Compressing VMDK and VHDX disk images with qemu-img convert

This is actually a kind of anniversary post. 10 years ago I wrote a howto about compressing Virtualbox VDI disk images by zeroing the unused space. Today, although the hypervisor, the OS, even my whole build environment has changed, I’m still using the same technique to keep virtual disk images as small as possible.

This time I’m reducing the size of VMDK disk images during a CI/CD pipeline build using the qemu-img tool.

The basic idea is the same as 10 years ago: create and setup the disk image; as soon as you are finished delete unnecessary files and zero-fill the free space. Then use ‘qemu-img’ to either compact the disk image (if this is supported) or create a copy of it minus the free space.

This works because most hypervisors support dynamically allocated virtual disk image formats. They grow every time a disk sector is written for the first time until the virtual disk reaches its maximum defined capacity. A big advantage because it allows the efficient storage of rather empty virtual image files (eg. right after their initial setup).

But their size is not automatically reduced when files are deleted. This is where the ‘zeroing the empty space’ step kicks in… by zeroing these sectors, they can be marked as ’empty’ in the virtual disk image file. Next time when ‘compacting’ the image or when creating a copy of it, they will not be copied over to the new image.

In case of a freshly installed Linux system you can fill up the empty space by creating a temporary file on the root partition, fill it with zeros until all remaining disk space is used up, then delete it and shut down the system.

dd if=/dev/zero of=/target/temp.img bs=4M; sync; rm /target/temp.img

I’m assuming here that the target partition on the virtual disk you want to compress is mounted as /target. The first part (disk dump) will write zeros to a temporary file and use up all disk space available on the partition. After synchronizing the file system (just to be safe everything was written), the temporary file is deleted again.

I’m then using qemu-img to create a copy of the disk image. Here I’m converting a VMDK image again into a VMDK:

qemu-img convert -p -f vmdk -O vmdk /path/to/src.vmdk /path/to/dest.vmdk

The option -p stands for an optional progress meter, -f for the source format, and -O for the destination format. The last two parameters are the input and output filenames.

If you prefer qcow2 formatted disk images you can make use of the compress option to even further reduce the image size. The source format here is again VMDK; the option -c stands for compress:

qemu-img convert -c -p -f vmdk -O qcow2 /path/to/src_image.vmdk /path/to/dest_image.qcow2

So in the end, by simply creating a copy and thereby getting rid of empty space, the resulting image can be reduced to only a bit more than the overall size of the files in the image.

Replacing the WiFi Card on an Asus ROG Strix B450-I Motherboard

I’ve recently got so fed up with the Linux driver support of the Realtek RTL8822BE network adapter on my motherboard (especially its poor Bluetooth support) that I seriously considered a hardware fix. In the end I’ve replaced it with a much better supported card, an Intel® Wi-Fi 6 AX200.

The Realtek linux driver in Ubuntu Desktop 20.04 up to 21.10 is crappy, to say the least – I’ve constantly had WiFi connection issues and connecting to my stereo via Bluetooth was not even working out of the box. Only after manually extracting the firmware from the Windows driver, copying it to the right location, and adapting a few settings, it worked. Poorly.

Network controller: Realtek Semiconductor Co., Ltd. RTL8822BE 802.11a/b/g/n/ac WiFi adapter
Subsystem: ASUSTeK Computer Inc. RTL8822BE 802.11a/b/g/n/ac WiFi adapter
Flags: bus master, fast devsel, latency 0, IRQ 74, IOMMU group 15
I/O ports at c000 [size=256]
Memory at fc500000 (64-bit, non-prefetchable) [size=64K]
Capabilities: <access denied>
Kernel driver in use: rtw_8822be
Kernel modules: rtw88_8822be

Having already dealt with these issues before on my Dell notebook by, surprise, replacing the Realtek card with an Intel AX200 about a year ago, I was eager to try the same on my Mini-ITX motherboard.

