On Fri, Feb 17, 2023 at 08:51:33AM -0800, Russ Allbery wrote:
> Adrian Bunk <b...@debian.org> writes:
>
> > The image creators could just set the features they enable to what they
> > copied from /etc/mke2fs.conf from the target distribution, a label with
> > a timestamp wouldn'tbring much benefit here.
>
> That's a very good point and I'm embarrassed it wasn't immediately obvious
> to me. Sorry about the noise.
One other thing I woud add here is (a) this whole discussion of
mke2fs.conf only helps for ext4, and the general problem is something
that extends to all file system. The immediate question may be ext4
specific, but as I mentioned earlier, XFS is enabling the "bigtime"
feature for the first time in Bookworm. So enabling what may be
convenient, but ultimately an anti-pattern is something that hopefully
in the long-term Debian should be striving towards. Yes, it's
annoying and and extra work. So is using build chroots if we are
building packages for a older Distro versions. But it's the right
thing to do.
Secondly, (b) there may be a misapprehension that it is possible to
get an identical file system just by controlling the contents of
mke2fs and/or specifying the file system features on the command line.
While this is mostly true, it is not the whole story. For example,
the size and location of the journal is determined hueristically, and
in the past, this has changed as we have discovered that (for example)
making the default journal size larger would result in better
performance. The location of the journal has also changed from the
beginning of storage device (low-numbered LBA's) to the middle of the
device.
So just as a binary compiled using the default gcc on Buster might be
different from a binary build using Sid, even if you you force the use
of older glibc shared libraries at link time or use -static to
statically link with the glibc installed in your random desktop
environment, the results from using mke2fs on Debian N may be
different from using mke2fs on Debian M, even if you control for thea
file system features. (And other things which _are_ controllable on
mke2fs.conf, but which go beyond mke file system features. For
example, in Bookworm starting with e2fsprogs 1.46.4, the default inode
size is forced to always be 256 bytes, even for small file systems.
This was not true in Buster and older Debian distributions.)
So if you want a bootable image that is identical to what would be
created by using the Buster or Bullseye debian-interaller, you
*really* want to use the mkfs that was supplied by Buster or Bullseye,
and that means running the mkfs from a chroot.
Best regards,
- Ted
