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I have a device that I can see in the output of fdisk -l:


(XX is a number.)

How do I know that I can use this /dev/dm-XX to add it to a RAID1 array? How can I be 100% sure that nothing is using this /dev/dm-XX?

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This is indeed a field in which Linux / LVM2 can improve. It requires a bit of puzzling. – jippie May 27 '12 at 16:28
up vote 1 down vote accepted
sudo fuser /dev/dm-31
/dev/dm-31:          24799
ps -fp 24799
vbox     24799     1  2 May18 ?        04:37:39 /usr/lib/virtualbox/VBoxHeadle.....

or when using LVM2 (on Ubuntu, SLES may vary in detail):

ls -l /dev/mapper | grep dm-1$
lrwxrwxrwx 1 root root       7 May 11 19:01 vg_diablo-var -> ../dm-1

You may also want to check man dmsetup eg. dmsetup info

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dmsetup info is the command to use - mainly the information about Open count. DM target can be opened/used for example by making a LVM volume group active, or by mounting a filesystem on LVM logical volume. – Marki555 May 27 '12 at 18:52

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