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SubjectRe: Ext4 and the "30 second window of death"
On Thu, Apr 02, 2009 at 07:38:06PM -0400, Theodore Tso wrote:

> What's been frustrating about this whole controversy is this implicit
> assumptions that users and applications should never change, and the
> filesystem should magically accomodate and Do The Right Thing.

This is the attitude that I have a significant problem with. Filesystems
exist to serve applications. Without applications, there's no reason to
have a filesystem. If a filesystem doesn't provide the behaviour that
applications want then that filesystem has no reason to exist. The aim
isn't to produce a platonically ideal filesystem. The aim is to produce
a filesystem that behaves well given the applications that use it.

Disagreeing with the behaviour of applications is a perfectly sensible
thing to do. However, it's something that should be done at the *start*
of a filesystem development cycle. Getting agreement from a broad
section of application developers means that you get to write a
filesystem that embodies a different set of assumptions and everyone
wins. Writing a filesystem and then bitching about application behaviour
after it's been merged to mainline is just pathological.

> The problem is, this is what the application programmers are telling
> the filesystem developers. They refuse to change their programs; and
> the features they want are sometimes mutually contradictory, or at
> least result in a overconstrained problem --- and then they throw the
> whole mess at the filesystem developers' feet and say, "you fix it!"

Which application developers did you speak to? Because, frankly, the
majority of the ones I know felt that ext3 embodied the pony that they'd
always dreamed of as a five year old. Stephen gave them that pony almost
a decade ago and now you're trying to take it to the glue factory. I
remember almost crying at that bit on Animal Farm, so I'm really not
surprised that you're getting pushback here.

> I'm not saying the filesystems are blameless, but give us a little
> slack, guys; we NEED some help from the application developers here.

Then having a discussion with application developers over the
expectations they can have would be a good first step. Just pointing at
POSIX isn't good enough - POSIX allows a bunch of behaviours
sufficiently pathological that a filesystem implementing them would be
less useful than /dev/null. We need to have a worthwhile conversation
about what guarantees Linux will provide above and beyond POSIX. The
filesystem summit next week isn't going to be that conversation. Perhaps
something to try at Plumbers?

Matthew Garrett |

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