7.2. Creating a snapshot of the original volume

Procédure

1. Create a logical volume named origin from the volume group vg001:

   # lvcreate -L 1G -n origin vg001
   Logical volume "origin" created.

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2. Create a snapshot logical volume named snap of /dev/vg001/origin that is 100 MB in size:

   # lvcreate --size 100M --name snap --snapshot /dev/vg001/origin
     Logical volume "snap" created.

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   If the original logical volume contains a file system, you can mount the snapshot logical volume on an arbitrary directory in order to access the contents of the file system to run a backup while the original file system continues to get updated.

3. Display the origin volume and the current percentage of the snapshot volume being used:

   # lvs -a -o +devices
     LV      VG    Attr       LSize  Pool Origin Data% Meta% Move Log Cpy%Sync Convert Devices
    origin vg001  owi-a-s---  1.00g                                                  /dev/sde1(0)
     snap vg001  swi-a-s--- 100.00m     origin 0.00                                 /dev/sde1(256)

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   You can also display the status of logical volume /dev/vg001/origin with all the snapshot logical volumes and their status, such as active or inactive by using the lvdisplay /dev/vg001/origin command.

   Avertissement

   Since the snapshot increases in size as the origin volume changes, it is important to monitor the percentage of the snapshot volume regularly with the lvs command to be sure it does not become full. A snapshot that is 100% full is lost completely, as a write to unchanged parts of the origin would be unable to succeed without corrupting the snapshot.

4. You can configure LVM to automatically extend a snapshot when its usage exceeds the snapshot_autoextend_threshold value to avoid the snapshot becoming invalid when it is 100% full. View the existing values for the snapshot_autoextend_threshold and snapshot_autoextend_percent options from the /etc/lvm.conf file and edit them as per your requirements.

   The following example, sets the snapshot_autoextend_threshold option to value less than 100 and snapshot_autoextend_percent option to the value depending on your requirement to extend the snapshot volume:

   # vi /etc/lvm.conf
   snapshot_autoextend_threshold = 70
   snapshot_autoextend_percent = 20

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   You can also extend this snapshot manually by executing the following command:

   # lvextend -L+100M /dev/vg001/snap

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   Note

   This feature requires unallocated space in the volume group. An automatic extension of a snapshot does not increase the size of a snapshot volume beyond the maximum calculated size that is necessary for the snapshot. Once a snapshot has grown large enough to cover the origin, it is no longer monitored for automatic extension.