Commits on May 31, 2021

1. nixos/lib/make-ext4-fs: Fix: `resize2fs -M' can leave insufficient slack …

   The root filesystem resizing step, `resize2fs -M', does not provide any
   control over the amount of slack left in the result.  It can produce an
   arbitrarily tight fit, depending on how well the payload aligns with
   ext4 data structures.
   
   This is problematic, as NixOS must create a few files and directories
   during its first boot, before the root is enlarged to match the size of
   the containing SD card.
   
   An overly tight fit can cause failures in the first stage:
   
       mkdir: can't create directory '/mnt-root/proc': No space left on device
   
   or in the second stage:
   
       install: cannot create directory '/var': No space left on device
   
   A previous version of `make-ext4-fs' (before PR NixOS#79368) was explicitly
   "reserving" 16 MiB of free space in the final filesystem.  Manually
   calculating the size of an ext4 filesystem is a perilous endeavor,
   however, and the method it employed was apparently unreliable.
   
   Reverting is consequently not a good option.
   
   A solution would be to create some sort of "balloon" occupying inodes
   and blocks in the image prior to invoking `resize2fs -M', and to remove
   these temporary files/directories before the compression step.
   
   This changeset takes the simpler approach of simply dropping the
   resizing step.
   
   Note that this does *not* result in a larger image in general, as the
   current procedure does not truncate the `.img' file anyway.  In fact, it
   has been observed to yield *smaller* compressed images---probably
   because of some "noise" left after resizing.  E.g., before-vs-after:
   
       -r--r--r-- 2 root root 607M  1. Jan 1970  nixos-sd-image-21.11pre-git-x86_64-linux.img.zst
   
       -r--r--r-- 2 root root 606M  1. Jan 1970  nixos-sd-image-21.11pre-git-x86_64-linux.img.zst

   

   Damien Diederen authored and ztzg committed May 31, 2021
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