Tuning HDD performance options in Linux

README.md

For long batches of unattended work performed by a single process, HDD access scheduling can be optimized to let the Linux kernel and the disk's driver save some HDD head movements when not necessary.

The script takes a number, where 0 are the system defaults, 1 is UI-oriented, and then 2 to 5 give gradually larger buffers and longer IO deadlines (to utilize caching to the max).

tune_sda_iosched.sh

#!/bin/bash
# Tune I/O scheduler for sda
# Usage: tune_sda_iosched.sh [multiplier]
#
# Workload: pure large sequential I/O (photo reads, snapshotter video frame writes)
# DB has been moved to NVMe SSD - no more random I/O on this disk.
#
# Multiplier 0   = kernel defaults
# Multiplier 1   = base tuned values (recommended for normal use)
# Multiplier 2   = 2× base (snapshotter bulk writing sessions)
# etc. (non-integer multipliers accepted, e.g. 1.5)
#
# Parameters scaled:
#   read_expire    (ms)  - base 750   (relaxed, sequential reads aren't latency-sensitive)
#   write_expire   (ms)  - base 5000  (same as default, writes are already sequential)
#   writes_starved       - base 2     (no competing workloads, keep default)
#   fifo_batch           - base 32    (less reordering needed, I/O is already sequential)
#   nr_requests          - base 128   (no deep queue needed for sequential workload)
#   read_ahead_kb        - base 8192  (8MB readahead, key for large sequential reads)
#   wbt_lat_usec         - inversely scaled: mult 0=75000, 1=75000, >=2=0 (disabled)
#   # front_merges       - boolean, not scaled; confirmed 1 (enabled) - correct for HDD

DEFAULTS="500 5000 2 16 60 128 75000 1"
BASES="750 5000 2 32 128 8192 75000 1"
LABELS="read_expire write_expire writes_starved fifo_batch nr_requests read_ahead_kb wbt_lat_usec front_merges"
PATHS=(
    /sys/block/sda/queue/iosched/read_expire
    /sys/block/sda/queue/iosched/write_expire
    /sys/block/sda/queue/iosched/writes_starved
    /sys/block/sda/queue/iosched/fifo_batch
    /sys/block/sda/queue/nr_requests
    /sys/block/sda/queue/read_ahead_kb
    /sys/block/sda/queue/wbt_lat_usec
    /sys/block/sda/queue/iosched/front_merges
)

MULT=${1:-2}

if ! [[ "$MULT" =~ ^[0-9]+(\.[0-9]+)?$ ]]; then
    echo "Usage: $0 [multiplier]  (non-negative number, default: 2)"
    echo "  0 = kernel defaults, 1 = base tuned, 2 = 2x base, etc."
    exit 1
fi

read -r -a DEF_ARR  <<< "$DEFAULTS"
read -r -a BASE_ARR <<< "$BASES"
read -r -a LBL_ARR  <<< "$LABELS"

# Compute the values to apply
APPLY=()
for i in "${!BASE_ARR[@]}"; do
    base=${BASE_ARR[$i]}
    def=${DEF_ARR[$i]}
    lbl=${LBL_ARR[$i]}

    if [[ "$MULT" == "0" ]]; then
        APPLY+=("$def")
    elif [[ "$lbl" == "wbt_lat_usec" ]]; then
        # Inverse: higher mult = lower latency cap; 0 = disabled
        # mult 1 = 75000 (default), mult 2+ = 0 (fully disabled)
        if (( $(echo "$MULT >= 2" | bc -l) )); then
            val=0
        else
            val=$(echo "scale=0; $base / $MULT / 1" | bc)
        fi
        APPLY+=("$val")
    elif [[ "$lbl" == "front_merges" ]]; then
        # Boolean - always enabled (1) for HDD; multiplier 0 also keeps it 1
        APPLY+=(1)
    else
        val=$(echo "scale=0; $base * $MULT / 1" | bc)
        APPLY+=("$val")
    fi
done

# Read current values
CUR=()
for path in "${PATHS[@]}"; do
    CUR+=("$(cat "$path" 2>/dev/null || echo '?')")
done

# Print header
printf "%-20s  %10s  %10s  %10s  %10s\n" "parameter" "default" "current" "will-apply" "after"
printf "%-20s  %10s  %10s  %10s  %10s\n" "--------------------" "----------" "----------" "----------" "----------"

# Apply and read back
AFTER=()
for i in "${!PATHS[@]}"; do
    path=${PATHS[$i]}
    val=${APPLY[$i]}
    echo "$val" > "$path" 2>/dev/null
    AFTER+=("$(cat "$path" 2>/dev/null || echo '?')")
done

# Print all rows
for i in "${!LBL_ARR[@]}"; do
    printf "%-20s  %10s  %10s  %10s  %10s\n" \
        "${LBL_ARR[$i]}" "${DEF_ARR[$i]}" "${CUR[$i]}" "${APPLY[$i]}" "${AFTER[$i]}"
done