Asus Wi-Fi GO! Card Socket

I haven’t found much information about the Asus Wi-Fi GO! card on the board. But I’ve seen quite a few of them on eBay and some were shown in a disassembled state. So I was quite confident that the RF shield could be opened and inside would be a standard PCIe M.2 WiFi card that could be replaced.

The RF shield is held on the board by two screws (from the bottom side) and it can be pulled off the PCIe connector after removing these screws. Another screw holds the two halves of the shield together.

Opened Wi-Fi GO! Card Shield
Removed Realtek RTL8822BE Card

Replacing the PCIe WiFi card should then be pretty straight forward. In my case I’ve bought an Intel AX200 card for under 20,- € (shipping included) as replacement. It should be possible to replace the PCIe card without completely disassembling the the two SMA connectors; I’ve done it for convenience. Also there’s a black rubber block stuck onto the card that you might want to reuse (peel it off carefully).

The reassembly with a new card is pretty much doing the same steps in reverse order, so I’m not describing or showing it here. In the end, it probably took me longer to get the motherboard out of the Mini-ITX case and back in again than disassembling the Wi-Fi card; less than half an hour for the actual card disassembly and replacement.

Backing up and restoring a bootable USB Stick

I recently needed to create a backup of a bootable USB stick. My usual approach is to simply use dd (disk dump) on a Linux system to create a full copy of the stick into an image file. This time I wanted to play around a bit and see if I could store only the relevant data, to restore the stick without storing empty sectors or – even worse – remnants of old, potentially sensitive data from previous uses.

The USB stick in this case was a 8 GByte Sandisk Cruzer Blade with a bootable FAT32 partition on it:

# fdisk -l /dev/sda
Disk /dev/sda: 7.45 GiB, 8004304896 bytes, 15633408 sectors
Disk model: Cruzer Blade    
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x01234567

Device     Boot Start      End  Sectors  Size Id Type
/dev/sda1  *     2048 15632383 15630336  7.5G  c W95 FAT32 (LBA)

Step 1: Backing up the Partition Layout

I started with creating a backup of the partition layout. sfdisk comes handy here as it is easily scriptable (not required in this case). You can pipe its output into a text file and use the same file to re-create the partition layout.

# sfdisk -d /dev/sda 
label: dos
label-id: 0x01234567
device: /dev/sda
unit: sectors
sector-size: 512

/dev/sda1 : start=        2048, size=    15630336, type=c, bootable

# sfdisk -d /dev/sda > partition-layout-sda.sfdisk

Step 2: Copying the Boot Sector and Hidden Data

You probably won’t need this step if your USB stick is not bootable. In my case it was a recovery stick so it contained a bootable MBR FAT32 partition. Backing up the boot sector is easy: create a disk dump of the first sector (512 bytes). This sector contains the master boot record (the first 446 bytes), partition table entry points (64 bytes), and two magic bytes (0x55, 0xAA) at the end.

# dd if=/dev/sda of=bootsect-sda.img bs=512 count=1
1+0 records in
1+0 records out
512 bytes copied, 0.00350891 s, 146 kB/s

The Master Boot Record only consists of the first 446 bytes. I’ve taken a shortcut here and stored the whole first sector. When restoring the MBR I will only restore the first 446 bytes of it.

Some disk images can contain additional (secret) data in the gap between the boot sector and the first sector of the first partition. The GRUB2 boot loader for example stores additional data in this area. I haven’t created a backup of these sectors in my test; but if you would need to, the command would look something like this…
(In my case the first partition starts at sector 2048, so there could be about 1 MByte of additional hidden data.)

# dd if=/dev/sda of=bootsect-and-additional-sectors.img bs=512 count=2048
2048+0 records in
2048+0 records out
1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.0616555 s, 17.0 MB/s

Step 3: Creating a backup of the files on the stick

So now we’re getting to the interesting part: creating a copy of the files on the USB stick. And only of the files. A really handy tool for this is partclone. It supports various partition formats and only stores the files without the ‘free’ space.

