Ubuntu server minimal 20.04 error: filesystem statistic error: cannot read /sys/class/block/root/stat -- No such file or directory

Issue #937 resolved
Radek Osmulski created an issue

I am using monit version 5.26 (but I tested this with 5.27 as well). I am on a VM running Ubuntu server minimal 20.04.

This is what is have in my config:

    check filesystem rootfs path /
      if space usage > 80% then alert

This is the error that appears in the log file:

filesystem statistic error: cannot read /sys/class/block/root/stat -- No such file or directory

This is what I have in /sys/class/block

Thank you very much for all your help!

Comments (10)

  1. Tildeslash repo owner

    It seems that your system has paravirtualized disks (vda). Please can you provide following data? :

    1. output of “mount”
    2. output of “ls -l /sys/class/block”

  2. Radek Osmulski reporter

    Thank you so much for looking into this @Tildeslash !

    output of mount:

    /dev/vda1 on / type ext4 (rw,relatime)
    devtmpfs on /dev type devtmpfs (rw,relatime,size=505664k,nr_inodes=126416,mode=755)
    sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
    proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
    securityfs on /sys/kernel/security type securityfs (rw,nosuid,nodev,noexec,relatime)
    tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
    devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
    tmpfs on /run type tmpfs (rw,nosuid,nodev,size=101500k,mode=755)
    tmpfs on /run/lock type tmpfs (rw,nosuid,nodev,noexec,relatime,size=5120k)
    tmpfs on /sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,mode=755)
    cgroup2 on /sys/fs/cgroup/unified type cgroup2 (rw,nosuid,nodev,noexec,relatime,nsdelegate)
    cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,xattr,name=systemd)
    none on /sys/fs/bpf type bpf (rw,nosuid,nodev,noexec,relatime,mode=700)
    cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,nosuid,nodev,noexec,relatime,cpuset)
    cgroup on /sys/fs/cgroup/net_cls,net_prio type cgroup (rw,nosuid,nodev,noexec,relatime,net_cls,net_prio)
    cgroup on /sys/fs/cgroup/perf_event type cgroup (rw,nosuid,nodev,noexec,relatime,perf_event)
    cgroup on /sys/fs/cgroup/freezer type cgroup (rw,nosuid,nodev,noexec,relatime,freezer)
    cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (rw,nosuid,nodev,noexec,relatime,cpu,cpuacct)
    cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory)
    cgroup on /sys/fs/cgroup/hugetlb type cgroup (rw,nosuid,nodev,noexec,relatime,hugetlb)
    cgroup on /sys/fs/cgroup/rdma type cgroup (rw,nosuid,nodev,noexec,relatime,rdma)
    cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,devices)
    cgroup on /sys/fs/cgroup/blkio type cgroup (rw,nosuid,nodev,noexec,relatime,blkio)
    cgroup on /sys/fs/cgroup/pids type cgroup (rw,nosuid,nodev,noexec,relatime,pids)
    hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime,pagesize=2M)
    mqueue on /dev/mqueue type mqueue (rw,nosuid,nodev,noexec,relatime)
    debugfs on /sys/kernel/debug type debugfs (rw,nosuid,nodev,noexec,relatime)
    tracefs on /sys/kernel/tracing type tracefs (rw,nosuid,nodev,noexec,relatime)
    /dev/vda15 on /boot/efi type vfat (rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
    tmpfs on /run/user/1000 type tmpfs (rw,nosuid,nodev,relatime,size=101496k,mode=700,uid=1000,gid=1000)
    

