of
> the Linux press might pick it up and stress test and benchmark this new
> filesystem.

I think the gains from early testing far outweigh the risks of bad early
press.

-chris


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[Adrian Bunk-3]

On Tue, Oct 07, 2008 at 12:01:24PM -0400, Chris Mason wrote:

> On Tue, 2008-10-07 at 18:27 +0300, Adrian Bunk wrote:
> > On Mon, Oct 06, 2008 at 09:40:03AM -0400, Chris Mason wrote:
> >  
> > >
> > > The btrfs timelines have always been aggressive, and as btrfs gets
> > > closer to feature complete, the testing matrix grows dramatically.  I
> > > can't promise my crazy timelines won't slip, but I've been hacking away
> > > in the basement for almost 18 months now and it's time for me to get off
> > > the pot and make it stable.
> > >
> > > Ext4 has always had to deal with the ghost of ext3.  Both from a
> > > compatibility point of view and everyone's expectations of stability.  I
> > > believe that most of us underestimated how difficult it would be to move
> > > ext4 forward.
> > >
> > > Btrfs is different for lots of reasons, and being in mainline will
> > > definitely increase the pressure on the btrfs developers to finish, and
> > > the resources available for us to finish with.
> >
> > Your last sentence does not make sense:
> >
> > According to your timeline btrfs 1.0 will be released in Q408 [1] - and
> > the merge window for 2.6.29 will be in Q109.
>
> Planning for mainline inclusion is always a guessing game.

The 2.6.29 merge window will start in January - everything else is much
more unlikely than an incompetent chick from Alaska getting only one
heart attack away from the big red button.

> Cutting 1.0
> is different from being in mainline, and the dates don't have to be the
> same.

Sure, but Andrew's "special-case treatment" suggestion does not apply if
1.0 gets released before the 2.6.29 merge window.

>...
> -chris

cu
Adrian

--

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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[David Sanders-6]

On Tuesday 07 October 2008 04:25:39 pm Adrian Bunk wrote:
> The 2.6.29 merge window will start in January - everything else is much
> more unlikely than an incompetent chick from Alaska getting only one
> heart attack away from the big red button.

These comments are inappropriate for this list.
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[Randy Dunlap-5]

On Tue, 7 Oct 2008, David Sanders wrote:

> On Tuesday 07 October 2008 04:25:39 pm Adrian Bunk wrote:
> > The 2.6.29 merge window will start in January - everything else is much
> > more unlikely than an incompetent chick from Alaska getting only one
> > heart attack away from the big red button.
>
> These comments are inappropriate for this list.

and you dropped several mailing list cc's.  Please don't do that.

--
~Randy
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[Christoph Hellwig]

On Tue, Oct 07, 2008 at 05:20:02PM -0400, David Sanders wrote:
> On Tuesday 07 October 2008 04:25:39 pm Adrian Bunk wrote:
> > The 2.6.29 merge window will start in January - everything else is much
> > more unlikely than an incompetent chick from Alaska getting only one
> > heart attack away from the big red button.
>
> These comments are inappropriate for this list.

Why?

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[David Sanders-6]

On Wednesday 08 October 2008 08:53:56 am Christoph Hellwig wrote:
> On Tue, Oct 07, 2008 at 05:20:02PM -0400, David Sanders wrote:
> > On Tuesday 07 October 2008 04:25:39 pm Adrian Bunk wrote:
> > > The 2.6.29 merge window will start in January - everything else is much
> > > more unlikely than an incompetent chick from Alaska getting only one
> > > heart attack away from the big red button.
> >
> > These comments are inappropriate for this list.
>
> Why?

I was talking about the Alaska chick thing.  The comment is wrong, but I
didn't object to his politics, just the fact it was on the kernel developer's
list.  The comments about the merge window are of course on topic.
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[Daniel Phillips-7]

On Sunday 05 October 2008 08:09, Adrian Bunk wrote:
> "accelerate its development and will broaden its developer base" is not
> about users/testers but about people doing code development.
>
> For people wanting to try WIP code you don't need it in mainline.

