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

Jeff Garzik
2006-06-09T15:40:13Z

Andrew Morton wrote:
> Ted&co have been pretty good at avoiding compatibility problems.

Well, extents and 48bit make that track record demonstrably worse.

Users are now forced to remember that, if they write to their filesystem
after using either $mmver or $korgver kernels, they are locked out of
using older kernels.

From the user's perspective, ext3 has no clear "metadata version 1",
"metadata version 2" division. Thus they are now forced to keep a
matrix of kernel versions and ext3 feature flag support, to know which
kernels are usable with which data. It is a support nightmare.

At no point is a user ever told, in big capital letters, "IF YOU WRITE
TO THIS FILESYSTEM, YOU CAN'T BOOT OLDER KERNELS." There is no "click
OK to continue with this dramatic event."

And as features continue to be added in this manner, this problem gets
_exponentially_ worse.

On the project management side of things, I see no indication that this
momentum slow -- which implies to me that people will keep slapping new
stuff into ext3, rather than directing energy towards a newer, cleaner
ext-NG filesystem.

Dragging around back-compat really constrains freedom, and you have to
have some sort of "pressure relief valve" (a massive, wildly
incompatible update) eventually.

In my mind, it's analagous to locking developers into developing and
deploying new features into a stable branch of software. The hacks just
get worse and worse, as you bend over backwards for back-compat.

Jeff