http://how-to.linuxcareer.com/create-and-restore-manual-logical-volume-snapshots
Logical volume manager allows you to create a Logical group consisting of multiple physical volumes. Physical volumes can be entire hard-drives or separate partitions. Physical volumes can reside on a single or multiple hard-drives, partitions , USBs, SAN's etc. To increase a Logical Volume size you can add additional physical volumes. Once you create Logical volume group you can then create multiple Logical volumes and at the same time completely disregard a physical volume layer. Logical volume group can be resized at any time by adding more physical volumes so new logical volumes can created or resized.
Creating a Snapshot
Now we are going to create a snapshot of logical volume "volume1". In the process Logical Volume Manager will create a new separate logical volume. This new logical volume will have size of 20MB and will be called "volume1_snapshot":
Now that the snapshot has been created we can start altering data on "volume1" for example by removing the entire content:
1. Introduction
By creating a Logical Volume snapshots you are able to freeze a current state of any of your logical volumes. This means that you can very easily create a backup and once needed rollback to a original logical volume state. This method is very similar to what you already know from using Virtualization software such as Virtualbox or VMware where you can simply take a snapshot of entire virtual machine and revert back in case something went wrong etc. Therefore, using LVM snapshots allows you to take a control of your system's logical volumes whether it is your personal laptop or server. This tutorial is self-contained as no previous experience with Logical Volume Manager is required.2. Scenario
In this article we will explain how to manually create and restore logical volume snapshots. Since we do not assume any previous experience with Logical Volume Manager we will start from a scratch using a dummy physical hard drive /dev/sdb with size of 1073 MB. Here are all steps in nutshell:- 1First we will create two partitions on our /dev/sdb drive. These partitions will be of "8e Linux LVM" type and will be used to create a physical volumes
- 2Once both partitions are created we use pvcreate command to create physical volumes
- 3In this step we create a new Logical Volume Group and a single 300MB in size logical volume using ext4 filesystem
- 4Mount our new logical volume and create some sample data
- 5Take a snapshot and remove sample data
- 6Rollback logical volume snapshot
3. Creating a Logical Volume
3.1. Logical Volume Manager Basics
Here is a quick start definition of logical volume manager:Logical volume manager allows you to create a Logical group consisting of multiple physical volumes. Physical volumes can be entire hard-drives or separate partitions. Physical volumes can reside on a single or multiple hard-drives, partitions , USBs, SAN's etc. To increase a Logical Volume size you can add additional physical volumes. Once you create Logical volume group you can then create multiple Logical volumes and at the same time completely disregard a physical volume layer. Logical volume group can be resized at any time by adding more physical volumes so new logical volumes can created or resized.
3.2. Create a partitions
First, we need to create a partitions and mark them as physical volumes. Here is our physical disk we are going to work with:# fdisk -l /dev/sdb Disk /dev/sdb: 1073 MB, 1073741824 bytes 255 heads, 63 sectors/track, 130 cylinders, total 2097152 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x335af99c Device Boot Start End Blocks Id SystemLet's create two primary partitions. Here we are using fdisk to do tis job. Feel free to use any other partitioning tool to do this job such as cfdisk, parted etc.
# fdisk /dev/sdbAll command are highlighted in bold:
Command (m for help): n
Partition type:
p primary (0 primary, 0 extended, 4 free)
e extended
Select (default p): p
Partition number (1-4, default 1):
Using default value 1
First sector (2048-2097151, default 2048):
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-2097151, default 2097151): +400M
Command (m for help): n
Partition type:
p primary (1 primary, 0 extended, 3 free)
e extended
Select (default p): p
Partition number (1-4, default 2): 2
First sector (821248-2097151, default 821248):
Using default value 821248
Last sector, +sectors or +size{K,M,G} (821248-2097151, default 2097151): +200M
Command (m for help): t
Partition number (1-4): 1
Hex code (type L to list codes): 8e
Changed system type of partition 1 to 8e (Linux LVM)
Command (m for help): t
Partition number (1-4): 2
Hex code (type L to list codes): 8e
Changed system type of partition 2 to 8e (Linux LVM)
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
If you followed the above steps, your new partition table on the disk /dev/sdb will now look similar to the one below:# fdisk -l /dev/sdb Disk /dev/sdb: 1073 MB, 1073741824 bytes 255 heads, 63 sectors/track, 130 cylinders, total 2097152 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x335af99c Device Boot Start End Blocks Id System /dev/sdb1 2048 821247 409600 8e Linux LVM /dev/sdb2 821248 1230847 204800 8e Linux LVM
3.3. Create Physical Volumes
At this point we mark both partitions as physical volumes. Please note that you do not have to follow the same pattern as in this tutorial. For example you could simply partition entire disk with a single partition instead of two. Use pvcreate to create physical volumes:# pvcreate /dev/sdb[1-2] Writing physical volume data to disk "/dev/sdb1" Physical volume "/dev/sdb1" successfully created Writing physical volume data to disk "/dev/sdb2" Physical volume "/dev/sdb2" successfully created
3.4. Create Volume Group
Now it is time to create a Volume Group. For this we use tool vgcreate. The new Volume Group will have a name "volume_group".# vgcreate volume_group /dev/sdb1 /dev/sdb2 Volume group "volume_group" successfully createdAfter execution of the above command you will have a new volume group created named "volume_group". This new volume group will consist of two physical volumes:
- /dev/sdb1
- /dev/sdb2
# vgdisplay --- Volume group --- VG Name volume_group System ID Format lvm2 Metadata Areas 2 Metadata Sequence No 1 VG Access read/write VG Status resizable MAX LV 0 Cur LV 0 Open LV 0 Max PV 0 Cur PV 2 Act PV 2 VG Size 592.00 MiB PE Size 4.00 MiB Total PE 148 Alloc PE / Size 0 / 0 Free PE / Size 148 / 592.00 MiB VG UUID 37jef7-3q3E-FyZS-lMPG-5Jzi-djdO-BgPIPa
3.5. Creating Logical Volumes
If all went smoothly we can now finally create a logical volume. The size of the logical volume must not exceed the size of your logical group. Let's create new logical volume called "volume1" of size 200 MB and format it with ext4 filesystem.# lvcreate -L 200 -n volume1 volume_group Logical volume "volume1" createdYou can see a definition of your new logical volume using lvdisplay command. Take a note of the LV Path value as you will need it when creating a filesystem on your new h"volume1" logical volume.
