Linux Logical Volume Manager (LVM) for on the fly disk operations

Introduction

Laymen explanation

Suppose that you are system admin and you have lots of real-time data flow. And you are using your hard-disk for this data and hard-disk is getting filled up often. In this case, you will need multiple hard-disks to work on. Will it be nice if you have to manage each hard-disk individually? I mean that while storing data, you need to take care of disk space in each hard-disk. I am sure you will want better. You will like to have all hard-disks appeared as a single storage. Underlying software should handle each hard-disk. LVM helps in this regard.

Moreover, you will definitely want to back-up this real-time data. Also, during backup, you don't want to stop real-time data generation. For this also, LVM helps.

Technical explanation

LVM stands for Logical Volume Management. It is a system of managing logical volumes, or filesystems, that is much more advanced and flexible than the traditional method of partitioning a disk into one or more segments and formatting that partition with a filesystem.

There are 3 concepts that LVM manages:

• • Physical Volumes

    • Physical Volumes correspond to disks; they are block devices that provide the space to store logical volumes.

  • Logical Volumes

    • Logical volumes correspond to partitions: they hold a filesystem. Unlike partitions though, logical volumes get names rather than numbers, they can span across multiple disks, and do not have to be physically contiguous.

  • Volume Groups

    • A Volume Group is a named collection of physical and logical volumes. Typical systems only need one Volume Group to contain all of the physical and logical volumes on the system

gparted tool versus LVM

Why anyone would want to use LVM when he can use gparted to resize and move partitions just fine. The answer is that LVM can do these things better, and some nifty new things that we just can't do otherwise.

One of the biggest advantages LVM has is that most operations can be done on the fly, while the system is running.

Snapshots

This is something you simply can not do without LVM. It allows you to freeze an existing Logical Volume in time, at any moment, even while the system is running. You can continue to use the original volume normally, but the snapshot volume appears to be an image of the original, frozen in time at the moment you created it. You can use this to get a consistent filesystem image to back up, without shutting down the system. You can also use it to save the state of the system, so that you can later return to that state if you mess things up. You can even mount the snapshot volume and make changes to it, without affecting the original.

Partition management

With gparted you can expand and shrink partitions, but only if they are not in use. LVM can expand a partition while it is mounted, if the filesystem used on it.

Moving partitions with gparted is usually only necessary in the first place because of the requirement that partitions be physically contiguous, so you probably won't ever need to do this with LVM. If you do, unlike gparted, LVM can move a partition while it is in use, and will not corrupt your data if it is interrupted.

LVM disk read/write flow

From the userspace point of view, it looks like another layer of "virtual stuff" on top of the disk, and it seems natural to imagine that all of the I/O has to now pass through this before it gets to or from the real hardware. But it's not like that. The kernel already needs to have a mapping (or several layers of mapping actually) which connects high level operations like "write this to a file" to the device drivers which in turn connect to actual blocks on disk. When LVM is in use, that lookup is changed, but that's all. (Since it has to happen anyway, doing it a bit differently is a negligible performance hit.) When it comes to actually writing the file, the bits take as direct a path to the physical media as they would otherwise.

LVM performance

LVM represents a new layer in the virtual file system hierarchy, which creates overheads for processing, thus has a negative impact on performances. The compensation of the LVM mapping overhead is partially performed by file-caching features, although it is always present in some form

LVM and Hypervisor

KVM, Xenserver and VMWare supports LVM formatted disks. In fact, Xenserver recommends it as default option(Refer here) starting Xenserver 5.5.

Reference

https://wiki.ubuntu.com/Lvm

https://askubuntu.com/questions/3596/what-is-lvm-and-what-is-it-used-for

https://discussions.citrix.com/topic/287071-2-xenserver-hosts-help-with-choosing-storage-type/

https://www.ibm.com/support/knowledgecenter/en/SSDV85_4.1.0/Admin/tasks/creatingLVMdeviceonhh.html

https://communities.vmware.com/thread/339495

https://www.researchgate.net/publication/284897601_LVM_in_the_Linux_environment_Performance_examination

https://unix.stackexchange.com/questions/7122/does-lvm-impact-performance