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make-disk-image: Account for the ext4 reserved space

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Reserved space includes:

 - inodes space in use (2 blocks per)
 - about 5.2% of the space

The 5.2% reserved space was computed empirically when working on a
previous EXT4 image builder. It seems to stabilize around 5% even for
much larger filesystems.
This commit is contained in:
Samuel Dionne-Riel 2021-04-23 23:18:08 -04:00
parent 05c13a03e2
commit 9b18a78c73
1 changed files with 19 additions and 5 deletions
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24
nixos/lib/make-disk-image.nix
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@ -186,6 +186,13 @@ let format' = format; in let
echo "$acc"
}
# Approximative percentage of reserved space in an ext4 fs over 512MiB.
# 0.05208587646484375
# × 1000, integer part: 52
compute_fudge() {
echo $(( $1 * 52 / 1000 ))
}
mkdir $out
root="$PWD/root"
@ -252,15 +259,22 @@ let format' = format; in let
''}
# Compute required space in filesystem blocks
requiredSpace=$(find . ! -type d -exec 'du' '--apparent-size' '--block-size' "${blockSize}" '{}' ';' | cut -f1 | sum_lines)
# Convert to bytes
requiredSpace=$(( requiredSpace * ${blockSize} ))
diskUsage=$(find . ! -type d -exec 'du' '--apparent-size' '--block-size' "${blockSize}" '{}' ';' | cut -f1 | sum_lines)
# Each inode takes space!
numInodes=$(find . | wc -l)
# Convert to bytes, inodes take two blocks each!
diskUsage=$(( (diskUsage + 2 * numInodes) * ${blockSize} ))
# Then increase the required space to account for the reserved blocks.
fudge=$(compute_fudge $diskUsage)
requiredFilesystemSpace=$(( diskUsage + fudge ))
diskSize=$(( requiredSpace + additionalSpace ))
diskSize=$(( requiredFilesystemSpace + additionalSpace ))
truncate -s "$diskSize" $diskImage
printf "Automatic disk size...\n"
printf " Space needed: %d bytes\n" $requiredSpace
printf " Closure space use: %d bytes\n" $diskUsage
printf " fudge: %d bytes\n" $fudge
printf " Filesystem size needed: %d bytes\n" $requiredFilesystemSpace
printf " Additional space: %d bytes\n" $additionalSpace
printf " Disk image size: %d bytes\n" $diskSize
'' else ''