reader.rb

#!/usr/bin/env ruby
require 'timeout'

abort "Usage: #{$0} <file> [duration] [rps]" unless ARGV.size >= 1

filename, duration, rps = ARGV
duration = (duration || 5).to_i
rps = (rps || 100).to_i

pause = 1.0 / rps
reads = failures = 0
endtime = Time.at(Time.now.to_i + duration)

while(Time.now < endtime) do
  reads += 1
  File.mtime(filename) rescue failures += 1
  sleep pause
end

puts "Reads: #{reads}, Failures: #{failures}"

test.sh

#!/bin/bash

swaps=1000

mkdir -p releases1 releases2
touch releases1/file releases2/file
rm -f releases
ln -s releases1 releases

################################################################################
echo "# Switching symlinks via rm && ln -s"
./reader.rb releases/file $1 $2 &
reader_pid="$!"

for i in `seq $swaps`
do
    rm -f releases
    ln -s releases2 releases
    rm -f releases
    ln -s releases1 releases
done

wait $reader_pid

################################################################################

echo "# Switching symlinks via ln -sf"
./reader.rb releases/file $1 $2 &
reader_pid="$!"

for i in `seq $swaps`
do
    ln -sfT releases1 releases
    ln -sfT releases2 releases
done

wait $reader_pid

################################################################################

echo "# Switching symlinks via ln -s && mv"
./reader.rb releases/file $1 $2 &
reader_pid="$!"

for i in `seq $swaps`
do
    ln -sfT releases2 releases_tmp
    mv -T releases_tmp releases
    ln -sfT releases1 releases_tmp
    mv -T releases_tmp releases
done

wait $reader_pid