    output of ls -l /sys/class/block

    total 0
    lrwxrwxrwx 1 root root 0 Oct  1 08:09 loop0 -> ../../devices/virtual/block/loop0
    lrwxrwxrwx 1 root root 0 Oct  1 08:09 loop1 -> ../../devices/virtual/block/loop1
    lrwxrwxrwx 1 root root 0 Oct  1 08:09 loop2 -> ../../devices/virtual/block/loop2
    lrwxrwxrwx 1 root root 0 Oct  1 08:09 loop3 -> ../../devices/virtual/block/loop3
    lrwxrwxrwx 1 root root 0 Oct  1 08:09 loop4 -> ../../devices/virtual/block/loop4
    lrwxrwxrwx 1 root root 0 Oct  1 08:09 loop5 -> ../../devices/virtual/block/loop5
    lrwxrwxrwx 1 root root 0 Oct  1 08:09 loop6 -> ../../devices/virtual/block/loop6
    lrwxrwxrwx 1 root root 0 Oct  1 08:09 loop7 -> ../../devices/virtual/block/loop7
    lrwxrwxrwx 1 root root 0 Oct  1 08:09 vda -> ../../devices/pci0000:00/0000:00:04.0/virtio1/block/vda
    lrwxrwxrwx 1 root root 0 Oct  1 08:09 vda1 -> ../../devices/pci0000:00/0000:00:04.0/virtio1/block/vda/vda1
    lrwxrwxrwx 1 root root 0 Oct  1 08:09 vda14 -> ../../devices/pci0000:00/0000:00:04.0/virtio1/block/vda/vda14
    lrwxrwxrwx 1 root root 0 Oct  1 08:09 vda15 -> ../../devices/pci0000:00/0000:00:04.0/virtio1/block/vda/vda15
    

  3. Tildeslash repo owner

    Thank you for data. It seems that for some reason, the mount entity was mapped to “root” instead of “vda1”. The “root” can come from filesystem label / device mapping.

    it would be best if you can provide us with access to some virtual machine, where the problem occurs (you can send details to support@mmonit.com).

    If no remote access is possible, please can you send yet the content of /etc/mtab?

  4. Tildeslash repo owner

    Thanks for update, i have reproduced the problem on ubuntu AWS.

    The filesystem is mounted using label:

    $ cat /etc/fstab
    LABEL=cloudimg-rootfs / ext4 defaults,discard 0 0
    

    Which points to e.g. /dev/xvda1:

    $ ls -l /dev/disk/by-label/
    lrwxrwxrwx 1 root root 11 Nov  1 14:43 cloudimg-rootfs -> ../../xvda1
    

    But the getmntent() API, which is used by Monit, and corresponds to content of /etc/mtab and /proc/mounts (which are links to /proc/self/mounts), shows /dev/root instead:

    $ cat /etc/mtab
    /dev/root / ext4 rw,relatime,discard 0 0

    ditto /proc/self/mountinfo:

    $ cat /proc/self/mountinfo
    24 1 202:1 / / rw,relatime shared:1 - ext4 /dev/root rw,discard
    

    The /dev/root is not symlink to /dev/xvda1, but an independent device entry for the same filesystem (with the same major/minor number):

    $ ls -l /dev/root /dev/xvda1
    brw------- 1 root root 202, 1 Nov  1 14:38 /dev/root
    brw-rw---- 1 root disk 202, 1 Nov  1 14:43 /dev/xvda1
    

    However, statistics is available via the xvda1 only:

    $ ls -l /sys/class/block
    …
    lrwxrwxrwx 1 root root 0 Nov  1 15:09 xvda1 -> ../../devices/vbd-768/block/xvda/xvda1
    

    => it’s quite tricky … procfs uses /dev/root, but sysfs uses /dev/xvda1 for the same device.

    It seems /proc/partitions could be the key for sysfs:

    $ cat /proc/partitions
    major minor  #blocks  name
    202        1    8387567 xvda1
    

    Will fix.

  5. Tildeslash repo owner

    1.) Fixed: Issue #937: If the filesystem check uses mountpoint instead of device and multiple devices are defined for the same filesystem with mismatch between /etc/mtab and device name in disk statistics path, Monit reported error and disk activity was not reported.

    2.) refactor the VxFS disk activity support: uses the common interface now, no need for special callback (revert most of code added on issue #877 - it was the same root cause for VxFS as for issue #937)

    → <<cset 7469b8c86d66>>

  6. Dev Dua

    Hi!

    I am still experiencing this error on Ubuntu 20.04 on AWS EC2.

    [UTC Jan 21 07:52:31] error    : filesystem statistic error: cannot read /sys/class/block/root/stat -- No such file or directory
    

    System info -

    # lsb_release -a
    No LSB modules are available.
    Distributor ID: Ubuntu
    Description:    Ubuntu 20.04.3 LTS
    Release:        20.04
    Codename:       focal
    # monit --version
    This is Monit version 5.26.0
    Built with ssl, with ipv6, with compression, with pam and with large files
    Copyright (C) 2001-2019 Tildeslash Ltd. All Rights Reserved.
    

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