Nov 20, 2017

The bind mounts on Linux - remount part of the file hierarchy somewhere else

The bind mounts

              Since Linux 2.4.0 it is possible to remount part of the file hierarchy somewhere else.  The call is:

                     mount --bind olddir newdir

              or by using this fstab entry:

                     /olddir /newdir none bind

              After this call the same contents are accessible in two places.  One can also remount a single file (on a single file).  It's also possible to use the bind  mount  to
              create a mountpoint from a regular directory, for example:

                     mount --bind foo foo

              The  bind mount call attaches only (part of) a single filesystem, not possible submounts.  The entire file hierarchy including submounts is attached a second place by

                     mount --rbind olddir newdir

              Note that the filesystem mount options will remain the same as those on the original mount point,  and  cannot  be  changed  by  passing  the  -o  option  along  with
              --bind/--rbind.  The mount options can be changed by a separate remount command, for example:

                     mount --bind olddir newdir
                     mount -o remount,ro newdir

              Note  that the behavior of the remount operation depends on the /etc/mtab file.  The first command stores the 'bind' flag in the /etc/mtab file and the second command
              reads the flag from the file.  If you have a system without the /etc/mtab file or if you explicitly define source and target for the remount  command  (then  mount(8)
              does not read /etc/mtab), then you have to use the bind flag (or option) for the remount command too.  For example:

                     mount --bind olddir newdir
                     mount -o remount,ro,bind olddir newdir

              Note  that remount,ro,bind will create a read-only mountpoint (VFS entry), but the original filesystem superblock will still be writable, meaning that the olddir will
              be writable, but the newdir will be read-only.

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