Rapido: A Glorified Wrapper for Dracut and QEMU

Introduction

I've blogged a few of times about how Dracut and QEMU can be combined to greatly improve Linux kernel dev/test turnaround.

• My first post covered the basics of building the kernel, running dracut, and booting the resultant image with qemu-kvm.
• A later post took a closer look at network configuration, and focused on bridging VMs with the hypervisor.
• Finally, my third post looked at how this technique could be combined with Ceph, to provide a similarly efficient workflow for Ceph development.

In bringing this series to a conclusion, I'd like to introduce the newly released Rapido project. Rapido combines all of the procedures and techniques described in the articles above into a handful of scripts, which can be used to test specific Linux kernel functionality, standalone or alongside other technologies such as Ceph.

Usage - Standalone Linux VM

The following procedure was tested on openSUSE Leap 42.3 and SLES 12SP3, but should work fine on many other Linux distributions.

Step 1: Checkout and Build

Checkout the Linux kernel and Rapido source repositories:

~/> cd ~
~/> git clone https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
~/> git clone https://github.com/rapido-linux/rapido.git

Build the kernel (using a config provided with the Rapido source):

~/> cp rapido/kernel/vanilla_config linux/.config
~/> cd linux
~/linux/> make -j6
~/linux/> make modules
~/linux/> INSTALL_MOD_PATH=./mods make modules_install

Step 2: Configuration 

Install Rapido dependencies: dracut and qemu.

Create a master rapido.conf configuration file using the example template:

~/linux/> cd ~/rapido
~/rapido/> cp rapido.conf.example rapido.conf
~/rapido/> vi rapido.conf

• set KERNEL_SRC="/home/<user>/linux"
• set KERNEL_INSTALL_MOD_PATH="${KERNEL_SRC}/mods"
• the remaining options can be left as is for now
  

Step 3: Image Generation 

Generate a minimal Linux VM image which includes binaries, libraries and kernel modules for filesystem testing:

~/rapido/> ./cut_fstests_local.sh
...
 dracut: *** Creating initramfs image file 'initrds/myinitrd' done ***
~/rapido/> ls -lah initrds/myinitrd
-rw-r--r-- 1 ddiss users 30M Dec 13 18:17 initrds/myinitrd

Step 4 - Boot!

 ~/rapido/> ./vm.sh
+ mount -t btrfs /dev/zram1 /mnt/scratch
[    3.542927] BTRFS info (device zram1): disk space caching is enabled
...
btrfs filesystem mounted at /mnt/test and /mnt/scratch
rapido1:/#  

In a whopping four seconds, or thereabouts, the VM should have booted to a rapido:/# bash prompt. Leaving you with two zram backed Btrfs filesystems mounted at /mnt/test and /mnt/scratch.

Everything, including the VM's root filesystem, is in memory, so any changes will not persist across reboot. Use the rapido.conf QEMU_EXTRA_ARGS parameter if you wish to add persistent storage to a VM.

Once you're done playing around, you can shutdown:

rapido1:/# shutdown
[  267.304313] sysrq: SysRq : sysrq: Power Off
rapido1:/# [  268.168447] ACPI: Preparing to enter system sleep state S5
[  268.169493] reboot: Power down
+ exit 0

Step 5: Network Configuration

The fstests_local VM above is networkless, so doesn't require bridge network configuration. For VMs that do (e.g. CephFS client below) edit rapido.conf:

• set TAP_USER="<user>"
• set MAC_ADDR1 to a valid MAC address, e.g. "b8:ac:24:45:c5:01"
• set MAC_ADDR2 to a valid MAC address, e.g. "b8:ac:24:45:c5:02"

Configure the isolated bridge and tap network devices. This must be done as root:

~/rapido/> sudo tools/br_setup.sh
~/rapido/> ip addr show br0
4: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    ...
    inet 192.168.155.1/24 scope global br0

Usage - Ceph vstart.sh cluster and CephFS client VM

This usage guide builds on the previous standalone Linux VM procedure, but this time adds Ceph to the mix. If you're not interested in Ceph (how could you not be!) then feel free to skip to the next section.

