- A critical production system was running out of disk space and things were going downhills scarcely fast, df was showing utilization at 99% already.
- The user decided to delete some big log files in order to free up some space.
- After that df was still showing 99% , however running du on the root directory would show less space being utilized.
What was happening here? Was df or du lying to the user?
Well... software in general doesn't lie, usually what actually happens is that we don't really understand how it works and therefore we expect a different behavior. The root cause of the problem mentioned before is the unlink syscall, why? we can get the answer by asking the oracle (man 2 unlink):
DESCRIPTION
unlink() deletes a name from the filesystem. If that name was the last
link to a file and no processes have the file open, the file is deleted
and the space it was using is made available for reuse.
If the name was the last link to a file but any processes still have
the file open, the file will remain in existence until the last file
descriptor referring to it is closed.
...
Yes :D, turns out that if you delete a file and there's at least one process that has a file descriptor pointing to that file, the space of the file won't be available right away.Lets reproduce this:
The easiest way to understand this is by actually reproducing the behavior. I just used dd to fill-up the drive and the result was this:
[ec2-user@ip-172-31-16-117 ~]$ df -h Filesystem Size Used Avail Use% Mounted on devtmpfs 3.9G 56K 3.9G 1% /dev tmpfs 3.9G 0 3.9G 0% /dev/shm /dev/xvda1 7.8G 7.7G 0 100% / [ec2-user@ip-172-31-16-117 ~]$ du -sch / 2>/dev/null 7.7G / 7.7G total [ec2-user@ip-172-31-16-117 ~]$ echo a > file -bash: echo: write error: No space left on device [ec2-user@ip-172-31-16-117 ~]$the system has clearly ran out of disk space at this point and we can't even add a single byte to a file. Now the next step would be to free up some space, so we check with du where we can find some files to delete:
[ec2-user@ip-172-31-16-117 ~]$ du -sch /* 2>/dev/null 7.0M /bin 25M /boot 4.0K /cgroup 56K /dev 9.1M /etc 6.7G /home 46M /lib 23M /lib64 4.0K /local 16K /lost+found 4.0K /media 4.0K /mnt 43M /opt 0 /proc 4.0K /root 8.0K /run 12M /sbin 4.0K /selinux 4.0K /srv 0 /sys 8.0K /tmp 802M /usr 48M /var 7.7G total [ec2-user@ip-172-31-16-117 ~]$the size of /home is a bit crazy considering the size of the partition itself so lets delete some files from /home/ec2-user:
[ec2-user@ip-172-31-16-117 ~]$ ls -lah total 6.7G drwx------ 3 ec2-user ec2-user 4.0K Nov 17 20:10 . drwxr-xr-x 3 root root 4.0K Nov 17 18:42 .. -rw-r--r-- 1 ec2-user ec2-user 18 Aug 15 23:52 .bash_logout -rw-r--r-- 1 ec2-user ec2-user 193 Aug 15 23:52 .bash_profile -rw-r--r-- 1 ec2-user ec2-user 124 Aug 15 23:52 .bashrc -rw-rw-r-- 1 ec2-user ec2-user 306M Nov 17 20:09 big_file -rw-rw-r-- 1 ec2-user ec2-user 0 Nov 17 20:10 file -rw-rw-r-- 1 ec2-user ec2-user 6.4G Nov 17 18:44 fillingup -rwxrwxr-x 1 ec2-user ec2-user 7.0K Nov 17 20:09 service -rw-rw-r-- 1 ec2-user ec2-user 309 Nov 17 20:09 service.c drwx------ 2 ec2-user ec2-user 4.0K Nov 17 18:42 .ssh -rw------- 1 ec2-user ec2-user 2.1K Nov 17 20:09 .viminfo [ec2-user@ip-172-31-16-117 ~]$ rm big_file [ec2-user@ip-172-31-16-117 ~]$ [ec2-user@ip-172-31-16-117 ~]$ echo a > file -bash: echo: write error: No space left on device [ec2-user@ip-172-31-16-117 ~]$I've just deleted a 306Mbytes file and can't even write 1 single byte!!! Having a look at df and du things look like:
[ec2-user@ip-172-31-16-117 ~]$ df -h Filesystem Size Used Avail Use% Mounted on devtmpfs 3.9G 56K 3.9G 1% /dev tmpfs 3.9G 0 3.9G 0% /dev/shm /dev/xvda1 7.8G 7.7G 0 100% / [ec2-user@ip-172-31-16-117 ~]$ du -sch / 2>/dev/null 7.4G / 7.4G total [ec2-user@ip-172-31-16-117 ~]$while du shows the expected size, df insists in saying the partition is at 100% utilization. Cool, with just a few steps we have reproduced the situation!
The reason behind the difference between df and du is in the fact that du calculates the size of the folders by adding up the size of the files inside and big_file does not exist anymore on the folder (has been removed from the directory entry), while df shows the real available space of the file system.
With all this in mind, the only reason why the file system size wouldn't change after deleting a file is because there's at least one process that keeps a file descriptor pointing to the deleted file (you can read more about open files and file descriptors in Linux limits 102 - Open Files). We can easily identify the process that holds the file descriptor by simple using our lovely lsof:
[ec2-user@ip-172-31-16-117 ~]$ lsof |grep "\(deleted\)" service 23177 ec2-user 3r REG 202,1 319934464 14708 /home/ec2-user/big_file (deleted) [ec2-user@ip-172-31-16-117 ~]$ja! turns out process "service" with PID 23177 has a file descriptor that points to file /home/ec2-user/big_file and that file has been deleted!!! So if we get rid of process service then the space should be finally available, lets confirm that:
[ec2-user@ip-172-31-16-117 ~]$ kill -9 23177 [ec2-user@ip-172-31-16-117 ~]$ [1]+ Killed ./service [ec2-user@ip-172-31-16-117 ~]$ df -h Filesystem Size Used Avail Use% Mounted on devtmpfs 3.9G 56K 3.9G 1% /dev tmpfs 3.9G 0 3.9G 0% /dev/shm /dev/xvda1 7.8G 7.4G 306M 97% / [ec2-user@ip-172-31-16-117 ~]$ du -sch / 2>/dev/null 7.4G / 7.4G total [ec2-user@ip-172-31-16-117 ~]$and magic happened!!! When the processes was terminated the fd was finally closed and the last reference to the file disappeared causing the space to bee available now and therefore df matches with du now.
Depending on which process is keeping the file open you may have other options besides killing it, for example:
- Restarting it if it's a service.
- Perhaps sending another signal (for example to rotate logs?).
- If for some reason (uninterruptible state) you can't kill the process then the only easy way here is to restart the whole system.
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