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# Tested on RHEL 6 & 7
# LVM allows to take read/write snapshots of a logical volume. An snapshot consists of a # point-in-time copy of a logical volume that can be written to independently from the # original volume. # They can be used for backup purposes or test changes, for example. # When a snapshot is created it points to the same block on disk as the original but when # a change is made to either the original or the snapshot, it will be copied and snapshot # will point to the copy instead. # To store changes a snapshot is given an initial space. Once it has used all the # available space it will be turned 'inactive'. To avoid this, we can configure snapshot # to automatically extend the space available when a defined threshold is reached. # We can define following variables in /etc/lvm/lvm.conf configuration file: # snapshot_autoextend_threshold # snapshot_autoextend_percent # # When usage goes above 'snapshot_autoextend_threshold' percent, it will be grown by # 'snapshot_autoextend_percent' of the original size. Once snapshot has reached the same # size as the original logical volume it will stop being extended. # # lvm2-monitor service has to be restarted for changes to take effect # Creating the snapshot of an existing logical volume LV=lv_test # Our existing logical volume SNAP=lv_snapshot # Snapshot VG=datavg # Volume group our logical volume belongs to SIZE=256M # Desired size for the snapshot - do your calculations before ;) lvcreate -s -n $SNAP -L $SIZE /dev/$VG/$LV # To check how much space has been used in a snapshot, we can use lvs lvs LV VG Attr LSize Pool Origin Data% Move Log Cpy%Sync Convert [...] lv_snapshot rootvg swi-a-s-- 256.00m lv_test 0.01 # Snapshot cat be mounted to make modifications mount /dev/$VG/$SNAP /mnt # Once modifications done, we can merge changes to origin logical volume (both original # lv and snapshot should be unmounted, if not next time lv will be de-activated and # re-activated it will be synchronized). If changes may be discarded we can 'lvremove' # snapshot # NOTE: Use snapshot merging carefully; merging will discard any change done to original # logical volume after taking the snapshot # Apart from that, one has to take into account that merging logical volumes will # definitely destroy snapshot. umount /dev/$VG/$LV umount /mnt lvconvert --merge /dev/$VG/$SNAP Merging of volume lv_snapshot started. lv_test: Merged: 100.0% lv_test: Merged: 100.0% Merge of snapshot into logical volume lv_test has finished. Logical volume "lv_snapshot" successfully removed # We may run merge in the background ('-b'). If the original volume is in use, the merge # will be started next time the original volume is activated. Obviously merging a snapshot # of the root file system requires a reboot of the server # Example of snapshot # ------------------------------------------------------------------------------------------ # lvs rootvg/lv_test LV VG Attr LSize Pool Origin Data% Move Log Cpy%Sync Convert lv_test rootvg -wi-ao--- 256.00m # ll /test/ total 12 -rw-r--r-- 1 root root 0 Aug 5 16:19 1.txt -rw-r--r-- 1 root root 0 Aug 5 16:19 2.txt drwx------ 2 root root 12288 Aug 5 16:06 lost+found # lvcreate -s -n lv_snapshot -L 32M /dev/rootvg/lv_test Logical volume "lv_snapshot" created # mount /dev/rootvg/lv_snapshot /mnt # ll /mnt/ total 12 -rw-r--r-- 1 root root 0 Aug 5 16:19 1.txt -rw-r--r-- 1 root root 0 Aug 5 16:19 2.txt drwx------ 2 root root 12288 Aug 5 16:06 lost+found # touch /mnt/3.txt # touch /mnt/4.txt # touch /test/5.txt # ll /test/ total 12 -rw-r--r-- 1 root root 0 Aug 5 16:19 1.txt -rw-r--r-- 1 root root 0 Aug 5 16:19 2.txt -rw-r--r-- 1 root root 0 Aug 5 16:21 5.txt drwx------ 2 root root 12288 Aug 5 16:06 lost+found # ll /mnt/ total 12 -rw-r--r-- 1 root root 0 Aug 5 16:19 1.txt -rw-r--r-- 1 root root 0 Aug 5 16:19 2.txt -rw-r--r-- 1 root root 0 Aug 5 16:21 3.txt -rw-r--r-- 1 root root 0 Aug 5 16:21 4.txt drwx------ 2 root root 12288 Aug 5 16:06 lost+found # lvs rootvg/lv_test rootvg/lv_snapshot LV VG Attr LSize Pool Origin Data% Move Log Cpy%Sync Convert lv_snapshot rootvg swi-aos-- 32.00m lv_test 0.12 lv_test rootvg owi-aos-- 256.00m # umount /test # umount /mnt # lvconvert --merge /dev/rootvg/lv_snapshot Merging of volume lv_snapshot started. lv_test: Merged: 100.0% Merge of snapshot into logical volume lv_test has finished. Logical volume "lv_snapshot" successfully removed # lvs rootvg/lv_test rootvg/lv_snapshot One or more specified logical volume(s) not found. LV VG Attr LSize Pool Origin Data% Move Log Cpy%Sync Convert lv_test rootvg -wi-a---- 256.00m # mount /dev/rootvg/lv_test /test # ll /test total 12 -rw-r--r-- 1 root root 0 Aug 5 16:19 1.txt -rw-r--r-- 1 root root 0 Aug 5 16:19 2.txt -rw-r--r-- 1 root root 0 Aug 5 16:21 3.txt -rw-r--r-- 1 root root 0 Aug 5 16:21 4.txt drwx------ 2 root root 12288 Aug 5 16:06 lost+found # Files 3.txt and 4.txt that were created on snapshot have been merged to original LV. # On the other hand file 5.txt, that was created on original LV after having taken the # snapshot, has been discarded during merge operation. |