貳. 檔案系統掛載 (Mount)

所有的檔案都存在不同的 partition 裡面﹐但 Linux 的檔案系統和Windows 的不太一樣,在windows裡﹐每一個 partition 都有各自的磁碟代號﹐從 C﹕開始一直到 Z﹕﹐超過了就讀不到了。在每一個磁碟代號裡面﹐都各自有其獨立的根目錄和子目錄﹐您必須明確指定哪一個磁碟代號才能找到相關檔案。

在 Linux 裡面, 沒有磁碟代號﹐一切都從 / 開始﹐/ 必須有自己的獨立的 partition﹐但其它 partition 則一律掛在 / 下面的目錄或子目錄中﹐我們稱這些用來掛載 partition 的目錄為 mount point﹐而將這些 partition 掛入相應的 mount point 的動作稱為 mount (掛載)。在掛入新的 parition 的時候﹐是無需重新開機的﹐任何時候都可以進行。當 partition 不再使用的時候﹐您也可以將之進行卸載(umount)。

在進行掛載的過程中﹐有一些規則我們是必須遵守的﹕

• 根目錄 / 是必須掛載的﹐而且一定要先於其它 mount point 被掛載進來。
• 其它 mount point 必須為已建立的目錄﹐可以任意指定﹐但一定要遵守必須的系統目錄架構原則。
• 所有 mount point 在同一時間之內﹐只能掛載一次。
• If you mount a file system to a directory that already has files and subdirectories in it, those will be bypassed, giving you access only to the files in the mounted file system. Unmounting the file system restores access to the original directory files.
• 所有 partition 在同一時間之內﹐只能掛載一次。
• 如若進行卸載﹐您必須先將工作目錄移到 mount point及其子目錄之外。

(1). Device Files: /dev

.To mount a file system, you have to specify its device name. The interfaces to devices that may be attached to your system are provided by special files known as device files. The names of these device files are the device names.

.Device files are located in the /dev directories and usually have abbreviated names ending with the number of the device.

     For example, the device name for your floppy drive is fd0 and is located in the directory /dev. /dev/fd0 references your floppy drive. If you have more than one floppy drive, they are represented by fd1, fd2, and so on.

.A complete listing of all device names can be found in the devices file located in the linux/doc/device-list directory at the www.kernel.org Web site. Table 10-1 lists several of the commonly used device names.

# mount /mnt/dos

    # umount device-or-mountpoint

# umount /dev/fd0

# umount /mydir

# mount /dev/hdc /mnt/cdrom
# cd /mnt/cdrom
# umount /mnt/cdrom
umount: /dev/hdd: device is busy
# cd /root
# umount /mnt/cdrom

      # mount /mnt/cdrom

      # umount /mnt/cdrom

      # mount /dev/hdc /mydir

     # mount -t ext3 /dev/hda4 /mnt/mydata

    # mount -t vfat /dev/hda1 /mnt/windows

(3). Mount Configuration: /etc/fstab

.You can simplify the process by placing mount information in the /etc/fstab configuration file. Using entries in this file, you can have certain file systems automatically mounted whenever your system boots.

.You can do this by directly and carefully editing the /etc/fstab file to type in a new entry.

.An entry in an fstab file contains several fields, each separated by a space or tab. These are described in the sequence shown here:

      <device> <mountpoint> <filesystemtype> <options> <dump> <fsck>

     1. The first field is the device name to be mounted. This usually begins with /dev, such as /dev/hda3 for the third hard disk partition.

     2. The next field is the directory in your file structure where you want the file system on this device to be attached.

     3. The third field is the type of file system being mounted. Table 10-3 provides a list of all the different types you can mount. The type for a standard Linux hard disk partition is ext3. The next example shows an entry for the main Linux hard disk partition. This entry is mounted at the root directory, /, and has a file type of ext2.

               3.1 The next example shows an entry for the main Linux hard disk partition. This entry is mounted at the root directory, /, and has a file type of ext2.

                      /dev/hda3       /                  ext2        defaults                   0   1

               3.2 The type of file system on a floppy drive could vary, often depending on the type floppy you are trying to mount. For example, you may want to read a Windows formatted floppy disk at one time and a Linux formatted floppy disk at another time. For this reason, the file system type specified for the floppy device is auto. With this option, the type of file system formatted on the floppy disk will be automatically detected and the appropriate file system type used.

            /dev/fd0  /mnt/floppy  auto   defaults,noauto   0 0

          4. The 4th field lists the different options for mounting the file system. You can list specific options next to each other separated by a comma (no spaces).

