ceph_test_librbd: 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/ceph_test_librbd - from package: ceph-test

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

On Debian possible default full paths are:

/usr/lib/ceph/bin/ceph_test_librbd - from package: ceph-test

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

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