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
Using a ram file system
mount -t tmpfs -o size=2G tmpfs /somewhere
is not a viable option because make check only supports btrfs, xfs and ext4 file systems and it will fail. eatmydata does not work for make check on Unbuntu 14.04.
Many thanks to Cyril Bouthors for his help in researching the best solution.