15 managing disk - understanding /etc/fstab xfs_admin mount -o rw,remount /

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the classical way to mount filelsystems automatically is to use fstab
there is another method which is based on systemd

 [root@svn ~]# cat /etc/fstab

 #
# /etc/fstab
# Created by anaconda on Wed Apr 26 17:25:59 2017
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
/dev/mapper/cl-root     /                       xfs     defaults        0 0
UUID=6e3708b2-09ea-43f3-b0f0-9b43a157b3b1 /boot                   xfs     defaults        0 0
/dev/mapper/cl-swap     swap                    swap    defaults        0 0
[root@svn ~]# 

#description
/boot - where you want to mount filesystem
xfs - filesystem
defaults - mount option
0 - backup (set 1 if filesystem needs to be backed up)
0 - fsck - filesystem check
As the filesystems in /etc/fstab will eventually be mounted using mount(8) it is not surprising that the options field simply contains a comma-separated list of options which will be passed directly to mount when it tries to mount the filesystem.
The options common to all filesystems are:
auto / noauto
With the auto option, the device will be mounted automatically at bootup or when the mount -a command is issued. auto is the default option. If you do not want the device to be mounted automatically, use the noauto option in /etc/fstab. With noauto, the device can be only mounted explicitly.
dev / nodev
Interpret/do not interpret block special devices on the filesystem.
exec / noexec
exec lets you execute binaries that are on that partition, whereas noexec does not let you do that. noexec might be useful for a partition that contains no binaries, like /var, or contains binaries you do not want to execute on your system, or that cannot even be executed on your system, as might be the case of a Windows partition.
rw / ro
Mount the filesystem in either read write or read only mode. Explicitly defining a file system as rw can alleviate some problems in file systems that default to read only, as can be the case with floppies or NTFS partitions.
sync / async
How the input and output to the filesystem should be done. sync means it is done synchronously. If you look at the example fstab, you will notice that this is the option used with the floppy. In plain English, this means that when you, for example, copy a file to the floppy, the changes are physically written to the floppy at the same time you issue the copy command.
suid / nosuid
Permit/Block the operation of suid, and sgid bits.
user / users / nouser
user permits any user to mount the filesystem. This automatically implies noexec, nosuid, nodev unless overridden. If nouser is specified, only root can mount the filesystem. If users is specified, every user in group users will be able to unmount the volume.
defaults
Use default settings. Default settings are defined per file system at the file system level. For ext3 file systems these can be set with the tune2fs command. The normal default for Ext3 file systems is equivalent to rw,suid,dev,exec,auto,nouser,async(no acl support). Modern Red Hat based systems set acl support as default on the root file system but not on user created Ext3 file systems. Some file systems such as XFS enable acls by default. Default file system mount attributes can be overridden in /etc/fstab.
owner (Linux-specific)
Permit the owner of device to mount.

Example[edit]

The following is an example of an fstab file on a typical Linux system.
# device-spec   mount-point     fs-type      options                                          dump pass
LABEL=/         /               ext4         defaults                                            1 1
/dev/sda6       none            swap         defaults                                            0 0
none            /dev/pts        devpts       gid=5,mode=620                                      0 0
none            /proc           proc         defaults                                            0 0
none            /dev/shm        tmpfs        defaults                                            0 0

# Removable media
/dev/cdrom      /mnt/cdrom      udf,iso9660  noauto,owner,ro                                     0 0

# NTFS Windows 7 partition
/dev/sda1       /mnt/Windows    ntfs-3g      quiet,defaults,locale=en_US.utf8,umask=0,noexec     0 0

# Partition shared by Windows and Linux
/dev/sda7       /mnt/shared     vfat         umask=000                                           0 0