Step I - Checkout and Build

We already have a clone of the Rapido and Linux kernel repositories. All that's needed for CephFS testing is a Ceph build:

~/> git clone https://github.com/ceph/ceph
~/> cd ceph
<install Ceph build dependencies>
~/ceph/> ./do_cmake.sh -DWITH_MANPAGE=0 -DWITH_OPENLDAP=0 -DWITH_FUSE=0 -DWITH_NSS=0 -DWITH_LTTNG=0
~/ceph/> cd build
~/ceph/build/> make -j4 

Step II - Start a vstart.sh Ceph "cluster"

Once Ceph has finished compiling, vstart.sh can be run with the following parameters to configure and locally start three OSDs, one monitor process, and one MDS.

~/ceph/build/> OSD=3 MON=1 RGW=0 MDS=1 ../src/vstart.sh -i 192.168.155.1 -n
...
~/ceph/build/> bin/ceph -c status
...
     health HEALTH_OK
     monmap e2: 1 mons at {a=192.168.155.1:40160/0}
            election epoch 4, quorum 0 a
      fsmap e5: 1/1/1 up {0=a=up:active}
        mgr no daemons active 
     osdmap e10: 3 osds: 3 up, 3 in

Step III - Rapido configuration

Edit rapido.conf, the master Rapido configuration file:

~/ceph/build/> cd ~/rapido
~/rapido/> vi rapido.conf

• set CEPH_SRC="/home/<user>/ceph/src"
• KERNEL_SRC and network parameters were configured earlier

Step IV - Image Generation

The cut_cephfs.sh script generates a VM image with the Ceph configuration and keyring from the vstart.sh cluster, as well as the CephFS kernel module.

~/rapido/> ./cut_cephfs.sh

...
 dracut: *** Creating initramfs image file 'initrds/myinitrd' done ***

Step V - Boot!

Booting the newly generated image should bring you to a shell prompt, with the vstart.sh provisioned CephFS filesystem mounted under /mnt/cephfs:

~/rapido/> ./vm.sh

...

+ mount -t ceph 192.168.155.1:40160:/ /mnt/cephfs -o name=admin,secret=...
[    3.492742] libceph: mon0 192.168.155.1:40160 session established
...
rapido1:/# df -h /mnt/cephfs
Filesystem             Size  Used Avail Use% Mounted on
192.168.155.1:40160:/  1.3T  611G  699G  47% /mnt/cephfs

CephFS is a clustered filesystem, in which case testing from multiple clients is also of interest. From another window, boot a second VM:

~/rapido/> ./vm.sh

 

 

Further Use Cases

Rapido ships with a bunch of scripts for testing different kernel components:

• cut_cephfs.sh (shown above)
• Image: includes Ceph config, credentials and CephFS kernel module
• Boot: mounts CephFS filesystem
• cut_cifs.sh
• Image: includes CIFS (SMB client) kernel module
• Boot: mounts share using details and credentials specified in rapido.conf
• cut_dropbear.sh
• Image: includes dropbear SSH server
• Boot: starts an SSH server with SSH_AUTHORIZED_KEY
• cut_fstests_cephfs.sh
• Image: includes xfstests and CephFS kernel client
• Boot: mounts CephFS filesystem and runs FSTESTS_AUTORUN_CMD
• cut_fstests_local.sh (shown above)
• Image: includes xfstests and local Btrfs and XFS dependencies
• Boot: provisions local xfstest zram devices. Runs FSTESTS_AUTORUN_CMD
• cut_lio_local.sh
• Image: includes LIO, loopback dev and dm-delay kernel modules
• Boot: provisions an iSCSI target, with three LUs exposed
• cut_lio_rbd.sh
• Image: includes LIO and Ceph RBD kernel modules
• Boot: provisions an iSCSI target backed by CEPH_RBD_IMAGE, using target_core_rbd
• cut_qemu_rbd.sh
• Image: CEPH_RBD_IMAGE is attached to the VM using qemu-block-rbd
• Boot: runs shell only
• cut_rbd.sh
• Image: includes Ceph config, credentials and Ceph RBD kernel module
• Boot: maps CEPH_RBD_IMAGE using the RBD kernel clien
• cut_samba_cephfs.sh
• Image: includes Ceph vstart config, credentials and libcephfs from CEPH_SRC, and additionally pulls in Samba from a (pre compiled) SAMBA_SRC
• Boot: configures smb.conf with a CephFS backed share and starts Samba
• cut_samba_local.sh
• Image: includes local kernel filesystem utils, and pulls in Samba from SAMBA_SRC
• Boot: configures smb.conf with a zram backed share and starts Samba
• cut_tcmu_rbd_loop.sh
• Image: includes Ceph config, librados, librbd, and pulls in tcmu-runner from TCMU_RUNNER_SRC
• Boot: starts tcmu-runner and configures a tcmu+rbd backstore exposing CEPH_RBD_IMAGE via the LIO loopback fabric
• cut_usb_rbd.sh (see https://github.com/ddiss/rbd-usb)
• Image: usb_f_mass_storage, zram, dm-crypt, and RBD_USB_SRC
• Boot: starts the conf-fs.sh script from RBD_USB_SRC

