Thin LVM Snapshots: Why Size Is Less Important

Traditionally with LVM snapshots you need to be especially careful when choosing how big to make your snapshots; if it is too small it will fill up and become invalid. If taking many snapshots with limited space then it becomes quite difficult to decide which snapshots need more space than others.

One approach has been to leave some extra space in the VG and let dmeventd periodically poll and lvextend the snapshot if necessary (I covered this in a previous post ). However, a reader of mine has pointed out that this polling mechanism does not work very well for small snapshots.

Fortunately, with the addition of thin logical volume support within LVM (I believe initially in RHEL/CentOS 6.4 and/or Fedora 17), size is much less important to consider when taking a snapshot. If you create a thin LV and then “snapshot” the thin LV, what you actually end up with are two thin LVs. They both use extents from the same pool and the size will grow dynamically as needed.

As always, examples help. In my system I have a 20G sdb. I’ll create a VG, vgthin, that uses sdb and then a 10G thin pool, lvpool, within vgthin.
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[root@localhost ~]# lsblk /dev/sdb
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sdb    8:16   0  20G  0 disk 
[root@localhost ~]# 
[root@localhost ~]# vgcreate vgthin /dev/sdb
  Volume group "vgthin" successfully created
[root@localhost ~]# 
[root@localhost ~]# lvcreate --thinpool lvpool --size 10G vgthin
  Logical volume "lvpool" created

Next, I’ll create a thin LV (lvthin), add a filesystem and mount it.
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[root@localhost ~]# lvcreate --name lvthin --virtualsize 5G --thin vgthin/lvpool
  Logical volume "lvthin" created
[root@localhost ~]# 
[root@localhost ~]# mkfs.ext4 /dev/vgthin/lvthin 
...
[root@localhost ~]# mkdir /mnt/origin
[root@localhost ~]# mount /dev/vgthin/lvthin /mnt/origin
[root@localhost ~]#  
[root@localhost ~]# lvs
  LV     VG     Attr      LSize  Pool   Origin Data%  Move Log Copy%  Convert
  lvpool vgthin twi-a-tz- 10.00g                 1.27                        
  lvthin vgthin Vwi-a-tz-  5.00g lvpool          2.54                        

I’ll go ahead and create the snapshot now, but just as a sanity check I’ll create a file, A, that exists before the snapshot. After the snapshot I’ll create a file, B. This file should NOT be visible in the snapshot if it is working properly.
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[root@localhost ~]# touch /mnt/origin/A
[root@localhost ~]# 
[root@localhost ~]# lvcreate --name lvsnap --snapshot vgthin/lvthin 
  Logical volume "lvsnap" created
[root@localhost ~]# 
[root@localhost ~]# mkdir /mnt/snapshot
[root@localhost ~]# mount /dev/vgthin/lvsnap /mnt/snapshot/
[root@localhost ~]# 
[root@localhost ~]# touch /mnt/origin/B
[root@localhost ~]# 
[root@localhost ~]# ls /mnt/origin/
A  B  lost+found
[root@localhost ~]# ls /mnt/snapshot/
A  lost+found

Perfect! Snapshotting is working as expected. What are our utilizations?
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[root@localhost ~]# lvs
  LV     VG     Attr      LSize  Pool   Origin Data%  Move Log Copy%  Convert
  lvpool vgthin twi-a-tz- 10.00g                 2.05                        
  lvsnap vgthin Vwi-aotz-  5.00g lvpool lvthin   4.10                        
  lvthin vgthin Vwi-aotz-  5.00g lvpool          4.10                        

Since we just created the snapshot our current utilization for both lvthin and lvsnap are the same. Take note also that the overall data usage for the entire pool actually shows us that lvthin and lvsnap are sharing the blocks that were present at the time the snapshot was taken. This will continue to be true as long as those blocks don’t change.

A few more sanity checks.. If we add a 1G file into the filesystem on lvthin we should see only the usage of lvthin increase.
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[root@localhost ~]# cp /root/1Gfile /mnt/origin/
[root@localhost ~]# lvs
  LV     VG     Attr      LSize  Pool   Origin Data%  Move Log Copy%  Convert
  lvpool vgthin twi-a-tz- 10.00g                12.06                        
  lvsnap vgthin Vwi-aotz-  5.00g lvpool lvthin   4.10                        
  lvthin vgthin Vwi-aotz-  5.00g lvpool         24.10                        

If we add a 512M file into the snapshot then we should see only the usage of lvsnap increase.
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[root@localhost ~]# cp /root/512Mfile /mnt/snapshot/
[root@localhost ~]# lvs
  LV     VG     Attr      LSize  Pool   Origin Data%  Move Log Copy%  Convert
  lvpool vgthin twi-a-tz- 10.00g                17.06                        
  lvsnap vgthin Vwi-aotz-  5.00g lvpool lvthin  14.10                        
  lvthin vgthin Vwi-aotz-  5.00g lvpool         24.10                        

And thats it.. Not that exciting, but it is dynamic allocation of snapshots (did I also mention there is support for snapshots of snapshots of snapshots?). As long as there is still space within the pool the snapshot will grow dynamically.

Cheers

Dusty