               4.1 You can specify a default set of options by simply entering defaults. The defaults option specifies that a device is read/write (rw), asynchronous (async), a block device (dev), cannot be mounted by ordinary users (user) , and that programs can be executed on it (exec).

               4.2  CD-ROM only has a few options listed for it: ro and noauto. ro specifies this is read-only; and noauto specifies this is not automatically mounted. The noauto option is used with both CD-ROMs and floppy drives, so they won't automatically mount because you do not know if you have anything in them when you start up.

               4.3 Table 10-4 lists the options for mounting a file system. 

               4.4 An example of CD-ROM and floppy drive entries follows. Notice the type for a CD-ROM file system is different from a hard disk partition, iso9660. The floppy drive entry also has all the default options of the hard disk partitions, with the exception that it is not automatically mounted.

             /dev/hdc /mnt/cdrom   iso9660 ro,noauto          0 0
             /dev/fd0 /mnt/floppy  auto    defaults,noauto    0 0

              5. The last two fields consist of an integer value. The first one is used by the dump command to determine if a file system needs to be dumped, backing up the file system. The last one is used by fsck to see if a file system should be checked at reboot and in what order. If the field has a value of 1, it indicates a boot partition, and 2 indicates other partitions. The 0 value means the fsck needn't check the file system.

.Example, A copy of an /etc/fstab file is shown here. Notice the first line is comment. All comment lines begin with a #. The entry for the /proc file system is a special entry used by your Linux operating system for managing its processes, and it is not an actual device. The /proc and swap partition entries are particularly critical.

/etc/fstab

# <device> <mountpoint> <filesystemtype> <options> <dump><fsck>
/dev/hda3  /            ext3             defaults  0     1
/dev/hdc   /mnt/cdrom   iso9660          ro,noauto 0     0
/dev/fd0   /mnt/floppy  auto             defaults,
                                         noauto    0 
/proc      /proc        proc             defaults
/dev/hda2  none         swap             sw
/dev/hda1  /mnt/windows vfat             defaults  0     0

.You can mount either MS-DOS or Windows 95 partitions onto your Linux file structure. You only have to specify the file type of vfat for Windows 95 and msdos for MS-DOS. To do this, you need to put an entry for your Windows partitions in your /etc/fstab file and give it the defaults option or be sure to include an auto option. The /mnt/windows directory would be a logical choice. The next example shows a standard MS-DOS partition entry for an /etc/fstab file.

         /dev/hda1 /mnt/windows vfat defaults 0 0

.If your /etc/fstab file ever becomes a line gets deleted accidentally then your system will boot into a maintenance mode, giving you read-only access to your partitions. To gain read/write access so you can fix your /etc/fstab file, you have to remount your main partition. The following command performs such an operation:

         # mount -n -o remount,rw /

.If you would want to make the CD-ROM user mountable, add the user option.

/dev/hdc /mnt/cdrom iso9660 ro,noauto,user 0 0

.The "automatic" mounting of file systems from /etc/fstab is actually implemented by executing a mount -a command in the /etc/rc.d/rc.sysinit file that is run whenever you boot. The mount -a command will mount any file system listed in your /etc/fstab file that does not have a noauto option. The umount -a option will unmount the file systems in /etc/fstab (which is executed when you shut down your system).

參. 磁碟空間管理(Disk Quota)

On your Linux system, unused disk space is held as a common resource that each user can access as they need it. As a user creates more files, they take the space they need from the pool of available disk space. In this sense, all the users are sharing this one resource of unused disk space. However, if one user were to use up all the remaining disk space, then none of the other users would be able to create files.

To counter this problem, you can create disk quotas on particular users, limiting the amount of available disk space they can use.

一. Concept

2. Soft Limit 、 Hard Limit & Grace Period
      Soft Limit、Hard Limit & Grace Period是管理者利用quota觀念，對user使用磁碟所設的三個限制。

    .Soft Limit
          Soft Limit 是user使用所能使用磁碟空間的大小，Soft Limit是一個警戒線，使用者使用磁碟空間超過這個警戒線，就會開始收到系統所給的警告。

    .Hard Limit
          Hard Limit代表的是user對於磁碟空間使用的絕對上限，使用者使用磁碟空間無法超越此一上限。

    .Grace period
          Grace period是user可被容許超過soft limit 的時間，假設Grace period設定為3天，超過soft limit的user在三天內未處理超過limit的檔案的話，user將無法再使用任何磁碟空間。
　

Quota Setup: Quotas are enabled using the quotacheck and quotaon programs. On Fedora core, they are executed in the /etc/rc.d/rc.sysinit script, which is run whenever you start up your system.

