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Date: Sun, 11 Oct 2020 21:57:04 -0400 From: Douglas Gilbert <dgilbert@...erlog.com> To: Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>, Linux List Kernel Mailing <linux-kernel@...r.kernel.org> Subject: Re: [question] What happens when dd writes data to a missing device? On 2020-10-11 3:46 p.m., Mikhail Gavrilov wrote: > Hi folks! > I have a question. > What happens when dd writes data to a missing device? > > For example: > # dd if=/home/mikhail/Downloads/Fedora-Workstation-Live-x86_64-Rawhide-20201010.n.0.iso > of=/dev/adb > > Today I and wrongly entered /dev/adb instead of /dev/sdb, > and what my surprise was when the data began to be written to the > /dev/adb device without errors. > > But my surprise was even greater when cat /dev/adb started to display > the written data. > > I have a question: > Where the data was written and could it damage the stored data in > memory or on disk? Others have answered your direct question. You may find 'oflag=nocreat' helpful if you (or others) do _not_ want a regular file created in /dev ; for example: if you have misspelt a device name. That flag may also be helpful in unstable systems (e.g. where device nodes are disappearing and re-appearing) as it can be a real pain if you manage to create a regular file with a name like /dev/sdc when the disk usually occupying that node is temporarily offline. When that disk comes back online then regular file '/dev/sdc' will stop device node '/dev/sdc' from being created. The solution is to remove the regular file /dev/sdc and you probably need to power cycle that disk. If this becomes a regular event then 'oflag=nocreat' is your friend [see 'man dd' for a little more information, it really should be expanded]. Doug Gilbert
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