Date:   Thu, 8 Nov 2018 17:59:18 +0000
From:   Elana Hashman <Elana.Hashman@...sigma.com>
To:     "'tytso@....edu'" <tytso@....edu>
CC:     "'linux-ext4@...r.kernel.org'" <linux-ext4@...r.kernel.org>
Subject: Phantom full ext4 root filesystems on 4.1 through 4.14 kernels

Hi Ted,

We've run into a mysterious "phantom" full filesystem issue on our Kubernetes fleet. We initially encountered this issue on kernel 4.1.35, but are still experiencing the problem after upgrading to 4.14.67. Essentially, `df` reports our root filesystems as full and they behave as though they are full, but the "used" space cannot be accounted for. Rebooting the system, remounting the root filesystem read-only and then remounting as read-write, or booting into single-user mode all free up the "used" space. The disk slowly fills up over time, suggesting that there might be some kind of leak; we previously saw this affecting hosts with ~200 days of uptime on the 4.1 kernel, but are now seeing it affect a 4.14 host with only ~70 days of uptime.

Here is some data from an example host, running the 4.14.67 kernel. The root disk is ext4.

$ uname -a
Linux <hostname> 4.14.67-ts1 #1 SMP Wed Aug 29 13:28:25 UTC 2018 x86_64 GNU/Linux
$ grep ' / ' /proc/mounts
/dev/disk/by-uuid/<some-uuid> / ext4 rw,relatime,errors=remount-ro,data=ordered 0 0

`df` reports 0 bytes free:

$ df -h /
Filesystem                                              Size  Used Avail Use% Mounted on
/dev/disk/by-uuid/<some-uuid>   50G   48G     0 100% /

Deleted, open files account for almost no disk capacity:

$ sudo lsof -a +L1 /
COMMAND    PID   USER   FD   TYPE DEVICE SIZE/OFF NLINK    NODE NAME
java      5313 user    3r   REG    8,3  6806312     0 1315847 /var/lib/sss/mc/passwd (deleted)
java      5313 user   11u   REG    8,3    55185     0 2494654 /tmp/classpath.1668Gp (deleted)
system_ar 5333 user    3r   REG    8,3  6806312     0 1315847 /var/lib/sss/mc/passwd (deleted)
java      5421 user    3r   REG    8,3  6806312     0 1315847 /var/lib/sss/mc/passwd (deleted)
java      5421 user   11u   REG    8,3   149313     0 2494486 /tmp/java.fzTwWp (deleted)
java      5421 tsdist   12u   REG    8,3    55185     0 2500513 /tmp/classpath.7AmxHO (deleted)

`du` can only account for 16GB of file usage:

$ sudo du -hxs /
16G     /

But what is most puzzling is the numbers reported by e2freefrag, which don't add up:

$ sudo e2freefrag /dev/disk/by-uuid/<some-uuid>
Device: /dev/disk/by-uuid/<some-uuid>
Blocksize: 4096 bytes
Total blocks: 13107200
Free blocks: 7778076 (59.3%)

Min. free extent: 4 KB
Max. free extent: 8876 KB
Avg. free extent: 224 KB
Num. free extent: 6098

HISTOGRAM OF FREE EXTENT SIZES:
Extent Size Range :  Free extents   Free Blocks  Percent
    4K...    8K-  :          1205          1205    0.02%
    8K...   16K-  :           980          2265    0.03%
   16K...   32K-  :           653          3419    0.04%
   32K...   64K-  :          1337         15374    0.20%
   64K...  128K-  :           631         14151    0.18%
  128K...  256K-  :           224         10205    0.13%
  256K...  512K-  :           261         23818    0.31%
  512K... 1024K-  :           303         56801    0.73%
    1M...    2M-  :           387        135907    1.75%
    2M...    4M-  :           103         64740    0.83%
    4M...    8M-  :            12         15005    0.19%
    8M...   16M-  :             2          4267    0.05%

This looks like a bug to me; the histogram in the manpage example has percentages that add up to 100% but this doesn't even add up to 5%.

After a reboot, `df` reflects real utilization:

$ df -h /
Filesystem                                              Size  Used Avail Use% Mounted on
/dev/disk/by-uuid/<some-uuid>   50G   16G   31G  34% /

We are using overlay2fs for Docker, as well as rbd mounts; I'm not sure how they might interact.

Thanks for your help,

--
Elana Hashman
ehashman@...sigma.com

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