How to unmount a busy device

Question

I ve got some samba drives that are being accessed by multiple users daily  I already have code to recognize shared drives  from a SQL table  and mount them in a special directory where all users can access them    I want to know  if I remove a drive from my SQL table  effectively taking it offline  how  or even is  there a way to unmount a busy device  So far I ve found that any form of umount does not work    Ignoring the possibility of destroying data - is it possible to unmount a device that is currently being read

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Niche Answer     If you have a zfs pool on that device  at least when it s a file-based pool  lsof will not show the usage  But you can simply run  sudo zpool export mypoo   and then unmount

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Try the following  but before running it note that the -k flag will kill any running processes keeping the device busy   The -i flag makes fuser ask before killing   fuser -kim  address    kill any processes accessing file unmount  address

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Another alternative when anything works is editing  etc fstab  adding noauto flag and rebooting the machine  The device won t be mounted  and when you re finished doing whatever  remove flag and reboot again

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Make sure that you aren t still in the mounted device when you are trying to umount

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Avoid umount -l  At the time of writing  the top-voted answer recommends using umount -l   umount -l is dangerous or at best unsafe  In summary    It doesn t actually unmount the device  it just removes the filesystem from the namespace  Writes to open files can continue  It can cause btrfs filesystem corruption   Work around   alternative  The useful behaviour of umount -l is hiding the filesystem from access by absolute pathnames  thereby minimising further moutpoint usage   This same behaviour can be achieved by mounting an empty directory with permissions 000 over the directory to be unmounted   Then any new accesses to filenames in the below the mountpoint will hit the newly overlaid directory with zero permissions - new blockers to the unmount are thereby prevented   First try to remount ro  The major unmount achievement to be unlocked is the read-only remount  When you gain the remount ro badge  you know that    All pending data has been written to disk All future write attempts will fail The data is in a consistent state  should you need to physcially disconnect the device    mount -o remount ro  dev device is guaranteed to fail if there are files open for writing  so try that straight up  You may be feeling lucky  punk   If you are unlucky  focus only on processes with files open for writing   lsof  f --  dev  lt devicename gt    awk  NR  1     4   0-9   uw -      You should then be able to remount the device read-only and ensure a consistent state   If you can t remount read-only at this point  investigate some of the other possible causes listed here   Read-only re-mount achievement unlocked    Congratulations  your data on the mountpoint is now consistent and protected from future writing   Why fuser is inferior to lsof  Why not use use fuser earlier  Well  you could have  but  fuser operates upon a directory  not a device  so if you wanted to remove the mountpoint from the file name space and still use fuser  you d need to    Temporarily duplicate the mountpoint with mount -o bind  media hdd  mnt to another location Hide the original mount point and block the namespace    Here s how   null dir   sudo mktemp --directory --tmpdir empty XXXXX   sudo chmod 000   null dir     A request to remount ro will fail on a  -o bind ro  duplicate if there are   still files open for writing on the original as each mounted instance is   checked   https   unix stackexchange com a 386570 143394   So  avoid remount  and bind mount instead  sudo mount -o bind ro   original    original duplicate     Don t propagate mirror the empty directory just about hide the original sudo mount --make-private   original duplicate     Hide the original mountpoint sudo mount -o bind ro   null dir    original    You d then have    The original namespace hidden  no more files could be opened  the problem can t get worse  A duplicate bind mounted directory  as opposed to a device  on which to run fuser    This is more convoluted 1   but allows you to use   fuser -vmMkiw  lt mountpoint gt    which will interactively ask to kill the processes with files open for writing  Of course  you could do this without hiding the mount point at all  but the above mimicks umount -l  without any of the dangers   The -w switch restricts to writing processes  and the -i is interactive  so after a read-only remount  if you re it a hurry you could then use   fuser -vmMk  lt mountpoint gt    to kill all remaining processes with files open under the mountpoint   Hopefully at this point  you can unmount the device   You ll need to run umount on the mountpoint twice if you ve bind mounted a mode 000 directory on top    Or use   fuser -vmMki  lt mountpoint gt    to interactively kill the remaining read-only processes blocking the unmount   Dammit  I still get target is busy   Open files aren t the only unmount blocker  See here and here for other causes and their remedies   Even if you ve got some lurking gremlin which is preventing you from fully unmounting the device  you have at least got your filesystem in a consistent state    You can then use lsof  f --  dev device to list all processes with open files on the device containing the filesystem  and then kill them      1  It is less convoluted to use mount --move  but that requires mount --make-private  parent-mount-point which has implications  Basically  if the mountpoint is mounted under the   filesystem  you d want to avoid this