This can be slightly improved by adding a layer of compression. In my case most of the contents was already compressed, so this step wasn’t a big improvement (only 0.1 GBytes or about 4%):

# partclone.fat32 -c -s /dev/sda1 | gzip -c -9 > sda1.img.gz
Partclone v0.3.13
Starting to clone device (/dev/sda1) to image (-)
Reading Super Block
Calculating bitmap... Please wait... 
Elapsed: 00:00:01, Remaining: 00:00:00, Completed: 100.00%                      
Total Time: 00:00:01, 100.00% completed!
File system:  FAT32
Device size:    8,0 GB = 15630336 Blocks
Space in use:   4,6 GB = 8976976 Blocks
Free Space:     3,4 GB = 6653360 Blocks
Block size:   512 Byte
Elapsed: 00:06:10, Remaining: 00:00:00, Completed: 100.00%, Rate: 745,33MB/min, 
current block:   15630336, total block:   15630336, Complete: 100.00%           
Total Time: 00:06:10, Ave. Rate:  745,3MB/min, 100.00% completed!
Syncing... OK!
Partclone successfully cloned the device (/dev/sda1) to the image (-)
Cloned successfully.

Schrödinger’s Backup, or: Restoring the USB Stick

Well, it’s always nice to have a backup; but you need to verify that it can be restored.

The condition of any backup is unknown until a restore is attempted.

Schrödinger’s Backup

My approach was to take another USB stick, erase its contents, and then try to restore the files and images I had created earlier. I probably could have combined these steps into a shell script, but as this was a one-off I simply typed them in.

The first step is to erase the old partition layout – if there is one as in my case.

# fdisk -l /dev/sda
Disk /dev/sda: 7.45 GiB, 8004304896 bytes, 15633408 sectors
Disk model: Cruzer Blade    
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xfedcba98

Device     Boot Start      End  Sectors  Size Id Type
/dev/sda1        2048 15632383 15630336  7.5G  b W95 FAT32

# dd if=/dev/zero of=/dev/sda bs=1M count=1; sync
1+0 records in
1+0 records out
1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.114768 s, 9.1 MB/s

Next I’ve recreated the partition layout by piping the ‘partition-layout-sda’ file back into sfdisk.

# sfdisk /dev/sda < partition-layout-sda.sfdisk 
Checking that no-one is using this disk right now ... OK

Disk /dev/sda: 7.45 GiB, 8004304896 bytes, 15633408 sectors
Disk model: Cruzer Blade    
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

>>> Script header accepted.
>>> Script header accepted.
>>> Script header accepted.
>>> Script header accepted.
>>> Script header accepted.
>>> Created a new DOS disklabel with disk identifier 0x01234567.
/dev/sda1: Created a new partition 1 of type 'W95 FAT32 (LBA)' and of size 7.5 GiB.
Partition #1 contains a vfat signature.
/dev/sda2: Done.

New situation:
Disklabel type: dos
Disk identifier: 0x01234567

Device     Boot Start      End  Sectors  Size Id Type
/dev/sda1  *     2048 15632383 15630336  7.5G  c W95 FAT32 (LBA)

The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

After that I recreated the MBR (the first 446 bytes of the boot sector).

# dd if=bootsect-sda.img of=/dev/sda bs=446 count=1
1+0 records in
1+0 records out
446 bytes copied, 0.00312617 s, 143 kB/s

The last step was to restore the files with partclone.

