Persistent compressed ramdisk manager

The script zdisk creates a compressed ramdisk (using zramdevices) which can be persisted in a squashfs filesystem, which can be restored on boot.

In this way we have the benefits of a ramdisk (speed, less stress for SSD drives), less memory usage (thanks to the zram devices) and without loosing contents on power off.

Moreover, the ramdisk can be very big: everytime the ramdisk contents are saved, they are added to the squashfs archive and the used ram becomes free (after a service restart).

Requirements

This utility requires the following tools:

• Linux kernel versione 3.18 or upper; tested with kernel 4.14
• kernel modules zram and overlay
• xfs filesystem (can be changed with environment variables)
• mksquashfs: usually provided by the squashfs-tools package
• zramctl: sometimes provided by te util.linux package
• bzip2, for compressing backups during quick saves.

If some tool is missing, use the package manager of your Linux distribution to install it.

Optionally, these utilities are used if foundin the system:

• pbzip2: used as substitution for bzip2 for parallel compression during quick saves

Install

To install the zdisk binary, download the zdisk script and run the following command:

    sudo ./zdisk install

which copies the zdisk binary into the /usr/local/bin folder.

Install in a systemd environment

Optionally, it is possibile to automatically install a systemd service:

    sudo zdisk install --systemd [service_name]

The service_name parameter (the default value is zdisk) is optional and can be used to install a custom zdisk service, usefull if you want more than one disk.

The above command does the following:

• copies the zdisk binary in /usr/local/bin
• if the service_name parameter is specified and different from zdisk, a link /usr/local/bin/service_name pointing to the zdisk script will be created
• creates the /etc/systemd/system/service_name.service file, to declare the systemd service
• creates the /etc/conf.d/service_name file, used to tune the zdisk options
• runs the systemctl daemon-reload command to inform systemd of the configuration change
• runs the systemctl enable service_name to enable the mount of the disk at system startup (can be disabled with --autostart off)

Uninstall

To uninstall, run the command:

    sudo zdisk uninstall

which removes the zdisk binary from /usr/local/bin. Please pay attention to use the previous command only if no other services are created (use the command in the next section).

Uninstall from a systemd environment

To remove a systemd service, run the following command:

    sudo zdisk uninstall--systemd [service_name]

The above command does the following:

• stops the service_name (default is zdisk) service, if running
• disables the service_name from autostarting
• removes the /etc/conf.d/service_name configuration file
• removes the /etc/systemd/system/service_name service descriptor
• removes the file /usr/local/bin/service_name (the binary command)

Usage

The usage is simple (all commands must be run as root, e.g. use sudo):

• zdisk help is your friend: use it to view a quick help on usage and available options
• zdisk start creates a persistent compressed ramdisk with size 25% of installe ram and mounted ad /mnt/zdisk
• zdisk stop saves the contentents of /mnt/zdisk into a squashfs filesystem stored in /var/lib; the stop can be slow, depending on the disk size and hardware (CPU and disks)
• zdisk stop --quick is similar to previous, but saves only the ram contents, not the whole contents of /mnt/zdisk; at next zdisk start the contents will be restored (squashed contents and quick saved). This is preferred if changed files from the whole contents are limited. This is the default when used as a systemd service.
• zdisk save can be used to trigger a save of the /mnt/zdisk contents without doing a zdisk stop (a kind of backup)

All commands can be run with an optional argument specifing the disk name; this can be usefull if you need more than one disk. For esmple:

    sudo zdisk start mydisk
    cp -a $HOME/mystuff /mnt/mydisk/
    # trigger a save into /var/lib/mydisk.sqfs
    sudo zdisk save mydisk 
    # stop and unmount the disk
    sudo zdisk stop mydisk

Configuration

The behaviour of the zdisk script can be changed with environment variables or changing the relative /etc/conf.d/zdisk configuration file.

The available variables are (default values are shown):

    # Where to mount the persistent compressed ramdisk
    # ZDISK_MOUNTPOINT="/mnt/zdisk"

    # File where to backup ramdisk contents
    # SAVED_DISK="/var/lib/zdisk.sqfs"

    # Use pbzip2, if installed, instead of bzip2 for parallel compression during quick saves.
    # USE_PBZIP2="true"

    # on stop, save disk contents
    # SAVE_ON_STOP="true"

    # On start, restore previous saved disk contents
    # RESTORE_ON_START="true"

    # zdisk size will be max 25% of total memory; used (compressed) memory is (hopefully) less.
    # ZDISK_PCTSIZE=25

    # Alternatively, the ramdisk size can be explicetly set
    # ZDISK_SIZE="1G"

    # Ramdisk compression algorithm; can be lzo o lz4
    # ZDISK_ALGORITHM="lzo"

    # Filesystem to use for the ramdisk. Usually xfs consumes less space.
    # ZDISK_FS="xfs"

Tests

In the test directory there is a test using the BATS framework and some extensions.

To run the tests you need to checkout some submodules (the test framework) with the following commands:

    git submodule init
    git submodule update

Next, you can run the tests to check if the script is working as expected:

    test/zdisk.bats