Linux:Using ZRam
=================

I recently discovered zram in Linux, and it felt like a mystical, futuristic
experience. Upon discovering it and learning how to use it, I immediately went
looking for uses.


What is Zram?
-------------

`Zram <https://www.kernel.org/doc/html/latest/admin-guide/blockdev/zram.html>`_
is a feature of the Linux kernel (``modprobe zram``) that allows the user to
create "compressed RAM-based block devices".

If that doesn't make much sense, it is very similar to RAM drives, but
compressed. I learned about using RAM drives a few years ago (``mount -t tmpfs
...``) and have been using them for all kinds of things on my systems to reduce
IO load on my storage, while maximizing performance. With RAM drives you
section off a portion of your RAM to use as "storage" [with a read/write speed
of ~6 GB/s]. ZRam is very similar, except that it compresses inline, making
even more "storage" available at the cost of less RAM, but slightly higher CPU.


How can we use zram?
--------------------

A package compiling layer (like /var/pkgmk)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Previously, I did this (in fstab, but here's the command)

.. code-block:: sh

   mount -t tmpfs -o size=10G none /var/pkgmk

That creates a 10 GB RAM device, mounted at ``/var/pkgmk``, where I compile
packages from source on my system (any Crux users out there?).

NOW however, I do this:

.. code-block:: sh

   dev=$(zramctl -f)  # Find the first available zram device
   zramctl "${dev}" -s 16384 -t $(( $(nproc) / 2 ))
   mkfs.ext4 "${dev}"
   mount "${dev}" /var/pkgmk

It's a bit more complicated to get set up (I have the above in a script), but
the end result is a 16 GB RAM drive, which takes significantly less ram, using
up to half of my CPU's cores to compress. I tend to see about 70% compression
when compiling source code, so while the drive allows for 16 GB of capacity
(the full amount in my laptop in fact), at its maximum it only takes about 5
GB; usually much less.


zswap (swapon... swapoff... swapon... swapoff)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I haven't used swap on my system in a long time. With computers coming with
more and more RAM, I haven't needed it in a long time. However, I recently ran
into some compiling tasks that, combined with my ram drive for storing
compiling source code, consumes more RAM than my laptop has.

Enter ``zswap``

Zswap is basically the same thing as a zram block device, except instead of
being formatted with a real filesystem, it is formatted for swap usage. To set
one of these up, I do this (again, this is distilled from the script I wrote):

.. code-block:: sh

   dev=$(zramctl -f)  # Find the first available zram device
   zramctl "${dev}" -s 10240 -t $(( $(nproc) / 2 ))
   mkswap "${dev}"
   swapon "${dev}"

That will create a new zram block device that is 10 GB in size, using up to
half of my CPU's cores for compression. This zram device is then formatted for
swap usage (``mkswap``), and enabled (``swapon``).



Conclusion
----------

These commands obviously need to be put into a nice shell script for easy setup
and teardown. I have my in an init script, supporting ``start``, ``stop``, and
``status`` commands to make things less cumbersome. You can see the full init
script I wrote for zswap setup over `here
<https://oper.io/src/nullspoon/zram-swap.git/>`_.

Do you have any other ideas for how to use zram? If so, feel free to email me
any ideas you might come up with. I'd love to hear them!