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If possible  let us locate identify the busy process  kill that process and then unmount the samba share  drive to minimize damage   lsof   grep   lt mountpoint of  dev sda1 gt    or whatever the mounted device is   pkill target process  kills busy proc  by name   kill PID   killall target process   umount  dev sda1  or whatever the mounted device is

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Someone has mentioned that if you are using terminal and your current directory is inside the path which you want to unmount  you will get the error  As a complementary  in this case  your lsof   grep path-to-be-unmounted must have below output   bash     path-to-be-unmounted

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Just in case someone has the same pb    I couldn t unmount the mount point  here  mnt  of a chroot jail  Here are the commands I typed to investigate      umount  mnt umount   mnt  target is busy    df -h   grep  mnt  dev mapper VGTout-rootFS  4 8G  976M  3 6G  22   mnt   fuser -vm  mnt                       USER        PID ACCESS COMMAND  mnt                 root     kernel mount  mnt   lsof  f --  dev mapper VGTout-rootFS    As you can notice  even lsof returns nothing  Then I had the idea to type this     df -ah   grep  mnt  dev mapper VGTout-rootFS  4 8G  976M  3 6G  22   mnt dev                        2 9G     0  2 9G   0   mnt dev   umount  mnt dev   umount  mnt   df -ah   grep  mnt    Here it was a  mnt dev bind to  dev that I had created to be able to repair my system inside from the chroot jail  After umounting it  my pb  is now solved

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Check out umount2      Linux 2 1 116 added the umount2   system call  which  like umount      unmounts a target  but allows additional flags controlling the   behaviour of the operation       MNT FORCE  since Linux 2 1 116  Force unmount even if busy   Only for   NFS mounts   MNT DETACH  since Linux 2 4 11  Perform a lazy unmount    make the mount point unavailable for new accesses  and actually   perform the unmount when the mount point ceases to be busy  MNT EXPIRE    since Linux 2 6 8  Mark the mount point as expired  If a mount point   is not currently in use  then an initial call to umount2   with this   flag fails with the error EAGAIN  but marks the mount point as   expired  The mount point remains expired as long as it isn t accessed   by any process  A second umount2   call specifying MNT EXPIRE unmounts   an expired mount point  This flag cannot be specified with either   MNT FORCE or MNT DETACH  Return Value      On success  zero is returned  On error  -1 is returned  and errno is   set appropriately

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Check for exported NFS file systems with exportfs -v  If found  remove with exportfs -d share  directory  These don t show up in the fuser lsof listing  and can prevent umount from succeeding

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YES   There is a way to detach a busy device immediately - even if it is busy and cannot be unmounted forcefully  You may cleanup all later  umount -l  PATH OF BUSY-DEVICE umount -f  PATH OF BUSY-NFS  NETWORK-FILE-SYSTEM   NOTE CAUTION  These commands can disrupt a running process  cause data loss OR corrupt open files  Programs accessing target DEVICE NFS files may throw errors OR could not work properly after force unmount  Do not execute above umount commands when inside mounted path  Folder Drive Device  itself  First  you may use pwd command to validate your current directory path  which should not be the mounted path   then use cd command to get out of the mounted path - to unmount it later using above commands

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Multiple mounts inside a folder An additional reason could be a secondary mount inside your primary mount folder  e g  after you worked on an SD card for an embedded device    mount  dev sdb2  mnt         root partition which contains  boot   mount  dev sdb1  mnt boot    boot partition  Unmounting  mnt will fail    umount  mnt umount   mnt  target is busy   First we have to unmount the boot folder and then the root    umount  mnt boot   umount  mnt

[linux] How to unmount a busy device

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