Main Situation :
You need to check the UUID under those files (answered in details on other answers…)
/etc/crypttab
/etc/fstab
/etc/grub.d/40_custom
/boot/grub2/grub.cfg
Alternative Situation I — Udev :
This could be caused by udev if you have a rule script under /etc/udev/rules.d/ that is not meant to run at boot time, if the script fail it will make that fstab step go on forever, just edit your script to match your needs or delete it.
Alternative situation II — Crypted Dev :
Crypted partitions can be confusing because the main partition have an UUID and the mapped Decrypted one have an other UUID different from the main one for a single partition they have to be defined in different place etc/crypttab and /etc/fstab
# lsblk -o name,uuid,mountpoint
├─sda2 727fa348-8804-4773-ae3d-f3e176d12dac
│ └─sda2_crypt (dm-0) P1kvJI-5iqv-s9gJ-8V2H-2EEO-q4aK-sx4aDi
Real UUID need to be specified in etc/crypttab
# cat /etc/crypttab
sda2_crypt UUID=727fa348-8804-4773-ae3d-f3e176d12dac none luks
Virtual UUID need to be at /etc/fstab
# cat /etc/fstab
UUID=P1kvJI-5iqv-s9gJ-8V2H-2EEO-q4aK-sx4aDi / ext4 defaults,errors=remount-ro 0 1
Alternative situation III — Ghost Dev :
A device that is setup to be mounted at boot time but is not present in the system or detached like an usb drive.
Checkout real connected devices with lsblk -o name,uuid,mountpoint and edit /etc/fstab to keep only the connected device
OR leave the unconnected device there but set them up to be ignored at boot with the option noauto and set the line like this
UUID=BLA-BLA-BLA /mount ext4 option,noauto,option 0 0
Checking the system logs
journalctl -ab
systemd-analyze blame
systemd-analyze critical-chain
systemctl status dev-mapper-crypt_sda2.device
systemctl status systemd-udev-settle.service
Sources: Linuxhacks.org
Disclosure: I am the owner of Linuxhacks.org
The problem can reproduced when you multi-boot linux systems with more than one swap is activated during linux install, to solve this issue run the following command to find out the UUID of your swap partition:
ls -l /dev/disk/by-uuid
Example of output ( sda4 = your swap partition):
lrwxrwxrwx 1 root root 10 Oct 27 13:16 cfa74c40-1234-4fee-a2d5-e8c789c629f0 -> ../../sda4
Edit your fstab and change the uuid:
Example:nano /etc/fstab :
# swap was on /dev/sda4 during installation
UUID=f97a6afb-3447-4616-bfa3-123be50f9fa5 none swap sw 0 0
Re-create the swap partition and assign the new uuid:
swapoff /dev/sda4
mkswap -U cfa74c40-1234-4fee-a2d5-e8c789c629f0 /dev/sda4
swapon -a
If you have more than 1 swap partition, comment out the other swap partitions on your fstab.
I recently performed a VM disk resize, followed by a Gparted resize of /dev/sda1, followed by a dist-upgrade for Debian 8. I had to increase the VM disk size to accommodate the extra storage required for the upgrade.
Now, whenever I boot or reboot the VM, I get the following message along with its 1:30 second delay. The message is A start job is running for dev-disk-byx2duui...{UUID}.device.
Searching for terms like «Debian start job check disk» tells me Debian (and Linux) does not use tools like check disk 
What do I need to do or run to stop this start job once and for all?
asked Sep 13, 2015 at 9:28
2
This message indicates a problem in /etc/fstab. Run the lsblk -f command to view a list of your devices and their UUIDs. Then make sure the entries in /etc/fstab are using the correct UUIDs.
When I encountered this problem, my /etc/fstab had an entry for a swap partition, but lsblk -f revealed that I had no swap partition! Removing that entry from /etc/fstab solved the problem.
Helpful link: A Start Job Is Running For dev-disk-by
answered Feb 19, 2016 at 15:34
Serp CSerp C
2082 silver badges6 bronze badges
2
EDIT:
This thread got a bit off topic as I found several errors with my system not relevant to the cause of this error for me. If you have this error at boot, I’ve summised the fix in the last post on the thread (post number 10), but double check this post too to see that you do have the same error first!
