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Let's assume this is based on device-mapper. How can I tell if a given block device is already in use? (For example, a PV of a currently in use VG, or a encrypted LUKS device that is currently decrypted).

Also - how can I tell whether a given block device comes from device-mapper, or is a physical one?

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    Have a look at /sys/class/block/<device>/holders – Stéphane Chazelas Jul 18 '16 at 12:46
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I think lsblk will answer these questions for you with a nice display.

ls /sys/class/block/*/holders/ | 
awk '/\/sys/ {line=$0; next} ! /^$/ {print line,"\t",$0} /^$/ {line=""}'

outputs the same information in a different format.

dmsetup deps has some complementary info.

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