ib_write_bw: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/ib_write_bw - from package: perftest

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/ib_write_bw - from package: perftest

You may install the required package with command apt-get install {package}

Error in other languages:
ib_write_bw: bevel nie gevind nie
ib_write_bw: comandă negăsită
ib_write_bw: comando não encontrado
ib_write_bw: commande introuvable
ib_write_bw: command not found
ib_write_bw: käsku ei ole
ib_write_bw: không tìm thấy lệnh
ib_write_bw: komanda nerasta
ib_write_bw: Kommando nicht gefunden.
ib_write_bw: kommandot finns inte
ib_write_bw: komut yok
ib_write_bw: nie znaleziono polecenia
ib_write_bw: níor aimsíodh an t-ordú
ib_write_bw: no se encontró la orden
ib_write_bw: no s'ha trobat l'ordre
ib_write_bw: opdracht niet gevonden
ib_write_bw: parancs nem található
ib_write_bw: perintah tidak ditemukan
ib_write_bw: príkaz nenájdený
ib_write_bw: příkaz nenalezen
ib_write_bw: команда не найдена
ib_write_bw: командата не е открита
ib_write_bw: 命令找不到
ib_write_bw: コマンドが見つかりません