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Creating a ram disk on Linux


The best way to create a ram disk on linux is tmpfs. It’s a filesystem living in ram, so there is no need for ext2. You can create a tmpfs of 16Gb size with:

mount -o size=16G -t tmpfs none /mnt/tmpfs

Linux is very efficient in using RAM. There is little surprise that you see little if any speedup with tmpfs. The largest pieces to read into memory (and thus able to slow the process down) are the tools (compiler, assembler, linker), and in a longish make they will be loaded into memory at startup and never leave it. What is left is reading in source (the writing out of the results won’t slow you down, unless severely memory constrained). Again, comon header files will stay around, only the user’s source will require reading. And that is unlikely to be more than a few megabytes. Creating a large RAMdisk (or even much use of tmpfs) can very well slow things down (by making the build memory constrained, the files on RAMdisk or on tmpfs can not be used directly from there).

Besides tmpfs and ramfs, another option is the /dev/ram0 block device. On recent Ubuntu versions, this device does not exist by default, but can be created via modprobe brd.

This approach is more predictable since it creates a real ext4 filesystem and never exceeds the limit you specify. But it takes more steps to set up, and uses RAM less efficiently.

Using brd kernel module (/dev/ram0)

To create and initialize a 4GB RAM disk:

mkdir /ramdisk

modprobe brd rd_nr=1 rd_size=$((4 * 1048576))
mkfs.ext4 /dev/ram0
mount /dev/ram0 /ramdisk

The rd_nr parameter specifies how many RAM disks to create (by default, it creates 16, i.e. /dev/ram0 through /dev/ram15). The rd_size parameter is size in kilobytes. The $(( ... )) syntax lets you do arithmetic in the shell.

To deallocate the RAM disk, unmount it and remove the brd kernel module:

umount /ramdisk
modprobe -r brd

Creating a block device inside ramfs

Alternatively, you can create a block device inside of ramfs:

mkdir /ramdisk-storage /ramdisk
mount -t ramfs ramfs /ramdisk-storage

truncate -s 4G /ramdisk-storage/ramdisk.img
mkfs.ext4 /ramdisk-storage/ramdisk.img
mount /ramdisk-storage/ramdisk.img /ramdisk

The truncate command creates an empty file of a given size such that it is initialized (i.e. consumes memory) on-demand.

To deallocate the RAM disk, umount it and delete the disk image:

umount /ramdisk
rm /ramdisk-storage/ramdisk.img

Comparison with tmpfs and ramfs

Although tmpfs and ramfs are more efficient than using a block device, below are some of their downsides.

tmpfs may swap to disk. This is more efficient, but there may be times you want a pure RAM disk:

  • The files you are working with are sensitive (e.g. files from an encrypted partition).
  • You are doing performance testing and you don’t want disk I/O to be a factor (SSD write times can vary a lot).
  • You are unpacking a large file and you don’t want to wear out your SSD.

ramfs is easy to set up, reclaims space once you delete files, and uses RAM more efficiently (the system does not buffer the files because it knows they are in RAM). But it has its own downsides and surprises:

  • The df utility does not report space usage:

    root@cello-linux:~# df -h /ramdisk
    Filesystem      Size  Used Avail Use% Mounted on
    ramfs              0     0     0    - /ramdisk
  • There is no size limit parameter. If you put too much in the ramdisk, your system will hang.

  • Sparse files can become unsparse when you least expect it. This morning, I copied a VM image (150G, but 49G used on disk) to ramfs (I have 128G of RAM). That worked. But when I copied from the ramfs to the destination, my system became unresponsive. The cp utility apparently filled the holes on read, but not on write.

Both tmpfs and ramfs may behave differently than a real ext4 filesystem. Creating a block device in RAM and initializing it with ext4 avoids this.

For a more in-depth comparison: https://www.kernel.org/doc/Documentation/filesystems/ramfs-rootfs-initramfs.txt

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