A software image is sized from the 4 KiB blocks its tree takes, with an inode table sized to the tree

[DevomB]

A software image is sized from the 4 KiB blocks its tree takes, with an inode table sized to the tree, so an image of many small files fits.

pack_image sized an image from du -sb, the bytes in its tree, plus an eighth and 16 MiB. Since every image is ext4 in 4096-byte blocks (#531), each file takes whole blocks. A tree of 100,000 one-KiB files counts as 100 MiB but needs 390 MiB of blocks, and mke2fs fails with no room. On top of that, mke2fs.conf's small type gives a file system under 512 MiB one inode per 4 KiB: 6.25 % of the image is inode table that a read-only image never fills.

• The size counts blocks:
  • each file rounded up to 4 KiB;
  • each directory one block at least (no inline data);
  • each symlink of 60 bytes or more one block;
  • then a sixteenth for extent and directory index blocks, 256 bytes an inode, and 16 MiB for the bitmaps, the group descriptors and their copies.
• make_ext4 takes the inode count as an optional sixth argument (-N): the tree's entries plus an eighth plus 64. The root image passes none, and keeps mke2fs.conf's ratio, as before.
• Hard links count once per name, and sparse files at their full length. Both overcount, which only leaves room.

Checked: bash -n. The awk was run on a sample of a file of 100 and one of 5000 bytes, directories of 4096, 0 and 12288 bytes, symlinks of 20 and 80 bytes and an empty file: 9 blocks, 8 entries. The images job's own checks still hold: two mke2fs runs give the same image, and the block size is 4096.

The images' own fingerprints (image_inputs in 04-images.sh) do not cover 06-iso.sh, so a cached image build stays current; stage 06 packs anew. Not urgent: today's images fit.