 

 

Conclusion

• Dracut and QEMU can be combined for super-fast Linux kernel testing and development.
• Rapido is mostly just a glorified wrapper around these utilities, but does provide some useful tools for automated testing of specific Linux kernel functionality.

If you run into any problems, or wish to provide any kind of feedback (always appreciated), please feel free to leave a message below, or raise a ticket in the Rapido issue tracker.

Update 20170106:

• Add cut_tcmu_rbd_loop.sh details and fix the example CEPH_SRC path. 

 Update 20180312:

• Use KERNEL_INSTALL_MOD_PATH instead of an ugly symlink
• Update Github links to refer to new project URL
• Remove old brctl and tunctl dependencies
• Split network setup into a separate section, as fstests_local VMs are now networkless
• Add cut_samba_cephfs.sh and cut_samba_local.sh details

Ceph USB Storage Gateway

Last week was Hackweek, a week full of fun and innovation at SUSE. I decided to use the time to work on a USB storage gateway for Ceph.

The concept is simple - create a USB device that, when configured and connected, exposes remote Ceph RADOS Block Device (RBD) images for access as USB mass storage, allowing for:

• Ceph storage usage by almost any system with a USB port
  • Including dumb systems such as TVs, MP3 players and mobile phones
• Boot from RBD images
  • Many systems are capable of booting from a USB mass storage device
• Minimal configuration
  • Network, Ceph credentials and image details should be all that's needed for configuration

Hardware

I already own a Cubietruck, which has the following desirable characteristics for this project:

• Works with a mainline Linux Kernel
• Is reasonably small and portable
• Supports power and data transfer via a single mini-USB port
• Performs relatively well
• Dual-core 1GHz processor and 2GB RAM
• Gigabit network adapter and WiFi 802.11 b/g/n

Possible alternatives worth evaluation include C.H.I.P (smaller and cheaper), NanoPi2, and UP (faster). I should take this opportunity to mention that I do gladly accept hardware donations!

Base System

I decided on using openSUSE Tumbleweed as the base operating system for this project. An openSUSE Tubleweed ARM port for the Cubietruck is available for download at:
http://download.opensuse.org/ports/armv7hl/factory/images/openSUSE-Tumbleweed-ARM-JeOS-cubietruck.armv7l-Current.raw.xz

Installation is as straightforward as copying the image to an SD card and booting - I documented the installation procedure on the openSUSE Wiki.
Releases prior to Build350 exhibit boot issues due to the U-Boot device-tree path. However, this has been fixed in recent builds.

Kernel

The Linux kernel currently shipped with the openSUSE image does not include support for acting as a USB mass storage gadget, nor does it include Ceph RBD support. In order to obtain these features, and also reduce the size of the base image, I built a mainline Linux kernel (4.4-rc4) using a minimal custom kernel configuration:

~/> sudo zypper install --no-recommends git-core gcc make ncurses-devel bc
~/> git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
~/> cd linux
~/linux/> wget https://raw.githubusercontent.com/ddiss/ceph_usb_gateway/master/.config
          # or `make sunxi_defconfig menuconfig`
          #     ->enable Ceph, sunxi and USB gadget modules
~/linux/> make oldconfig
~/linux/> make -j2 zImage dtbs modules
~/linux/> sudo make install modules_install
~/linux/> sudo cp arch/arm/boot/zImage /boot/zImage-$(make kernelrelease)
~/linux/> sudo cp arch/arm/boot/dts/sun7i-a20-cubietruck.dtb /boot/dtb-4.3.0-2/
~/linux/> sudo cp arch/arm/boot/dts/sun7i-a20-cubietruck.dtb /boot/dtb/

This build procedure takes a long time to complete. Cross compilation could be used to improve build times.
I plan on publishing a USB gadget enabled ARM kernel on the Open Build Service in the future, which would allow for simple installation via zypper - watch this space!