# gzip -c -d sda1.img.gz | partclone.fat32 -r -o /dev/sda1 
Partclone v0.3.13
Starting to restore image (-) to device (/dev/sda1)
Calculating bitmap... Please wait...
File system:  FAT32
Device size:    8.0 GB = 15630336 Blocks
Space in use:   4.6 GB = 8976976 Blocks
Free Space:     3.4 GB = 6653360 Blocks
Block size:   512 Byte
Elapsed: 00:03:20, Remaining: 00:00:00, Completed: 100.00%, Rate:   1.38GB/min, 
current block:   15630336, total block:   15630336, Complete: 100.00%           
Total Time: 00:03:20, Ave. Rate:    1.4GB/min, 100.00% completed!
Syncing... OK!
Partclone successfully restored the image (-) to the device (/dev/sda1)
Cloned successfully.

It’s alive

The new stick is bootable. Checksum comparisons during boot did not report any errors. And I was able to significantly reduce the stored data. (I probably would have saved even more on larger USB sticks.)

So in the end, was it worth the time to go through all these steps? Maybe. It definitely was fun to do.

I will continue to simply create disk dumps via dd. But it is good to know that you can reduce the size of such a bootable USB stick if you, for example, need to distribute personalized images to customers and ‘every byte counts’. For everyday use cases I would recommend Clonezilla though. It not only covers all the steps from above, but is also far better field-tested and adapted to various different disk and partition layouts.

LED Grow Light Station for Chilis and Tomatos

Days are getting longer, and in the afternoons the temperature rises above 10°C… but if I’ve learned a lesson the last two years then that growing chilis and tomatos in our living room (although located on the south side of our house) is still far from enough for these seedlings…

So I’ve taken a few hours and built an LED grow light station for them. 😀

Two LED panels (bought on Amazon for ~ 23 € each, hanging above two tubs with seedlings.
These photos show the seedlings two weeks after sowing the tomato and chili seeds.

KVM over IP Switch: Cross-vendor Firmware Upgrade via hidden Option adds VNC Server

At work we have a KVM over IP switch from Inter-Tech, a KVM IP-KVM101. It is really a small and versatile device that, in combination with 16-port KVM switches, allows us to control a complete rack with test hardware.

The KVM switch provides a web interface with a Java Web Start application for remote access. But the latest browser security updates disabled and removed the Java Web Start support . This move was announced quite some time ago (Oracle White Paper). Still, the vendor of the KVM switch, Inter-Tech, was not able to provide a valuable solution to cope with this problem.

After a bit of Googling I found an Amazon review about this particular KVM switch where the reviewer stated to have installed the firmware from another brand, the LINDY KVM over IP Switch. The LINDY switch looks identical (besides a different sticker) and – even better – their latest firmware provides a VNC server. This would allow us a browser and even operating system independent access to the KVM.
Continue reading

WD PiDrive Cable USB 3 to USB 2 Hack with a Saw

I literally wanted to start this post with “I literally hacked the PiDrive cable…” but I actually have sawn it instead. Duh!

I’m using quite a few Raspberry Pi boards as servers, gateways, etc. and recently started to ‘polish’ their design. Instead of hard drives hanging beside them with a bunch of loose cables I’ve started to buy WD PiDrive Cases and Cables to pack all of them into nice ‘little boxes’.

The PiDrive Cables are really neat: they are designed to be positioned between a Raspberry Pi, its power supply, and an USB hard drive. The cable is designed for USB 3.0 hard drives, but in one case (hah) I wanted to use an external USB 2.0 drive I had lying around. So without further ado I’ve removed the USB 3.0 extension of the connector with a saw. It looks crude but works perfect…

Aliexpress search customization and optimization (via hidden sort options in the URL)

About a year ago (2016) Aliexpress decided to cripple simplify their search engine. They’ve removed really useful search options and added (at least in my eyes) unnecessary ones.

These useful search options were removed from the search panel but they are still available as URL parameters. I’ve mainly written this post to document these missing options for my everyday searches — but I’m pretty sure others may find them useful as well.