# mounting tmpfs
tmpfs           /mnt/tmpfschk   tmpfs        size=100m                                           0 0

# mounting cifs
//pingu/ashare  /store/pingu    cifs         credentials=/root/smbpass.txt                       0 0

# mounting NFS
pingu:/store    /store          nfs          rw                                                  0 0
The order of records in fstab is important because fsck(8), mount(8), and umount(8) sequentially iterate through fstab doing their thing.[1]
Blank lines and comment lines beginning with a "#" are ignored.
The space- or tab-separated fields within each row (typically aligned in columns, as above, but this is not a requirement) must appear in a specific order, as follows:
  1. device-spec – The device name, label, UUID, or other means of specifying the partition or data source this entry refers to.
  2. mount-point – Where the contents of the device may be accessed after mounting; for swap partitions or files, this is set to none.
  3. fs-type – The type of file system to be mounted.
  4. options – Options describing various other aspects of the file system, such as whether it is automatically mounted at boot, which users may mount or access it, whether it may be written to or only read from, its size, and so forth; the special option defaults refers to a predetermined set of options depending on the file system type.
  5. dump – A number indicating whether and how often the file system should be backed up by the dump program; a zero indicates the file system will never be automatically backed up.
  6. pass – A number indicating the order in which the fsck program will check the devices for errors at boot time; this is 1 for the root file system and either 2 (meaning check after root) or 0 (do not check) for all other devices.

[root@svn ~]# blkid
/dev/sda1: UUID="6e3708b2-09ea-43f3-b0f0-9b43a157b3b1" TYPE="xfs" 
/dev/sda2: UUID="98h644-2gqM-2SsL-q0lA-qUEE-Ivz8-TUZULc" TYPE="LVM2_member" 
/dev/sdb1: LABEL="myfs" UUID="05dbcff3-e639-40cb-bbe7-0da595186f1a" TYPE="xfs" 
/dev/sr0: UUID="2016-12-05-13-52-39-00" LABEL="CentOS 7 x86_64" TYPE="iso9660" PTTYPE="dos" 
/dev/mapper/cl-root: UUID="da05cdf9-1418-4e5a-80d7-7757ab1b9a9d" TYPE="xfs" 
/dev/mapper/cl-swap: UUID="8ac067b9-d1e7-4180-9a2f-2a3a929362f4" TYPE="swap" 
[root@svn ~]# 


UUID never changes, that why it's a good idea to mount file systems based on UUID

[root@svn ~]# cat /etc/fstab 

 #
# /etc/fstab
# Created by anaconda on Wed Apr 26 17:25:59 2017
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
/dev/mapper/cl-root     /                       xfs     defaults        0 0
UUID=6e3708b2-09ea-43f3-b0f0-9b43a157b3b1 /boot                   xfs     defaults        0 0
/dev/mapper/cl-swap     swap                    swap    defaults        0 0
/dev/sdb1 /data xfs defaults 1 2
[root@svn ~]# 

[root@svn ~]# mount -a
mount: mount point /data does not exist
[root@svn ~]# 

[root@svn ~]# mkdir /data
[root@svn ~]# mount -a
[root@svn ~]# mount | grep ^/dev/
/dev/mapper/cl-root on / type xfs (rw,relatime,seclabel,attr2,inode64,noquota)
/dev/sda1 on /boot type xfs (rw,relatime,seclabel,attr2,inode64,noquota)
/dev/sr0 on /run/media/svn/CentOS 7 x86_64 type iso9660 (ro,nosuid,nodev,relatime,uid=1000,gid=1000,iocharset=utf8,mode=0400,dmode=0500,uhelper=udisks2)
/dev/sdb1 on /data type xfs (rw,relatime,seclabel,attr2,inode64,noquota)
[root@svn ~]# 


# /etc/fstab
# Created by anaconda on Wed Apr 26 17:25:59 2017
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
/dev/mapper/cl-root     /                       xfs     defaults        0 0
UUID=6e3708b2-09ea-43f3-b0f0-9b43a157b3b1 /boot                   xfs     defaults        0 0
/dev/mapper/cl-swap     swap                    swap    defaults        0 0
LABEL=myfs /data xfs defaults 1 2
[root@svn ~]# 

labels and UUIDs is more robust as it safe mounting in case of any storage re-organization

LABEL can be added with xfs_admin command

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