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Oct 132013
 

I discovered this cool feature of Linux quite by accident. zRAM is a block device (i.e. a “disk”) where the contents are compressed and stored in memory, which makes it sound rather mundane and hardly very interesting. However in use, it does appear to be quite nifty; sufficiently so that Google are enabling it for Chrome OS. So why?

The way that it is usually configured is as a swap space … so in effect, zRAM is used to compress normal memory, trading processor utilisation for more memory. What should happen is that instead of hitting the performance brick wall of suddenly paging to disk when you hit the memory limits of your machine, the zRAM is used instead eating a bit of processor time but with any luck keeping everything within memory rather than going to disk. It should have no effect during normal operation, but during temporary surges of memory utilisation, it should allow things to proceed at more or less normal performance.

That’s the theory anyway; but if it were not the case would Google be enabling it by default?

Of course in addition to using it as a swap device, there are other possible uses for zRAM devices :-

  1. As an L2ARC cache device for those using ZFS.
  2. To use as a block device for very hot disk spots in examples such as Exim’s retry database – which can be safely discarded on reboot.
  3. Or any other cache whose contents can be safely discarded at any point.

The last point is worth remembering. Because zRAM devices are contained within main memory, their contents are discarded when the power goes away.

Configuration

To use zRAM, we need to load the zRAM module, and choose how many devices to make at the same time. Some people believe that it makes sense to create as many devices as you have cores, as that gives each core (or thread) a device to spend it’s time compressing. To do this, we add the following to the /etc/rc.local file (assuming a Debian system) :-

/sbin/modprobe zram zram_num_devices=$(cat /proc/cpuinfo | grep processor | wc -l)

By default the zRAM will allocate 25% of the main memory to all of the zRAM devices; personally I think that is reasonable enough. However it seems that as soon as you set the number of devices, the size defaults to zero … so we have to set the size of the device as we configure it. Once created, you will have to decide how to use the devices. In my case, I wanted to use half of the devices for swap and half for L2ARC, which I did by adding the following to /etc/rc.local :-

size=$(( ($(cat /proc/meminfo | awk '/^MemTotal/ {print $2}')*1024) / (4 * $(cat /proc/cpuinfo| grep "^processor" | wc -l)) ))
#       Complex way of determining the size of each zRAM device
for dev in /dev/zram*
do
  base=$(basename $dev)
  echo $size > /sys/block/${base}/disksize
  odd=$(( $(echo $dev | sed -e "s/^.*zram//") % 2 ))
  if [ $odd = 0 ]
  then
    /sbin/mkswap $dev
    /sbin/swapon -p 32767 $dev
  else
    zpool remove pool0 $dev > /dev/null 2>&1
    zpool add pool0 cache $dev
  fi
done

This is a rather complex way of doing it, and doesn’t contain much in the way of error checking, but it does work.

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