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I was playing with qemu on Linux when I discovered that I could just run

sudo qemu -hda /dev/sda

And it would boot up my system again! I closed qemu immediately after Grub was loaded successfully though, because the command is run using sudo and I was afraid it would damage my /dev/sda.

Could anyone explain how dangerous this could be?

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It did break my system on Ubuntu 14.04 AMD64: it reboot into the root partition anymore :-) – Ciro Santilli 巴拿馬文件 六四事件 法轮功 Aug 24 '15 at 10:01
up vote 10 down vote accepted

I would say it is incredibly dangerous.

You are right in your supposition that it would likely damage your /dev/sda.

Two systems accessing the same block device, each with their own cache and buffers, will definitely mean that each has a different idea of what is actually on the filesystem - changes made by one operating system will not be propagated properly to the other and you will start getting files overwriting each other.

All in all a horrible mess.

Now, if you had 2 operating systems installed on 2 different partitions (dual booting) you could use qemu to boot one of them inside the other, but you must never ever ever boot the same OS twice (unless it's a read-only OS like a live CD image for instance)

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You can safely run this command by using the -snapshot switch.

qemu usage: -snapshot write to temporary files instead of disk image files

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This is glorious. – xiaq Dec 5 '13 at 11:55

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