Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Programming
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Databases
Mail Systems
openSolaris
Eclipse Documentation
Techotopia.com
Virtuatopia.com
Answertopia.com

How To Guides
Virtualization
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Windows
Problem Solutions
Privacy Policy

  




 

 

3.8. Snapshots

A wonderful facility provided by LVM is 'snapshots'. This allows the administrator to create a new block device which presents an exact copy of a logical volume, frozen at some point in time. Typically this would be used when some batch processing, a backup for instance, needs to be performed on the logical volume, but you don't want to halt a live system that is changing the data. When the snapshot device has been finished with the system administrator can just remove the device. This facility does require that the snapshot be made at a time when the data on the logical volume is in a consistent state - the VFS-lock patch for LVM1 makes sure that some filesystems do this automatically when a snapshot is created, and many of the filesystems in the 2.6 kernel do this automatically when a snapshot is created without patching.

LVM1 has read-only snapshots. Read-only snapshots work by creating an exception table, which is used to keep track of which blocks have been changed. If a block is to be changed on the origin, it is first copied to the snapshot, marked as copied in the exception table, and then the new data is written to the original volume.

In LVM2, snapshots are read/write by default. Read/write snapshots work like read-only snapshots, with the additional feature that if data is written to the snapshot, that block is marked in the exception table as used, and never gets copied from the original volume. This opens up many new possibilities that were not possible with LVM1's read-only snapshots. One example is to snapshot a volume, mount the snapshot, and try an experimental program that change files on that volume. If you don't like what it did, you can unmount the snapshot, remove it, and mount the original filesystem in its place. It is also useful for creating volumes for use with Xen. You can create a disk image, then snapshot it and modify the snapshot for a particular domU instance. You can then create another snapshot of the original volume, and modify that one for a different domU instance. Since the only storage used by a snapshot is blocks that were changed on the origin or the snapshot, the majority of the volume is shared by the domU's.

WarningLVM 1 -> LVM 2 Upgrade Info
 

Make sure to remove snapshot LVs before upgrading from LVM 1 to LVM 2. (See Section 4.1)

 
 
  Published under the terms of the GNU General Public License Design by Interspire