Device Mapper and applications


What is device-mapper ?

Device mapper is a modular driver for the linux kernel 2.6. It can be said as a framework which helps to create or map logical sectors of a pseudo block device to an underlying physical block device. So what device-mapper do is keep a table of mappings which equate the logical block devices to the physical block devices.

Applications such as LVM2, EVMS, software raid aka dmraid, multipathing, block encryption mechanisms such as cryptsetup etc… use device-mapper to work. All these applications excluding EVMS use the libdevmapper library to communicate with device-mapper.

The applications communicate with device-mapper’s API to create the mapping. Due to this feature, device-mapper does not need to know what LVM or dmraid is, how it works, what LVM metadata is, etc… It is upto the application to create the pseudo devices pointing to the physical volumes using one of device-mapper’s targets and then update the mapper table.

The device-mapper mapping table :

The mapping table used by device-mapper doesn’t take too much space and is a list created using a ‘btree’. A btree or a ‘Binary Search Tree‘ is a data-structure from which data can be added, removed or queried.

In order to know more on what a btree is and the concept behind it, read :

http://en.wikipedia.org/wiki/Binary_search_tree

http://en.wikipedia.org/wiki/B-tree

Types of device-mapper targets :

Applications which use device-mapper actually use one or more of its target methods to achieve their purpose. Targets can be said as a method or type of mapping used by device-mapper. The general mapping targets are :

a) Linear – Used by linear logical volumes, ie.. the default data layout method used by LVM2.

b) Striped – Used by striped logical volumes as well as software RAID0.

c) Mirror – Used by software RAID1 and LVM mirroring.

d) Crypt – Used by disk encryption utilties.

e) Snapshot – Used to take online snapshots of block devices, an example is LVM snapshot.

f) Multipath – Used by device-mapper-multipath.

g) RAID45 – Software raid using device-mapper, ie.. dmraid

h) Error – Sectors of the pseudo device mapped with this target causes the I/O to fail.

There are a few more mappings such as ‘flaky’ which is not used much.

I’ll write on how device-mapper works in LVM, in the next post…

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