Free tutors on Clustering Storage, data and information The service subsystem examples of disk-based storage architectures

Free tutors on  Clustering Storage, data and information The service subsystem examples of disk-based storage architectures

A cluster is defined in the SNIA Shared Storage Model as a combination of resources with the objective of increasing scalability, availability and management within the shared storage environment (Section 6.4.1). The individual nodes of the cluster can share their resources via distributed volume managers (multi-node LVM) and cluster file systems (Figure 10.8, Section 4.3).

 Storage, data and information

The SNIA Shared Storage Model differentiates strictly between storage, data and information. Storage is space – so-called containers – provided by storage units, on which the data is stored. The bytes stored in containers on the storage units are called data. Information is the meaning – the semantics – of the data. The SNIA Shared Storage Model names the following examples in which data–container relationships arise (Table 10.1).

10.1.14 Resource and data sharing In a shared storage environment, in which the storage devices are connected to the host via a storage network, every host can access every storage device and the data stored upon it (Section 1.2). This sharing is called resource sharing or data sharing in the SNIA model, depending upon the level at which the sharing takes place (Figure 10.9). If exclusively the storage systems – and not their data content – are shared, then we talk of resource sharing. This is found in the physical resources, such as disk subsystems and tape libraries, but also within the network. Data sharing denotes the sharing of data between different hosts. Data sharing is significantly more difficult to implement, since the shared data must always be kept consistent, particularly when distributed caching is used. Heterogeneous environments also require additional conversion steps in order to convert the data into a format that the host can understand. Protocols such as NFS or CIFS are used in the more frequently used data sharing within the file/record layers

. For data sharing in the block layer, server clusters with shared disk file systems or parallel databases are used

 The service subsystem

Up to now we have concerned ourselves with the concepts within the layers of the SNIA Shared Storage Model. Let us now consider the service subsystem (Figure 10.10). Within the service subsystem we find the management tasks which occur in a shared storage environment and which we have, for the most part, already discussed in Chapter 8. In this connection, the SNIA Technical Council mention:

• discovery and monitoring

• resource management

• configuration

• security

• billing (charge-back)

• redundancy management, for example, by network back-up

• high availability

• capacity planning.

The individual subjects are not yet dealt with in more detail in the SNIA Shared Storage Model, since the required definitions, specifications and interfaces are still being developed (Section 8.7.3). At this point we expressly refer once again to the check list in the

Appendix B, which reflects a cross-section of the questions that crop up here.

EXAMPLES OF DISK-BASED STORAGE ARCHITECTURES

In this section we will present a few examples of typical storage architectures and their properties, advantages and disadvantages, as they are represented by the SNIA in the Shared Storage Model. First of all, we will discuss block-based architectures, such as the direct connection of storage to the host (Section 10.2.1), connection via a storage network (Section 10.2.2), symmetric and asymmetric storage virtualization in the network

(Section 10.2.3 and Section 10.2.4) and a multi-site architecture such as is used for data replication between several locations (Section 10.2.5). We then move on to the file/record layer and consider the graphical representation of a file server (Section 10.2.6), a NAS head (Section 10.2.7), the use of metadata controllers for asymmetric file level virtualization (Section 10.2.8) and an object-based storage device (OSD), in which the position

data of the files and their access rights is moved to a separate device, a solution that combines file sharing with increased performance due to direct file access and central metadata management of the files (Section 10.2.9). 10.2.1 Direct attached block storage Figure 10.11 shows the direct connection from storage to the host in a server-centric architecture. The following properties are characteristic of this structure:

• No connection devices, such as switches or hubs, are needed.

• The host generally communicates with the storage device via a protocol on block level.

• Block aggregation functions are possible both in the disk subsystem and on the host.