#!/bin/sh

PREREQ=""

prereqs()
{
    echo "$PREREQ"
}

case "$1" in
    prereqs)
        prereqs
        exit 0
        ;;
esac

# Most file systems use 128 KB readahead. NFS had a bug and used 15 MB.
# In LTSP, we want all network file systems to use only 4 KB readahead,
# as it lowers the network traffic to about half.
# I.e. NFS, NBD, SSHFS, and loop devices.
# https://github.com/ltsp/ltsp/issues/27#issuecomment-533976774
set_readahead() {
    local mpoint devs dev rakf rak

    mpoint=$1
    if [ -n "$mpoint" ]; then
        devs=$(awk '$5 =="'"$mpoint"'" { print $3 }' </proc/self/mountinfo)
    else
        devs=$(awk '/^v[0-9]/ { print $4 }' </proc/fs/nfsfs/volumes)
    fi
    for dev in $devs; do
        for rakf in \
            "/sys/class/bdi/$dev/read_ahead_kb" \
            "/sys/dev/block/$dev/../bdi/read_ahead_kb"
        do
            test -e "$rakf" || continue
            read -r rak <"$rakf"
            test "$rak" != "${READ_AHEAD_KB:-4}" || continue
            echo "${READ_AHEAD_KB:-4}" >"$rakf"
            break
        done
    done
}

set_readahead