The udev rules I've created so far only deal with devices being added or removed, i.e.:

ACTION=="add"... or ACTION=="remove"...

I've come across an example of a rule that seems to deal with device state changes as well:

ACTION=="add|change", KERNEL=="sd[b-z]", ATTR{queue/rotational}=="1", RUN+="/usr/bin/hdparm -B 127 -S 12 /dev/%k"

I take it that the above rule applies whenever a matching device is added OR its state changes.

Question: What kind of state changes are possible (generally and specific to a USB hard drive)?

I've checked all udev documentation I can find and there's barely any mention of, or usage guidance, in respect of device state changes or specifically ACTION="change".


"change" corresponds, for example, to removing or inserting an sdcard in a sdcard reader, or changing the hard disc inside a usb-to-sata enclosure. The device itself is not added nor removed, but the media is no longer the same.


udev change event gets triggered when media changer (regards to tape - Netbackup like environment) or sd card reader is used in continuation with different media or sd card. When a same device is re-added (a san disk is re-added again or you do switchdisable/enable on the switch side or target port disable/enable from target side )

Below are the few change udev events that we see on the host side, when I did target port disable (from Vexata VX100 array) for an existing san lun

KERNEL[34942.047543] change   /devices/virtual/block/dm-0 (block)
KERNEL[34942.047596] change   /devices/virtual/block/dm-1 (block)
KERNEL[34942.047613] change   /devices/virtual/block/dm-2 (block)
KERNEL[34942.047631] change   /devices/virtual/block/dm-3 (block)
KERNEL[34942.047678] change   /devices/virtual/block/dm-4 (block)
UDEV  [34942.053448] change   /devices/virtual/block/dm-3 (block)
UDEV  [34942.053486] change   /devices/virtual/block/dm-4 (block)
UDEV  [34942.053507] change   /devices/virtual/block/dm-0 (block)
UDEV  [34942.053536] change   /devices/virtual/block/dm-2 (block)

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