Step1: To enables quota controls for users and groups

• Each partition then needs to be mounted with the quota options, usrquota or grpquota. usrquota enables quota controls for users, and grpquota works for groups. These options are usually placed in the mount entry in the /etc/fstab file for a particular partition.

  For example, to mount the /dev/hda6 hard disk partition mounted to the /home directory with support for user and group quotas, you would require a mount entry like the following:

  /dev/hda6 /home ext3 defaults,usrquota,grpquota 1 1

  
  For example, modify /etc/fstab as following:
   
  /dev/hda1        /umsdos            defaults                   1    1
  /dev/hda2        /home1   ext3    derfaults,usrquota    1    1
  /dev/swapfile   swap      swap   defaults                  1    1
  none               /proc       proc    defaults                  1    1
  　

• 注意不可跳行, 否則會無法開機.
  　

Step 2: To create quota.user and quota.group files, these are the quota databases used to hold the quota information for each user and group.

• 方法一:  You can create these files by running the quotacheck command with the -a option or the device name of the file system where you want to enable quotas.
• 方法二: You could create these files by yourself.

           For example, create quota record "quota.user" on the base of new file system(/home1)
                                 cc201:/etc#touch  /home1/quota.user
                                 cc201:/etc#chmod  600 /home1/quota.user
　

Step3: To reboot

• Reboot for the change we have made.
  系統重開之後，/home1 file system就會被自動mount，同時具有disk quota 功能。

Enforce disk quota on users

Step 1: 空間設定

• 下面例子設定user woody可以使用file system 2 M or 4000 blocks空間(soft limit)，絕對上限
  為2.25M or 4500 blocks(hard limit)﹔可以儲存檔案數為400個，檔案個數絕對上限為420個。
  cc201:~# edquota woody
  Quotas for user woody:
  /dev/hda2: blocks in use: 0, limits (soft = 4000, hard = 4500)
  inodes in use: 0, limits (soft = 400, hard = 420)
  ~
  ~
  ~

Step 2: 時間設定

• 下面例子設定系統block(空間)Grace Period為4天，inode(file數) Grace Period為3天。
• cc201:/home1# edquota -t
  Time units may be: days, hours, minutes, or seconds
  Grace period before enforcing soft limits for users:
  /dev/hda2: block grace period:4 days, file grace period: 3 days
  ~
  ~
  ~
  　

其他相關的命令

Each partition that has quotas enabled will have its own quota database. You can check the validity of your quota database with the quotacheck command. You can turn quotas on an off using the quotaon and quotaoff commands. When you start up your system, quotacheck is run to check the quota databases and then quotaon is run to turn on quotas.

As the system administrator, you can use the repquota command to generate a summary of memory usage, checking to see what users are approaching or exceeding quota limits. Individual users can use the quota command to check their memory use and how much disk space they have left in their quota.

• Enable Disk Quota
  cc201:/home1# quotaon –a
  quota on之後系統才會在每一次user使用磁碟時，立即check disk quota。
  //加上-a是自動尋找有設定quota的linux檔案系統。
  
  　
• Run Quota Check Manually
  cc201:/home1# quotacheck -a
  通常於系統開機時進行之，以確定disk quota記錄是符合最新狀況的。
  //加上-a是自動檢查所有已掛載的linux檔案系統。
  　

        cc201:/home1# quotacheck -af
          //加上-f是強制執行。
　

• Check quota usage – by user
  [woody@test /]$quota –v                                              #### woody檢查自己quota使用情形
  Disk quotas for user woody (uid 501):
  Filesystem blocks   quota   limit   grace   files   quota   limit   grace
  /dev/hda2     270     4000   4200               68     400     450
  //user woody has used 270 blocks of space and 68 files (Still under soft limt, grace period is empty!)
  
  　
• Check quota usage – by root
  cc201:/home1/woody# repquota -a   /dev/hda2               #### root可查詢所有帳號使用空間的情形
  Block limits File limits
  User          used   soft   hard   grace   used   soft   hard   grace
  root              6       0        0                    6       0       0
  woody       270  4000  4200                68     400  450
  edu            131     0        0                     8       0       0
  cc201:/home1/woody#
  //請注意上例中user root及edu因未設quota所以soft & hard Limit均為0。
  //Limit設為零表示該user不受disk quota限制。