为什么文件系统比逻辑卷大?

为什么文件系统比逻辑卷大?

在机器上,有一个大小为 500MiB 的逻辑卷 /dev/lvm/tmp,其大小相同的分区 /dev/mapper/tmp 也安装在 /tmp 上。

检查逻辑卷:

# lvdisplay /dev/lvm/tmp
  --- Logical volume ---
  LV Path                /dev/lvm/tmp
  LV Name                tmp
  VG Name                lvm
  LV UUID                R1a6Yq-b5jt-Y5rF-sCma-50yh-rg19-LB21qs
  LV Write Access        read/write
  LV Creation host, time anarchy, 2021-04-01 13:28:12 -0400
  LV Status              available
  # open                 1
  LV Size                500.00 MiB
  Current LE             125
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:0

但是,检查/tmp的大小显示其大小为7.8G:

# df -h /tmp
Filesystem       Size  Used Avail Use% Mounted on
/dev/mapper/tmp  7.8G   34M  7.8G   1% /tmp

/etc/crypttab 中的相关行:

tmp        /dev/lvm/tmp        /dev/urandom tmp,cipher=aes-xts-plain64,size=256

/etc/fstab 中的相关行:

/dev/mapper/tmp        /tmp        tmpfs   defaults       0 0 

附加信息:

# cryptsetup status tmp
/dev/mapper/tmp is active.
  type:    PLAIN
  cipher:  aes-xts-plain64
  keysize: 256 bits
  key location: dm-crypt
  device:  /dev/mapper/lvm-tmp
  sector size:  512
  offset:  0 sectors
  size:    1024000 sectors
  mode:    read/write

为什么/tmp的大小是7.8G而不是500M?

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