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[深入理解linux] 3. what is /dev/dm-0

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问题描述#

使用df -h 发现/dev/dm-0 已使用100% 

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root@xxx-dev:# df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/dm-0       7.8G  7.8G   20K 100% /
udev             10M     0   10M   0% /dev
tmpfs           3.2G  329M  2.9G  11% /run
tmpfs           7.9G   72K  7.9G   1% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           7.9G     0  7.9G   0% /sys/fs/cgroup
/dev/vdc1       200G   46G  155G  23% /home
/dev/vda1       236M   34M  190M  15% /boot

分析#

  1. dmsetup查看对应的设备 

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    root@xxx-dev:# dmsetup info /dev/dm-0
    Name:              disk1-root
    State:             ACTIVE
    Read Ahead:        256
    Tables present:    LIVE
    Open count:        1
    Event number:      0
    Major, minor:      253, 0
    Number of targets: 1
    UUID: LVM-ytdyq65oG0cO4O95LsrZ1asrcdQSqvLBzepNHp4miyl5hLFuXzisTCvSzuUQihY5

  2. lsblk查看disks树形结构 

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    root@xxx-dev:# lsblk

    NAME           MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
    vda            254:0    0    8G  0 disk
    ├─vda1         254:1    0  243M  0 part /boot
    └─vda2         254:2    0  7.8G  0 part
      └─disk1-root 253:0    0  7.8G  0 lvm  /      
    vdb            254:16   0    2G  0 disk [SWAP]
    vdc            254:32   0  200G  0 disk
    └─vdc1         254:33   0  200G  0 part /home

  3. lvdisplay显示lvm详细信息 

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    root@xxx-dev:# lvdisplay
      --- Logical volume ---
      LV Path                /dev/disk1/root
      LV Name                root
      VG Name                disk1
      LV UUID                zepNHp-4miy-l5hL-FuXz-isTC-vSzu-UQihY5
      LV Write Access        read/write
      LV Creation host, time debian-owncloud-opt, 2015-06-15 16:00:36 +0800
      LV Status              available
      # open                 1
      LV Size                7.76 GiB
      Current LE             1986
      Segments               1
      Allocation             inherit
      Read ahead sectors     auto
      - currently set to     256
      Block device           253:0

    root@xxx-dev:# lvdisplay|awk  '/LV Name/{n=$3} /Block device/{d=$3; sub(".*:","dm-",d); print d,n;}'  (使用awk提取每个dm映射的地址)
    dm-0 root

  4. iostat查看读写性能 

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    root@xxx-dev:# iostat
    Linux 3.16.0-4-amd64 (xxx-dev)         06/10/2021      _x86_64_        (8 CPU)

    avg-cpu:  %user   %nice %system %iowait  %steal   %idle
               2.22    0.00    0.64    0.02    0.09   97.02

    Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
    vda               2.56         1.21        15.32   26871618  338839230
    vdb               0.14         0.21         0.71    4626304   15647968
    vdc               0.34         1.10         9.18   24298844  202995646
    dm-0              2.50         1.21        15.32   26828725  338839189

  5. dm对应的设备，一般都位于 /dev/mapper下面 

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    root@xxx-dev:# ls -l /dev/mapper
    total 0
    crw------- 1 root root 10, 236 Sep 27  2020 control
    lrwxrwxrwx 1 root root       7 Sep 27  2020 disk1-root -> ../dm-0

reference#

  1. What is this dm-0 device?
  2. dm0 in iostat???