Ceph RADOS Block Device (RBD) mapping

To again save space, I avoided installation of user-space Ceph packages by using the bare kernel sysfs interface for RBD image mapping.
The Ceph RBD kernel module must be loaded prior to use:

# modprobe rbd

Ceph RADOS block devices can be mapped using the following command:

# echo -n "${MON_IP}:6789 name=${AUTH_NAME},secret=${AUTH_SECRET} " \

          "${CEPH_POOL} ${CEPH_IMG} -" > /sys/bus/rbd/add

$MON_IP can be obtained from ceph.conf. Similarly, the $AUTH_NAME and $AUTH_SECRET credentials can be retrieved from a regular Ceph keyring file.
$CEPH_POOL and $CEPH_IMG correspond to the location of the RBD image.

A locally mapped RBD block device can be subsequently removed via:

# echo -n "${DEV_ID}" > /sys/bus/rbd/remove

$DEV_ID can be determined from the numeric suffix assigned to the /dev/rbdX device path.

Images can't be provisioned without the Ceph user-space utilities installed, so should be performed on a separate system (e.g. an OSD) prior to mapping on the Cubietruck. E.g. To provision a 10GB image:

# rbd create --size=10240 --pool ${CEPH_POOL} ${CEPH_IMG}

With my Cubietruck connected to the network via the ethernet adapter, I observed streaming read (/dev/rbd -> /dev/null) throughput at ~37MB/s, and the same value for streaming writes (/dev/zero -> /dev/rbd). Performance appears to be constrained by limitations of the Cubietruck hardware.

USB Mass Storage Gadget

The Linux kernel mass storage gadget module is configured via configfs. A device can be exposed as a USB mass storage device with the following procedure:

# modprobe sunxi configfs libcomposite usb_f_mass_storage

# mount -t configfs configfs /sys/kernel/config
# cd /sys/kernel/config/usb_gadget/
# mkdir -p ceph
# cd ceph

# mkdir -p strings/0x409
# echo "fedcba9876543210" > strings/0x409/serialnumber
# echo "openSUSE" > strings/0x409/manufacturer
# echo "Ceph USB Drive" > strings/0x409/product

# mkdir -p functions/mass_storage.usb0
# echo 1 > functions/mass_storage.usb0/stall
# echo 0 > functions/mass_storage.usb0/lun.0/cdrom
# echo 0 > functions/mass_storage.usb0/lun.0/ro
# echo 0 > functions/mass_storage.usb0/lun.0/nofua
# echo "$DEV" > functions/mass_storage.usb0/lun.0/file

# mkdir -p configs/c.1/strings/0x409
# echo "Config 1: mass-storage" > configs/c.1/strings/0x409/configuration
# echo 250 > configs/c.1/MaxPower
# ln -s functions/mass_storage.usb0 configs/c.1/

# ls /sys/class/udc > UDC

$DEV corresponds to a /dev/X device path, which should be a locally mapped RBD device path. The module can however also use local files as backing for USB mass storage.

Boot-Time Automation

By default, Cubietruck boots when the board is connected to a USB host via the mini-USB connection.
With RBD image mapping and USB mass storage exposure now working, the process can be run automatically on boot via a simple script: rbd_usb_gw.sh
Furthermore, a systemd service can be added:

[Unit]
Wants=network-online.target
After=network-online.target

[Service]
# XXX assume that rbd_usb_gw.sh is present in /bin
ExecStart=/bin/rbd_usb_gw.sh %i
Type=oneshot
RemainAfterExit=yes

Finally, this service can be triggered by Wicked when the network interface comes online, with the following entry added to /etc/sysconfig/network/config:

POST_UP_SCRIPT="systemd:rbd-mapper@.service"

Boot Performance Optimisation

A significant reduction in boot time can be achieved by running everything from initramfs, rather than booting to the full Linux distribution.
Generating a minimal initramfs image, with support for mapping and exposing RBD images is straightforward, thanks to the Dracut utility:

