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Skip the boot balance when it cannot free a chunk #4

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Skip the boot balance when it cannot free a chunk #4

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5da5e31
Skip the boot balance when it cannot free a chunk
DenisCDev Oct 27, 2026
a754bd8
Use the real chunk size on filesystems under 10 GiB
DenisCDev Oct 27, 2026
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37 changes: 35 additions & 2 deletions auto-rules-io/usr/bin/big-btrfs-balance
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Original file line number Diff line number Diff line change
Expand Up @@ -7,12 +7,45 @@ FolderRoot=$(btrfs device stats / | tail -n1 | cut -f1 -d] | cut -f2 -d[)

FolderHome=$(btrfs device stats /home | tail -n1 | cut -f1 -d] | cut -f2 -d[)

# A usage filter also picks the last, partly filled chunk that the previous
# balance just packed, so a filesystem that changed little since then has the
# same data rewritten on every boot and frees nothing. Freeing a chunk needs
# at least a chunk of allocated-but-unused space. With single, DUP or RAID1
# profiles, data chunks are at most 1 GiB, and the kernel reports the
# metadata chunk size, which is at least 256 MiB (its data chunk size there
# is only the upper limit). On smaller filesystems both are 10% of the size,
# rounded up to 16 MiB.
balance() {
local type slack size tenth data_chunk=1073741824 meta_chunk filters=()
meta_chunk=$(cat "/sys/fs/btrfs/$(findmnt -no UUID "$1")/allocation/metadata/chunk_size" 2>/dev/null)
meta_chunk=${meta_chunk:-268435456}
size=$(btrfs filesystem usage -b "$1" 2>/dev/null | awk '/Device size:/ {print $3; exit}')
if (( size )); then
tenth=$(( (size / 10 + 16777215) / 16777216 * 16777216 ))
(( tenth < data_chunk )) && data_chunk=$tenth
(( tenth < meta_chunk )) && meta_chunk=$tenth
fi
while read -r type slack; do
case $type in
Data) (( slack >= data_chunk )) && filters+=(-dusage=50) ;;
Metadata) (( slack >= meta_chunk )) && filters+=(-musage=50) ;;
Data+Metadata) filters+=(-dusage=50 -musage=50) ;;
esac
done < <(btrfs filesystem df -b "$1" | awk -F'[ ,=]+' '$1 == "Data" || $1 == "Metadata" || $1 == "Data+Metadata" {
for (i = 2; i < NF; i++) { if ($i == "total") t[$1] += $(i+1); if ($i == "used") u[$1] += $(i+1) }
} END { for (k in t) print k, t[k] - u[k] }')

if (( ${#filters[@]} )); then
nice -n+20 ionice -c 3 btrfs balance start "${filters[@]}" "$1"
fi
}

if [ "$FolderRoot" != "" ]; then
nice -n+20 ionice -c 3 btrfs balance start -dusage=50 -musage=50 /
balance /
fi

if [ "$FolderHome" != "" ] && [ "$FolderHome" != "$FolderRoot" ]; then
nice -n+20 ionice -c 3 btrfs balance start -dusage=50 -musage=50 /home
balance /home
fi