———————————————————————————————————————————-
Hi all,
So I finished my first ever Arch Linux install using the wiki beginners guide and other wiki pages, so just want to start with a big thank you to everyone who contributes and maintains the information; I’ve managed to get all set up with xfce and lxdm and get all my hardware working only having prior experience with Ubuntu/Ubuntu clones using just the wiki and one cry for help with my WiFi card, so you are doing a fantastic job!
My one remaining niggle is that when I boot, my boot pauses for 1min 30secs for ‘Job dev-disk-byx2duuid-b072209dx2d279ax2d41cdx’ before continuting as normal and seeming to work. using blkid, I’ve matched the UUID to my swap partition and I looked up the error messages:
$ journalctl -b
...
dev-disk-byx2duuid-b072209dx2d279ax2d41cdx2db4b3x2d16eab1a84d60.device: Job dev-disk-byx2duuid-b072209dx2d279ax2d41cdx
Timed out waiting for device dev-disk-byx2duuid-b072209dx2d279ax2d41cdx2db4b3x2d16eab1a84d60.device.
Dependency failed for /dev/disk/by-uuid/b072209d-279a-41cd-b4b3-16eab1a84d60.
Dependency failed for Swap.
swap.target: Job swap.target/start failed with result 'dependency'.
dev-disk-byx2duuid-b072209dx2d279ax2d41cdx2db4b3x2d16eab1a84d60.swap: Job dev-disk-byx2duuid-b072209dx2d279ax2d41cdx2d
dev-disk-byx2duuid-b072209dx2d279ax2d41cdx2db4b3x2d16eab1a84d60.device: Job dev-disk-byx2duuid-b072209dx2d279ax2d41cdx
...Next I checked that my swap partition is working:
$ swapon && free
NAME TYPE SIZE USED PRIO
/dev/sdb9 partition 11.2G 0B -1
total used free shared buff/cache available
Mem: 12190888 801380 10708160 181104 681348 11135672
Swap: 11718652 0 11718652I was concerned that my swap may not be working because of the 0B used, so I decided to open a LOT of tabs in chromium and change swappiness to test my swap:
$ sudo sysctl vm.swappiness=100
vm.swappiness = 100Give it a minute…
$ free
total used free shared buff/cache available
Mem: 12190888 1677076 9769028 142820 744784 10295964
Swap: 11718652 0 11718652I also tried to hibernate the computer, but it just shutdown. I’m only guessing here, but it doesn’t seem like swap is working properly?
My fstab entry for swap is:
# Swap Partition /dev/sdb9
UUID=b072209d-279a-41cd-b4b3-16eab1a84d60 none swap defaults 0 0Any help appreciated thanks all!
JJ
Last edited by Jj (2015-09-01 03:36:37)
Recently, I encountered a strange problem when booting up my Linux OS.
a start job is running for dev-disk-by uuid <disk-partition-uuid>
This process lasts for 1 minute and 30s ! OMG! That’s unbearable. And then it told me:
Timed out waiting for device dev-disk-by<disk-partition-uuid> Denpendency failed for /dev/disk/by-uuid/<disk-partition-uuid> Denpendency failed for Swap
Take a look at the third line. As you can clearly see, the disk partition causing this problem is the swap partition.
After my Linux OS booted into desktop, I opened up a terminal and check my swap partition with this command:
swapon --show
As you can see from the screenshot, my swap partition wasn’t activated. I also issued the following command to check mounted partitions and my swap partition wasn’t there.
mount
So why did swap partition fail to mount on system boot time? As I later found out, the UUID of my swap partition has changed. Enter this command to check the real UUID of swap partition.
sudo blkid
then check the UUID of swap partition in /etc/fstab.
sudo nano /etc/fstab
If the two UUIDs are different, then the /etc/fstab file is corrupted and you need to replace the UUID in /etc/fstab with the UUID returned by sudo blkid.
Save and close /etc/fstab file. Reboot your computer.
Now the “a start job is running for dev-disk-by” message was gone and you saved 1 minute and 30s of your life.
The same error occurred on my Arch Linux, Fedora and Ubuntu machine. One thing I don’t like about Ubuntu is that it veils system messages with its logo when booting up. For a long time I did’t know why it takes such a long time for Ubuntu to boot into Unity desktop. Finally I found the reason.