{"id":6,"date":"2018-11-18T21:36:00","date_gmt":"2018-11-18T21:36:00","guid":{"rendered":""},"modified":"2021-08-31T10:12:24","modified_gmt":"2021-08-31T09:12:24","slug":"create-loop-device-volumes-on-centos-7-from-files","status":"publish","type":"post","link":"https:\/\/www.srivittal.com\/wp\/?p=6","title":{"rendered":"Create Loop device volumes on CentOS 7 from files"},"content":{"rendered":"
It has been a while since I wrote my previous article. The substance for this article came from the video on YouTube here:<\/p>\n

https:\/\/www.youtube.com\/watch?v=dd08qoOyTCA<\/a><\/p>\n

What was I trying to do:<\/b><\/h4>\n

As a part of production docker configuration, I was playing with devicemapper storage drivers for creating a persistent storage area for containers. Docker documentation allows you to configure both block volumes and loop-lvm volumes. loop-lvm volumes are mainly used for testing purposes and hence the research.<\/p>\n

How does this work:<\/h4>\n
A bit of an intro to volumes in CentOS 7 Linux.<\/div>\n
<\/div>\n
One or more physical Volumes are added to Logical Volume Groups and then within the Logical Volume Group, one or more Logical volumes are created.<\/div>\n
<\/div>\n
In our case the situation is a bit different:<\/div>\n
<\/div>\n
Remember to run all the following commands as root user<\/div>\n
\n
    \n
  • We create a simple file called raid-0 using the touch command like so<\/li>\n<\/ul>\n

