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On a device with 2GB storage every MB is counted.

I wonder what is this /var/lib/ureadahead/debugfs doing on my system and is there a way to avoid its existence.

Note this mount point eats up 1.2 GB

# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1             1.8G  1.2G  546M  68% /
none                  995M  224K  995M   1% /dev
none                 1000M     0 1000M   0% /dev/shm
none                 1000M  136K 1000M   1% /var/run
none                 1000M     0 1000M   0% /var/lock
none                 1000M     0 1000M   0% /lib/init/rw
none                  1.8G  1.2G  546M  68% /var/lib/ureadahead/debugfs


# cat /proc/self/mountinfo
15 19 0:0 / /sys rw,nosuid,nodev,noexec,relatime - sysfs none rw
16 19 0:3 / /proc rw,nosuid,nodev,noexec,relatime - proc none rw
17 19 0:5 / /dev rw,relatime - devtmpfs none rw,size=1018860k,nr_inodes=254715,mode=755
18 17 0:12 / /dev/pts rw,nosuid,noexec,relatime - devpts none rw,gid=5,mode=620,ptmxmode=000
19 1 8:1 / / rw,relatime - ext4 /dev/disk/by-uuid/68bf9687-2dec-49fd-a37d-5532c19f3a39 rw,errors=remount-ro,barrier=1,data=ordered
21 15 0:15 / /sys/fs/fuse/connections rw,relatime - fusectl none rw
22 15 0:7 / /sys/kernel/debug rw,relatime - debugfs none rw
23 15 0:10 / /sys/kernel/security rw,relatime - securityfs none rw
24 17 0:16 / /dev/shm rw,nosuid,nodev,relatime - tmpfs none rw
25 19 0:17 / /var/run rw,nosuid,relatime - tmpfs none rw,mode=755
26 19 0:18 / /var/lock rw,nosuid,nodev,noexec,relatime - tmpfs none rw
27 19 0:19 / /lib/init/rw rw,nosuid,relatime - tmpfs none rw,mode=755

# du -a /var | sort -n -r | head -n 10
138576  /var
87496   /var/lib
61164   /var/lib/apt
61120   /var/lib/apt/lists
34120   /var/cache
30668   /var/cache/apt
26108   /var/lib/apt/lists/us.archive.ubuntu.com_ubuntu_dists_lucid_universe_binary-amd64_Packages
18996   /var/lib/dpkg
16952   /var/lib/dpkg/info
15320   /var/cache/apt/srcpkgcache.bin
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up vote 7 down vote accepted

It looks like df is a bit confused here rather than there being an actual problem.

According to your mountinfo, there is no file system mounted under /var/lib/ureadahead/debugfs, and its df statistics are identical to those for /. So it wouldn't be surprising if df is just reporting the statistics for /dev/sda1 a second time. It probably doesn't represent another 1.2GB of disk usage.

The debugfs file system is a virtual file system used to communicate debugging and tracing information from the kernel to user space. It does not consume disk space. While debugfs is usually mounted under /sys/kernel/debug, but it looks like the ureadahead program may temporarily mount it under /var/lib/ureadahead/debugfs in certain circumstances. It is possible that this caused the confusion.

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that was the reason I originally posted cat /proc/self/mountinfo output However, I added now du output which might give more info –  Tzury Bar Yochay Jul 12 '11 at 2:19
    
So the du output shows that the /var tree as a whole is taking up ~ 140MB, which is far less than the 1.2GB you asked about in your question. That seems to be consistent with the answer I gave you. –  James Henstridge Jul 12 '11 at 2:40
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