When running tests from the Ceph sources, the disk is used intensively and a ram disk can be used to reduce the latency. The kernel must be rebooted to set the ramdisk maximum size to 16GB. For instance on Ubuntu 14.04 in /etc/default/grub (the module name which could be rb or brd depending).
GRUB_CMDLINE_LINUX="brd.rd_size=16777216" # 16GB in KB
the grub configuration must then be updated with
sudo update-grub
After reboot the ram disk is formatted as an ext4 file system and mounted:
$ cat /sys/module/brd/parameters/rd_size
16777216
$ sudo mkfs -t ext4 /dev/ram1
$ sudo mount /dev/ram1 /srv
$ df -h /srv
Filesystem Size Used Avail Use% Mounted on
/dev/ram1 16G 44M 15G 1% /srv
$ free -g
total used free shared buffers cached
Mem: 31 0 31 0 0 0
-/+ buffers/cache: 0 31
Cloning ceph, compiling and running tests should now take less than 15 minutes with
$ git clone https://github.com/ceph/ceph $ cd ceph $ ./run-make-check.sh
When the ram disk is umounted, some of the memory used by the ram disk is still in use
$ free -g
total used free shared buffers cached
Mem: 31 27 4 0 0 17
-/+ buffers/cache: 9 22
$ sudo umount /srv
$ free -g
total used free shared buffers cached
Mem: 31 18 13 0 0 8
-/+ buffers/cache: 9 22
It can be flushed with
$ sudo blockdev --flushbufs /dev/ram1
$ free -g
total used free shared buffers cached
Mem: 31 9 22 0 0 8
-/+ buffers/cache: 0 31