    touch raid-0<\/span><\/p>\n

      \n
    • Then we use the dd command to copy some portion of the root device to this file. We are creating a file that is 200 MB in size (2M * 100)<\/li>\n<\/ul>\n
      dd if=\/dev\/sda of=raid-0 bs=2M count=100<\/span><\/div>\n
        \n
      • We then copy this raid-0 file 3 times to create 4 files of 200MB each<\/li>\n<\/ul>\n
        cp raid-0 raid-1<\/span><\/div>\n
        cp raid-0 raid-2<\/span><\/div>\n
        cp raid-0 raid-3<\/span><\/div>\n
          \n
        • We then create 4 loop devices using the losetup command from these 4 files<\/li>\n<\/ul>\n
          losetup \/dev\/loop0 raid-0<\/span><\/div>\n
          losetup \/dev\/loop1 raid-1<\/span><\/div>\n
          losetup \/dev\/loop2 raid-2<\/span><\/div>\n
          losetup \/dev\/loop3 raid-3<\/span><\/div>\n
            \n
          • Verify that the devices are created correctly<\/li>\n<\/ul>\n
            \n
            [root@centos75 ~]# losetup -a<\/span><\/div>\n
            \/dev\/loop0: [64768]:17786836 (\/root\/raid-0)<\/span><\/div>\n
            \/dev\/loop1: [64768]:17786805 (\/root\/raid-1)<\/span><\/div>\n
            \/dev\/loop2: [64768]:17013266 (\/root\/raid-2)<\/span><\/div>\n
            \/dev\/loop3: [64768]:17013268 (\/root\/raid-3)<\/span><\/div>\n<\/div>\n
              \n
            • Create 4 physical volumes based on these loop devices<\/li>\n<\/ul>\n
              pvcreate \/dev\/loop{0,1,2,3}<\/span><\/div>\n
                \n
              • Verify that the physical devices exist. Ignore the original device volume \/dev\/sda2 below and also a step below that – which adds the 4 physical volumes to the same Logical Volume Group<\/li>\n<\/ul>\n
                \n
                [root@centos75 ~]# pvs<\/span><\/div>\n
                  PV         VG              Fmt  Attr PSize   PFree<\/span><\/div>\n
                  \/dev\/loop0 vg00            lvm2 a–  196.00m    0 <\/span><\/div>\n
                  \/dev\/loop1 vg00            lvm2 a–  196.00m    0 <\/span><\/div>\n
                  \/dev\/loop2 vg00            lvm2 a–  196.00m    0 <\/span><\/div>\n
                  \/dev\/loop3 vg00            lvm2 a–  196.00m    0 <\/span><\/div>\n
                  \/dev\/sda2  centos_centos75 lvm2 a–   18.00g    0 <\/span><\/div>\n<\/div>\n
                <\/div>\n
                  \n
                • Create a volume group out of these 4 physical devices now. See the above message after the command has been executed<\/li>\n<\/ul>\n
                  \n
                  vgcreate vg00 \/dev\/loop{0,1,2,3}<\/span><\/div>\n<\/div>\n
                    \n
                  • The following command will confirm the created Logical Volume Group. Ignore the root volume group starting with centos..<\/li>\n<\/ul>\n
                    \n
                    [root@centos75 ~]# vgs<\/span><\/div>\n
                      VG              #PV #LV #SN Attr   VSize   VFree<\/span><\/div>\n
                      centos_centos75   1   2   0 wz–n-  18.00g    0 <\/span><\/div>\n
                      vg00              4   3   0 wz–n- 784.00m    0 <\/span><\/div>\n<\/div>\n
                      \n
                    • Issuing a pvscan command will also show the full arrangement<\/li>\n<\/ul>\n
                      \n
                      [root@centos75 ~]# pvscan<\/span><\/div>\n
                        PV \/dev\/loop0   VG vg00              lvm2 [196.00 MiB \/ 0    free]<\/span><\/div>\n
                        PV \/dev\/loop1   VG vg00              lvm2 [196.00 MiB \/ 0    free]<\/span><\/div>\n
                        PV \/dev\/loop2   VG vg00              lvm2 [196.00 MiB \/ 0    free]<\/span><\/div>\n
                        PV \/dev\/loop3   VG vg00              lvm2 [196.00 MiB \/ 0    free]<\/span><\/div>\n
                        PV \/dev\/sda2    VG centos_centos75   lvm2 [18.00 GiB \/ 0    free]<\/span><\/div>\n
                        Total: 5 [<18 .77=\"\" 0=\"\" 5=\"\" font=\"\" gib=\"\" in=\"\" nbsp=\"\" no=\"\" use:=\"\" vg:=\"\"><\/18><\/span><\/div>\n<\/div>\n
                        \n
                      • Issue the following commands to create 3 different logical volumes within the same volume group to use the entire size of the volume group. Please note: <\/b>The maximum size that can be used by a volume group will be slightly less due to the amount of space taken for storing metadata about these volumes <\/li>\n<\/ul>\n
                        lvcreate -L 300M -n lv00 vg00<\/span><\/div>\n
                        lvcreate -L 150M -n lv01 vg00<\/span><\/div>\n
                        lvcreate -L 332M -n lv02 vg00<\/span><\/div>\n
                          \n
                        • Issuing an lsblk command shows the entire arrangement<\/li>\n<\/ul>\n
                          \n
                          [root@centos75 ~]# lsblk<\/span><\/div>\n
                          NAME        MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT<\/span><\/div>\n
                          sda           8:0    0   20G  0 disk <\/span><\/div>\n
                          \u251c\u2500sda1        8:1    0  1.9G  0 part \/boot<\/span><\/div>\n
                          \u2514\u2500sda2        8:2    0   18G  0 part <\/span><\/div>\n
                            \u251c\u2500centos_centos75-root<\/span><\/div>\n
                                      253:0    0   16G  0 lvm  \/<\/span><\/div>\n
                            \u2514\u2500centos_centos75-swap<\/span><\/div>\n
                                      253:1    0    2G  0 lvm  [SWAP]<\/span><\/div>\n
                          sdb           8:16   0    8G  0 disk <\/span><\/div>\n
                          sr0          11:0    1 55.3M  0 rom  \/run\/media\/user\/VBox_GAs_5.2.<\/span><\/div>\n
                          loop0         7:0    0  200M  0 loop <\/span><\/div>\n
                          \u2514\u2500vg00-lv00 253:2    0  300M  0 lvm  <\/span><\/div>\n
                          loop1         7:1    0  200M  0 loop <\/span><\/div>\n
                          \u251c\u2500vg00-lv00 253:2    0  300M  0 lvm  <\/span><\/div>\n
                          \u2514\u2500vg00-lv02 253:4    0  332M  0 lvm  <\/span><\/div>\n
                          loop2         7:2    0  200M  0 loop <\/span><\/div>\n
                          \u251c\u2500vg00-lv01 253:3    0  152M  0 lvm  <\/span><\/div>\n
                          \u2514\u2500vg00-lv02 253:4    0  332M  0 lvm  <\/span><\/div>\n
                          loop3         7:3    0  200M  0 loop <\/span><\/div>\n
                          \u2514\u2500vg00-lv02 253:4    0  332M  0 lvm  <\/span><\/div>\n<\/div>\n
                            \n
                          • As can be seen above, we have created 4 physical volumes called loop0, 1, 2 and 3. We have added all the 4 physical volumes to a single Logical volume group called vg00. Within the volume group called vg00, we have created 3 logical volumes lv00, lv01 and lv02 with 300M, 150M and 332M sizes respectively. Please note: <\/b> Logical volume lv00 spans physical volumes loop0 and loop1, Logical volume lv01 is contained within physical volume loop2 and logical volume lv02 spans physical volumes loop1, loop2 and loop3 – nice!<\/li>\n
                          • Issue the following command to see the entire arrangement<\/li>\n<\/ul>\n
                            \n
                            [root@centos75 ~]# lvmdiskscan<\/span><\/div>\n
                              \/dev\/loop0                [     200.00 MiB] LVM physical volume<\/span><\/div>\n
                              \/dev\/centos_centos75\/root [      16.00 GiB] <\/span><\/div>\n
                              \/dev\/loop1                [     200.00 MiB] LVM physical volume<\/span><\/div>\n
                              \/dev\/sda1                 [       1.86 GiB] <\/span><\/div>\n
                              \/dev\/centos_centos75\/swap [       2.00 GiB] <\/span><\/div>\n
                              \/dev\/loop2                [     200.00 MiB] LVM physical volume<\/span><\/div>\n
                              \/dev\/sda2                 [      18.00 GiB] LVM physical volume<\/span><\/div>\n
                              \/dev\/vg00\/lv00            [     300.00 MiB] <\/span><\/div>\n
                              \/dev\/loop3                [     200.00 MiB] LVM physical volume<\/span><\/div>\n
                              \/dev\/vg00\/lv01            [     152.00 MiB] <\/span><\/div>\n
                              \/dev\/vg00\/lv02            [     332.00 MiB] <\/span><\/div>\n
                              \/dev\/sdb                  [       8.00 GiB] <\/span><\/div>\n
                              3 disks<\/span><\/div>\n
                              4 partitions<\/span><\/div>\n
                              0 LVM physical volume whole disks<\/span><\/div>\n
                              5 LVM physical volumes<\/span><\/div>\n<\/div>\n
                              \n
                            • To format them using the xfs filesystem, issue the following commands<\/li>\n<\/ul>\n
                              \n
                              [root@centos75 ~]# mkfs.xfs \/dev\/vg00\/lv00<\/span><\/div>\n
                              meta-data=\/dev\/vg00\/lv00         isize=512    agcount=4, agsize=19200 blks<\/span><\/div>\n
                                       =                       sectsz=512   attr=2, projid32bit=1<\/span><\/div>\n
                                       =                       crc=1        finobt=0, sparse=0<\/span><\/div>\n
                              data     =                       bsize=4096   blocks=76800, imaxpct=25<\/span><\/div>\n
                                       =                       sunit=0      swidth=0 blks<\/span><\/div>\n
                              naming   =version 2              bsize=4096   ascii-ci=0 ftype=1<\/span><\/div>\n
                              log      =internal log           bsize=4096   blocks=855, version=2<\/span><\/div>\n
                                       =                       sectsz=512   sunit=0 blks, lazy-count=1<\/span><\/div>\n
                              realtime =none                   extsz=4096   blocks=0, rtextents=0<\/span><\/div>\n<\/div>\n
                              <\/div>\n
                                \n
                              • Repeat the same command for the remaining 2 logical volumes<\/li>\n<\/ul>\n
                                mkfs.xfs \/dev\/vg00\/lv01<\/span><\/div>\n
                                mkfs.xfs \/dev\/vg00\/lv02<\/span><\/div>\n
                                  \n
                                • If you want to mount these volumes into some branch do the following<\/li>\n<\/ul>\n
                                  mkdir \/lvm<\/span><\/div>\n
                                  mkdir \/lvm\/disk1<\/span><\/div>\n
                                  mkdir \/lvm\/disk2<\/span><\/div>\n
                                  mkdir \/lvm\/disk3<\/span><\/div>\n
                                    \n
                                  • Mount the volumes now<\/li>\n<\/ul>\n
                                    mount \/dev\/vg00\/lv00 \/lvm\/disk1<\/span>
                                    mount \/dev\/vg00\/lv01 \/lvm\/disk2<\/span>
                                    mount \/dev\/vg00\/lv02 \/lvm\/disk3<\/span><\/p>\n