# lvdisplay --- Logical volume --- LV Path /dev/volume_group/volume1 LV Name volume1 VG Name volume_group LV UUID YcPtZH-mZ1J-OQQu-B4nj-MWo0-yC18-m77Vuz LV Write Access read/write LV Creation host, time debian, 2013-05-08 12:53:17 +1000 LV Status available # open 0 LV Size 200.00 MiB Current LE 50 Segments 1 Allocation inherit Read ahead sectors auto - currently set to 256 Block device 254:0Now you can create an ext4 filesystem on your logical volume:
# mkfs.ext4 /dev/volume_group/volume1
4. Logical Volume Snapshot
Finally, we have come to the point where we can take a snapshot of our logical volume created in previous section. For this we will also need some sample data on our Logical Volume "volume1" so once we revert from the snapshot we can confirm entire process by comparing original data with data recovered from the snapshot.4.1. Understanding Snaphosts
In order to understand how snapshots work we first need to understand what logical volume consists of and how data are stored. This concept is similar to well known symbolic links. When you create a symbolic link to a file you are not creating a copy of the actual file but instead you simply create only a reference to it. Logical volume stores data in a similar fashion and it consists of two essential parts:- metadata pointers
- data block
4.2. Sample Data
First, create a new mount point directory for "volume1" and mount it :# mkdir /mnt/volume1 # mount /dev/volume_group/volume1 /mnt/volume1Enter "volume1" mount point and copy some sample data:
# cd /mnt/volume1 # cp -r /sbin/ . # du -s sbin/ 8264 sbin/Using previous commands we have copied entire /sbin directory into /mnt/volume1. The size of /mnt/volume1/sbin/ is currently 8264 KB.
Creating a Snapshot
Now we are going to create a snapshot of logical volume "volume1". In the process Logical Volume Manager will create a new separate logical volume. This new logical volume will have size of 20MB and will be called "volume1_snapshot":
# lvcreate -s -L 20M -n volume1_snapshot /dev/volume_group/volume1 Logical volume "volume1_snapshot" createdExecute lvs command to confirm that new volume snapshot has been created:
# lvs LV VG Attr LSize Pool Origin Data% Move Log Copy% Convert volume1 volume_group owi-aos- 200.00m volume1_snapshot volume_group swi-a-s- 20.00m volume1 0.06
Now that the snapshot has been created we can start altering data on "volume1" for example by removing the entire content:
# cd /mnt/volume1 # rm -fr # rm -fr sbin/After this operation you can consult again lvs command and see that Data% on the volume1_snap is now increased. If you want to, you can now mount your snapshot volume to confirm that the original data from "volume1" still exists.
4.3. Revert Logical Volume Snapshot
Before we revert our logical volume snapshot, let's first confirm that our /mnt/volume1/sbin data are still missing:# du -s /mnt/volume1/sbin du: cannot access `/mnt/volume1/sbin': No such file or directoryRecovering a Logical Volume snapshots consists of two steps:
- scheduling a snapshot recovery after next logical volume activation
- deactivate and activate logical volume
# lvconvert --merge /dev/volume_group/volume1_snapshot Can't merge over open origin volume Merging of snapshot volume1_snapshot will start next activation.After execution of the above command the logical volume "volume1" will rollback once it is activated. Therefore, what needs to be done next is to re-activate "volume1". First, make sure that you unmount your "volume1"
# umount /mnt/volume1Deactivate and activate you volume:
# lvchange -a n /dev/volume_group/volume1 # lvchange -a y /dev/volume_group/volume1As a last step mount again your logical volume "volume1" and confirm that data all has been recovered:
# mount /dev/volume_group/volume1 /mnt/volume1 # du -s /mnt/volume1/sbin 8264 /mnt/volume1/sbin
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