# dracut --no-compress  \
         --kver "`uname -r" \
         --install "ps rmdir dd vim grep find df modinfo" \
         --add-drivers "rbd musb_hdrc sunxi configfs" \
         --no-hostonly --no-hostonly-cmdline \
         --modules "bash base network ifcfg" \
         --include /bin/rbd_usb_gw.sh /lib/dracut/hooks/emergency/02_rbd_usb_gw.sh \
         myinitrd

The rbd_usb_gw.sh script is installed into the initramfs image as a Dracut emergency hook, which sees it executed as soon as initramfs has booted.

To ensure that the network is up prior to the launch of rbd_usb_gw.sh, the kernel DHCP client (CONFIG_IP_PNP_DHCP) can be used by appending ip=dhcp to the boot-time kernel parameters. This can be set from the U-Boot bootloader prompt:

=> setenv append 'ip=dhcp'

=> boot

The new initramfs image must be committed to the boot partition via:

# cp myinitrd /boot/

# rm /boot/initrd

# sudo ln -s /boot/myinitrd /boot/initrd

Note: In order to boot back to the full Linux distribution, you will have to mount the /boot partition and revert the /boot/initrd symlink to its previous target.

Future Improvements

• Support configuration of the device without requiring console access
• Run an embedded web-server, or expose a configuration filesystem via USB 
• Install the operating system onto on-board NAND storage,
• Further improve boot times
• Avoid U-Boot device probes
• Experiment with the new f_tcm USB gadget module
• Expose RBD images via USB and iSCSI

Credits

Many thanks to:

• My employer, SUSE Linux, for encouraging me to work on projects like this during Hackweek.
• The linux-sunxi community, for their excellent contributions to the mainline Linux kernel.
• Colleagues Dirk, Bernhard, Alex and Andreas for their help in bringing up openSUSE Tumbleweed on my Cubietruck board.

Edits

• 2025-04-02: fix architecture image background

Accénts & Ümlauts - A Custom Keyboard Layout on Linux

As a native English speaker living in Germany, I need to be able to reach the full Germanic alphabet without using long key combinations or (gasp) resorting to a German keyboard layout.

Accents and umlauts on US keyboards aren't only useful for expats. They're also enjoyed (or abused) by a number of English speaking subcultures:

• Hipsters: "This is such a naïve café."
• Metal heads: "Did you hear Spın̈al Tap are touring with Motörhead this year?"
• Teenage gamers: "über pwnage!"

The standard US system keyboard layout can be enhanced to offer German characters via the following key mappings:

Key	Key + Shift	Key + AltGr (Right Alt)	Key + AltGr + Shift
e	E	é	É
u	U	ü	Ü
o	O	ö	Ö
a	A	ä	Ä
s	S	ß	ß
5	%	€

With openSUSE and some other Linux distributions, this can be configured by first defining the mappings in /usr/share/X11/xkb/symbols/us_de:

partial default alphanumeric_keys
xkb_symbols "basic" {
    name[Group1]= "US/ASCII";
    include "us"

    key <AD03> {[e,          E,           eacute,         Eacute]};
    key <AD07> {[u,          U,           udiaeresis,     Udiaeresis]};
    key <AD09> {[o,          O,           odiaeresis,     Odiaeresis]};
    key <AC01> {[a,          A,           adiaeresis,     Adiaeresis]};
    key <AC02> {[s,          S,           ssharp,         ssharp]};
    key <AE05> {[NoSymbol, NoSymbol,      EuroSign]};

    key <RALT> {type[Group1]="TWO_LEVEL",
                [ISO_Level3_Shift, ISO_Level3_Shift]};

    modifier_map Mod5   {<RALT>};
};

Secondly, specify the keyboard layout as the system default in /etc/X11/xorg.conf.d/00-keyboard.conf:

Section "InputClass"
        Identifier "system-keyboard"
        MatchIsKeyboard "on"
        Option "XkbLayout" "us_de"
EndSection

Once in place, the key mappings can be easily modified to suit specific tastes or languages - viel Spaß!

Edits

• 2025-04-02: drop IBus config override section. IBus doesn't appear to be around anymore.