EVMS defines a layered architecture where plug-ins in each layer create abstractions of the layer or layers below. EVMS also allows most plug-ins to create abstractions of objects within the same layer. The following list defines these layers from the bottom up.
The first (bottom) layer consists of device managers. These plug-ins communicate with hardware device drivers to create the first EVMS objects. Currently, all devices are handled by a single plug-in. Future releases of EVMS might need additional device managers for network device management (for example, to manage disks on a storage area network (SAN)).
The second layer consists of segment managers. These plug-ins handle the segmenting, or partitioning, of disk drives. The Engine components can replace partitioning programs, such as fdisk and Disk Druid, and EVMS uses Device Mapper to replace the in-kernel disk partitioning code. Segment managers can also be "stacked," meaning that one segment manager can take as input the output from another segment manager.
EVMS provides the following segment managers: DOS, GPT, System/390® (S/390), Cluster, BSD, Mac, and BBR. Other segment manager plug-ins can be added to support other partitioning schemes.
The third layer consists of region managers. This layer provides a place for plug-ins that ensure compatibility with existing volume management schemes in Linux and other operating systems. Region managers are intended to model systems that provide a logical abstraction above disks or partitions.
Like segment managers, region managers can also be stacked. Therefore, the input object(s) to a region manager can be disks, segments, or other regions.
There are currently three region manager plug-ins in EVMS: Linux LVM, LVM2, and Multi-Disk (MD).
The Linux LVM plug-in provides compatibility with the Linux LVM and allows the creation of volume groups (known in EVMS as containers) and logical volumes (known in EVMS as regions).
The LVM2 plug-in provides compatibility with the new volume format introduced by the LVM2 tools from Red Hat. This plug-in is very similar in functionality to the LVM plug-in. The primary difference is the new, improved metadata format.
The Multi-Disk (MD) plug-in for RAID provides RAID levels linear, 0, 1, 4, and 5 in software. MD is one plug-in that displays as four region managers that you can choose from.
The next layer consists of EVMS features. This layer is where new EVMS-native functionality is implemented. EVMS features can be built on any object in the system, including disks, segments, regions, or other feature objects. All EVMS features share a common type of metadata, which makes discovery of feature objects much more efficient, and recovery of broken features objects much more reliable. There are three features currently available in EVMS: drive linking, Bad Block Relocation, and snapshotting.
Drive linking allows any number of objects to be linearly concatenated together into a single object. A drive linked volume can be expanded by adding another storage object to the end or shrunk by removing the last object.
Bad Block Relocation (BBR) monitors its I/O path and detects write failures (which can be caused by a damaged disk). In the event of such a failure, the data from that request is stored in a new location. BBR keeps track of this remapping. Additional I/Os to that location are redirected to the new location.
The Snapshotting feature provides a mechanism for creating a "frozen" copy of a volume at a single instant in time, without having to take that volume off-line. This is useful for performing backups on a live system. Snapshots work with any volume (EVMS or compatibility), and can use any other available object as a backing store. After a snapshot is created and made into an EVMS volume, writes to the "original" volume cause the original contents of that location to be copied to the snapshot's storage object. Reads to the snapshot volume look like they come from the original at the time the snapshot was created.
File System Interface Modules (FSIMs) provide coordination with the file systems during certain volume management operations. For instance, when expanding or shrinking a volume, the file system must also be expanded or shrunk to the appropriate size. Ordering in this example is also important; a file system cannot be expanded before the volume, and a volume cannot be shrunk before the file system. The FSIMs allow EVMS to ensure this coordination and ordering.
FSIMs also perform file system operations from one of the EVMS user interfaces. For instance, a user can make new file systems and check existing file systems by interacting with the FSIM.
Cluster Manager Interface Modules, also known as the EVMS Clustered Engine (ECE), interface with the local cluster manager installed on the system. The ECE provides a standardized ECE API to the Engine while hiding cluster manager details from the Engine.