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I found that if I detach a disk from my Linux server (CentOS 7), the related /dev/sd* file will disappear automatically about 10 seconds later.

I'm wondering how does Linux know a disk has been detached? Is there something like a sweeper keep scanning all the devices?

And is it possible to make this quicker?

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The delay is likely caused by udisk2 & udev.

Research

$ ps -eaf|grep [u]disk
root     17041     1  0 09:48 ?        00:00:00 /usr/libexec/udisks2/udisksd

Can query it for storage devices like so:

$ udisksctl status
MODEL                     REVISION  SERIAL               DEVICE
--------------------------------------------------------------------------
VBOX HARDDISK             1.0       VBc5aaf476-f419b1f1  sda

If you look at the udisk2 process:

$ lsof -p $(pidof udisksd) | tail
udisksd 17041 root    3u     unix 0xffff88003a49d400      0t0     611852 socket
udisksd 17041 root    4u  a_inode                0,9        0       4852 [eventfd]
udisksd 17041 root    5u  a_inode                0,9        0       4852 [eventfd]
udisksd 17041 root    6u     unix 0xffff88003a49c000      0t0     611853 socket
udisksd 17041 root    7u  a_inode                0,9        0       4852 [eventfd]
udisksd 17041 root    8r      REG                0,3        0     611907 /proc/17041/mountinfo
udisksd 17041 root    9r      REG                0,3        0 4026532019 /proc/swaps
udisksd 17041 root   10r  a_inode                0,9        0       4852 inotify
udisksd 17041 root   11u  netlink                         0t0     611910 KOBJECT_UEVENT
udisksd 17041 root   12u  a_inode                0,9        0       4852 [eventfd]

Not a lot to go on there, the thing that catches my eye there is inotify. Whenever I see that, I immediately think udev.

Looking for udev rules

$ find /etc/udev/rules.d/ /usr/lib/udev/rules.d  | grep sto
/usr/lib/udev/rules.d/90-alsa-restore.rules
/usr/lib/udev/rules.d/60-persistent-storage.rules
/usr/lib/udev/rules.d/60-persistent-storage-tape.rules

the 2nd file looks interesting, take a look inside. This line looks like the cause:

$ cat /usr/lib/udev/rules.d/60-persistent-storage.rules
...
# enable in-kernel media-presence polling
ACTION=="add", SUBSYSTEM=="module", KERNEL=="block", ATTR{parameters/events_dfl_poll_msecs}=="0", ATTR{parameters/events_dfl_poll_msecs}="2000"

ATTR{parameters/events_dfl_poll_msecs}=="0", ATTR{parameters/events_dfl_poll_msecs}="2000"

References

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