Archiving Data with Snapshots in LVM2
Sometimes we use a technology even though we're unaware of its full features and capabilities and how they may be able to benefit us. One such feature is the data snapshot. The snapshot is a single state (that is, a copy) of a storage volume at a particular point in time. A volume can refer to a disk device or partition. The snapshot is primarily a data backup technology. Directed toward larger storage capacities, utilizing such a technique has advantages. For instance, full backups of an entire volume can take a long time and also use large amounts of storage space, even for files that will remain unchanged for some time to come. Also, when performing a data backup of entire volumes or subsets of volumes in a symmetric multiprocessing environment, write operations still may continue to modify the file data on that volume, preventing atomicity and, in turn, possibly leading to data corruption. There are ways around the latter in which the volume can be taken off-line or marked as read-only prior to the archival process, but in high-availability production environments, that may never be an option. This is where the snapshot comes in.
Used to avoid downtime and retain atomicity, the snapshot provides a read-only copy of a specified volume at a specific point in time, usually implemented by a copy-on-write mechanism. Some vendors and software implementations are known to support write commands via a concept known as branching snapshots, in which diverging versions of data are created via an extremely complex system of pointers, all based on the original snapshot. When you write to a snapshot or the original volume, the write will not be seen by the other. The way this works is when a volume marked for snapshot gets written to and data is modified, the original and unchanged data block(s) or file data (in the case of a file-based snapshot) will be copied to the space allocated for the snapshot. After all original and unmodified data are copied over to the snapshot, the original volume will be updated with the new data. When the snapshot volume needs to be mounted, using a system of pointers, the snapshot will reference the parent volume with the original data saved in the snapshot. With such a technique, it now becomes possible to archive valuable data incrementally without losing productivity or the risk of suffering from any data corruption.
The use of snapshot technologies can be seen in a variety of environments, ranging from external storage controllers, filesystems, virtual machines (such as VMware, VirtualBox and so on), databases and even volume managers, which is the focus of this article. Here, I cover the snapshot feature found in LVM2 and how to manage it, all from the command line.
LVM2 refers to a collection of user-space tools that provide logical volume management on Linux.
The second generation of the Linux Logical Volume Manager (LVM2) is a logical volume manager capable of pooling multiple storage devices together to represent a single volume or volumes, either in a striped or mirrored fashion. Everything is created and managed on a layer-by-layer basis. First is the physical volume. It is followed by the volume group and then the mountable logical volume itself. Most mainstream Linux distributions usually have the LVM2 userland tools preinstalled. If you find that it's not installed on your distribution, download and install it via your distribution's package repository.
The idea is almost similar in concept to the Redundant Array of Independent Disks (RAID), and although LVM2 does not support any parity-driven striping, it still adds additional value. For instance, LVM2 allows for the uninterrupted addition, removal and replacement of storage devices. It makes for easy dynamic resizing of volume groups and logical volumes. Most important, it supports the snapshot—the focus of this article. As of LVM2, write operations are supported to snapshot volumes.
As mentioned earlier, LVM2 volumes utilize a layered structure—that is, physical volumes (or PVs) must be created from a physical disk device. This can be accomplished with the pvcreate command followed by the list of physical partitions to label for LVM2 usage:
$ sudo pvcreate /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1
With the newly labeled physical volumes, volume groups (or VGs) need to be created with the vgcreate command, followed by a name for the volume group and then a list of all physical volumes to use:
$ sudo vgcreate vg0 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1 Volume group "vg0" successfully created
By default, the volume groups are located in the /dev directory path. It is with this volume group that logical volumes (or LVs) can be created, formatted with a filesystem, mounted and, in turn, used for file I/O operations. The best feature of creating logical volumes is that you can use some or all available capacity of the VG. For instance, if a 1GB LV needs to be created from the 4GB VG, the lvcreate command needs to be used followed by the name of the VG and then a size for the LV. When an LV is created, it will create a node name for accessibility in the /dev directory path under the volume group's name:
$ sudo lvcreate --name /dev/vg0/test_vg --size 1G Logical volume "test_vg" created
The example above showcases the creation of a nonredundant LV. To create an LV with mirroring capabilities, invoke the lvcreate command with the -m option. The example below creates a 500MB-mirrored LV:
$ sudo lvcreate --size 500M -m1 --name mirrorlv vg0 ↪/dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1 Logical volume "mirrorlv" created
You can remove logical volumes, volume groups and physical volumes easily with the lvremove, vgremove and pvremove commands followed by their respective volume names:
$ sudo lvremove /dev/vg0/test_vg Do you really want to remove active logical volume "test_vg"? [y/n]: y Logical volume "test_vg" successfully removed
Note that a list of all logical volumes, volume groups and physical volumes with detailed volume information can be displayed with the lvdisplay, vgdisplay and pvdisplay commands.
Petros Koutoupis is a full-time Linux kernel, device-driver and application developer for embedded and server platforms. He has been working in the data storage industry for more than six years and enjoys discussing the same technologies.
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