df command percentage error

BiFo · 05-14-2012, 01:41 PM

Hi.

Code:

root@ServerHostname:/# df -h
Filesystem                      Size  Used Avail Use% Mounted on
/dev/mapper/VolGroup00-Backups  425G  377G   27G  94% /Backups

Look carefully the result of my "df" command...
Size = 425G
Used = 377G
Available = 27G
Used + Available = 404G != 425G

¿Why is Used + Available different from Total Size? This is not happening in any other server.
I've tried remounting and rebooting the system.
The "du" command show that the "Used" space in the "/Backups" directory from the "df" command is correct!

Details:
* Slackware 13.37.0
* Linux 2.6.37.6-smp #1 SMP Sat Apr 9 14:01:14 CDT 2011 i686 Intel(R) Xeon(R) CPU E5530 @ 2.40GHz GenuineIntel GNU/Linux
* HP DL360 G6

Thanks in advance.
Bye!

Kustom42 · 05-14-2012, 03:40 PM

This happens frequently with the way df reads disk usage statistics by using file descriptors. Follow this example to see what I'm talking about.

You have a 10GB partition for /var/.

You have an application that spawns a process that logs data to /var/app/log which is always running and will rotate/remove old log files as needed.

Your process has created several log files of 1GB each /var/app/log.1 /var/app/log.2 that totals to 9GB. Your process sees this and removes 5 old log files to bring you down to 4GB of total disk space consumption.

You run du on /var/ and it comes back with 4GB.

You run df -h and /var/ shows using 9GB still.

You kill your logging process which allows df to read the new file descriptors since the process is no longer using them and reports the statistics correctly.

Excerpt from link below

df will include open files (in memory, but not on disk), data/index files (used for data management - sometimes using approximately 2 to 5% of each filesystem) and unnamed files in its size calculation. This is one reason why, sometimes (although not very often), df can show a larger amount of disk used than du does.

Check out http://linuxshellaccount.blogspot.co...nt-values.html

at the end of the day trust du and not df if you have to choose which one is more correct.

neillohit · 05-14-2012, 04:25 PM

Kustom42 covered most of it, but other thing to consider is the reserved blocks for journaling for any journaling based file system like ext3 or ext4.
You can change that by tune2fs -m 1 <filesystem name>
The above command will change journaling blocks to 1% and you would see more available in df -h. Specially in larger file syatems, you might not need the default 5%.

Reuti · 05-20-2012, 09:39 AM

Quote:

Originally Posted by neillohit

Kustom42 covered most of it, but other thing to consider is the reserved blocks for journaling for any journaling based file system like ext3 or ext4.
You can change that by tune2fs -m 1 <filesystem name>
The above command will change journaling blocks to 1% and you would see more available in df -h. Specially in larger file syatems, you might not need the default 5%.

While I agree to use -m to lower the amount of reserved blocks (as it fits perfect to the missing 23G), this has nothing to do with journaling. These are reserved blocks for privileged processes running under the root account. For some filesystems like /home it can even be set to 0.

neillohit · 05-21-2012, 02:36 PM

Thanks Reuti for the correction.