But it saves time for the user, who does not have to run around chasing
links, carefully checking for a kernel match, downloading, patching,
building and installing a single purpose kernel, and bringing it up on
a machine that would probably have only required one click on the new
filesystem option otherwise.  The considerable time thus saved can be
invested profitably in running test cases and filing bug reports.
 
> Stable kernels will anyway usually contain months old code of the
> WIP filesystem that is not usable for testing, so for any meaningful
> testing you will still have to follow the btrfs tree and not mainline.

True, but the trick here is getting started.  It is much easier to
justify the effort of going out and getting the latest patch if one
knows from experience that it basically already works.

> This is not meant as a statement on the quality of ext4 or btrfs, or any
> comparison of the development times of ext4 and btrfs, but for ext4 the
> advantages Andrew thinks would happen with an early btrfs merge do not
> seem to have happened.

Are you sure about that?  I see 33 messages on linux-ext4 yesterday,
from a broad range of contributors.  Versus eight from a much narrower
range of contributors, Oct 4 a year ago.

There is little question that an early merge helps both developers and
users employ their time more efficiently, once a project is past the
point where we wonder about its value and/or viability.  In my opinion,
Btrfs clearly has both.  Particularly because we need a way to stem the
loss of mindshare to ZFS in the storage space, which is significant at
the moment.  And Btrfs is closest to the finish line in that regard.
It needs all the help it can get.

Regards,

Daniel
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[Adrian Bunk-3]

On Wed, Oct 08, 2008 at 02:33:32PM -0700, Daniel Phillips wrote:

> On Sunday 05 October 2008 08:09, Adrian Bunk wrote:
> > "accelerate its development and will broaden its developer base" is not
> > about users/testers but about people doing code development.
> >
> > For people wanting to try WIP code you don't need it in mainline.
>
> But it saves time for the user, who does not have to run around chasing
> links, carefully checking for a kernel match, downloading, patching,
> building and installing a single purpose kernel, and bringing it up on
> a machine that would probably have only required one click on the new
> filesystem option otherwise.  The considerable time thus saved can be
> invested profitably in running test cases and filing bug reports.

Bug reports against a 3-6 months old snapshot of a filesystem being
under heavy development.

Ted said back in August in the announcement of an ext4 patchset:

"As before I've also released updated the patch set vs. the 2.6.26 stock
 kernel, for those people who don't want to play with development
 kernels but who still want to test out ext4." [1]

When running stable kernels you still have to patch, build and install
a single purpose kernel for testing ext4 although ext4 is in mainline.

>...
> > This is not meant as a statement on the quality of ext4 or btrfs, or any
> > comparison of the development times of ext4 and btrfs, but for ext4 the
> > advantages Andrew thinks would happen with an early btrfs merge do not
> > seem to have happened.
>
> Are you sure about that?  I see 33 messages on linux-ext4 yesterday,
> from a broad range of contributors.  Versus eight from a much narrower
> range of contributors, Oct 4 a year ago.

There are lies, damn lies, and statistics.  ;)

Single day statistics about mailing list postings are not very good
indicators for anything.

And since linux-ext4 is for all of ext2/ext3/ext4 the data you gave
could equally be used to prove that ext3 recently became much more
buggy or that ext2 development vastly increased...

> There is little question that an early merge helps both developers and
> users employ their time more efficiently,

Regarding users see my comment above.

Regarding developers it would be interesting to hear some experiences
from ext4 developers about their experiences (or get a pointer to them
in case I missed that they already expressed it somewhere).

> once a project is past the
> point where we wonder about its value and/or viability.  In my opinion,
> Btrfs clearly has both.
>...

2 years ago ext4 was in a similar situation of being regarded as an
important future filesystem.

> Regards,
>
> Daniel

cu
Adrian

BTW: My comments are not in any way meant against btrfs or ext4.
     I just question the advantages of merging them early.


[1] http://lwn.net/Articles/294784/

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                                       Pearl S. Buck - Dragon Seed

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