                                    Now – how I am going to map this to devicemapper will be updated below shortly … Have to save this page before I lose power .. <\/p><\/div>\n

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                                    It has been a while since I wrote my previous article. The substance for this article came from the video on YouTube here: https:\/\/www.youtube.com\/watch?v=dd08qoOyTCA What was I trying to do: As a part of production docker configuration, I was playing with devicemapper storage drivers for creating a persistent storage area for containers. Docker documentation allows …<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3,4,6,2,5],"tags":[],"class_list":["post-6","post","type-post","status-publish","format-standard","hentry","category-centos","category-docker","category-lvm","category-redhat-linux","category-storage","entry entry-center"],"_links":{"self":[{"href":"https:\/\/www.srivittal.com\/wp\/index.php?rest_route=\/wp\/v2\/posts\/6","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.srivittal.com\/wp\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.srivittal.com\/wp\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.srivittal.com\/wp\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.srivittal.com\/wp\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=6"}],"version-history":[{"count":1,"href":"https:\/\/www.srivittal.com\/wp\/index.php?rest_route=\/wp\/v2\/posts\/6\/revisions"}],"predecessor-version":[{"id":53,"href":"https:\/\/www.srivittal.com\/wp\/index.php?rest_route=\/wp\/v2\/posts\/6\/revisions\/53"}],"wp:attachment":[{"href":"https:\/\/www.srivittal.com\/wp\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.srivittal.com\/wp\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.srivittal.com\/wp\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}