Aliexpress Hidden Search Options
Parameter Declaration Description
isUnitPrice y Show Price per Piece
minQuantity value Minimal Quantity in Lot
maxQuantity value Maximal Quantity in Lot
minPrice value (e.g. 1.23) Minimum Price
maxPrice value (e.g. 9.87) Maximum Price
isFavorite y Results with 4+ Rating
isFreeShip y Free Shipping only
sortType price_asc/price_desc Ascending/Descending Prices
shipCountry de/us/… Destination Country
shipFromCountry cn/de/us/… Country of Origin

Especially the option isUnitPrice is helpful when searching for the best price per piece and not price per lot. I really cannot understand why this option is not available per default.

So when I’m searching for the lowest price per piece within a certain quantity range (here: 10 – 50 pieces) with free shipping I would add to the URL (Example Search):


Side Note: It also makes a difference which keywords you are using when searching. The following three searches will (at least the last time I’ve checked) give different results:

  • “stm32f103c8t6 lqfp
  • “stm32f103c8t6 lqfp48
  • “stm32f103c8t6 lqfp-48

So even slight variations can make a big difference. Unlike the search on eBay, the search on Alibaba and Aliexpress does not split up keywords into smaller ‘chunks’ which are used for searching.

2018-08-02: Updated the List of Search Options

Windows 10 Update can brick a Dual-Boot Setup when using Primary Partitions only

TL/DR: Use extended partitions for Linux in a dual-boot setup with Windows 10 — otherwise you might not be able to boot after a Windows update.

A few days ago I had to fix a dual-boot setup consisting of a Windows 10 and a Linux (Ubuntu) system. After deploying the latest Windows 10 Anniversary Update neither Linux nor Windows was accessible anymore. Only the GRUB rescue prompt appeared:

error: unknown filesystem.
Entering rescue mode...
grub rescue>

GRUB wasn’t even able to manually boot into the Windows or Linux partition. Continue reading

30th Anniversary of the Chernobyl Tragedy – or – Building a PIN Diode Geiger Counter

Exactly 30 years ago a great disaster struck the region of Chernobyl: a nuclear accident occurred that released a large quantity of radioactive particles into the atmosphere. And it is only five years ago that, with the Fukushima Daiichi nuclear disaster, a second similar catastrophic event has taken place.

These anniversaries did not directly let me build a PIN Photodiode based Geiger Counter, it is more or less a coincidence. The main drive to build such a device was my curiosity and (please forgive me) a fascinating green glow I’ve seen on various fluorescent Uranium minerals under UV light. But in this context it should not be forgotten that at present there still is a significant increase in background radiation in some regions and some agricultural products due to these events.

There are lots of descriptions of how to build such a device; even cheap commercial products (e.g. the Smart Geiger) use such a design. Especially two sites caught my attention: and Das Gammastrahlen-Mikrofon (German). The design I’ve chosen is based on these sources but I’ve begun to further modify it. In this post I’m showing the design I’ve started with. It mainly relies on two BPW34 (Vishay Datasheet) photodiodes connected in parallel, and two transistors to amplify the voltage fluctuations of beta and/or gamma rays striking the diodes. A 9 Volt battery was added to increase the pulse height.

The common approach to protect the photodiodes from light is to use one layer of tin foil and connect it to ground. This should also protect the circuit against electromagnetic radiation. I’ve started with something different and dipped the diodes three times into liquid rubber (Plasti Dip). My hope was to at least allow some beta particles to reach the semiconductor material.

Photodiode with Liquid Rubber Coating

So far I’ve tested the basic design shown above and had mostly noise on my microphone input. I’d say that sporadic crackling has more to do with the 1 hour hacked together design than beta or gamma rays. The liquid rubber seems to block of light, but the simple design is sensitive to electromagnetic radiation. Waving your hand or even movements in about 1 m distance is visible in the sound profile. An additional tin foil shield connected to ground did not change the noise profile, although the EMR influence was reduced. I’ve tested it with two different sound cards (microphone inputs).

I’m currently redesigning the whole approach and expect better results. So stay tuned…