module 3 advanced storage technology : network attached ... · over a network file system. •file...

Module 3 Advanced Storage Technology : NETWORK ATTACHED STORAGE

• Local File Systems

• Network File systems and file servers

• Shared Disk File systems: Case study

• Comparison: NAS, FC SAN and iSCSI SAN

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Local File Systems

• File systems form an intermediate layer between block-oriented hard disks and applications, with a volume manager often being used between the file system and the hard disk

• Together, these manage the blocks of the disk and make these available to users and applications via the familiar directories and files

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Figure.1 File system and volume manager manage the blocks of the block-oriented hard disks.

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File systems and databases

• File systems and volume manager provide their services to numerous applications with various load profiles.

• This means that they are generic applications; their performance is not generally optimised for a specific application.

• Database systems such as DB2 or Oracle can get around the file system and manage the blocks of the hard disk themselves

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Figure.2 To increase performance, databases can get around the file system and manage the blocks themselves

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Journaling

• In addition to the basic services, modern file systems provide three functions – journaling, snapshots and dynamic file system expansion.

• Journaling is a mechanism that guarantees the consistency of the file system even after a system crash.

• To this end, the file system first of all writes every change to a log file that is invisible to applications and end users, before making the change in the file system itself.

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Journaling

• After a system crash the file system only has to run through the end of the log file in order to recreate the consistency of the file system.

• In file systems without journaling, typically older file systems like Microsoft’s FAT32 file system or the Unix File System (UFS) that is widespread in Unix systems, the consistency of the entire file system has to be checked after a system crash (file system check)

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Snapshots

• Snapshots represent the same function as the instant copies function that is familiar from disk subsystems .

• Snapshots freeze the state of a file system at a given point in time. Applications and end users can access the frozen copy via a special path.

• As is the case for instant copies, the creation of the copy only takes a few seconds.

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Volume manager

• The volume manager is an intermediate layer within the operating system between the file system or database and the actual hard disks.

• The most important basic function of the volume manager is to aggregate several hard disks to form a large virtual hard disk and make just this virtual hard disk visible to higher layers.

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Volume manager

• Most volume managers provide the option of breaking this virtual disk back down into several smaller virtual hard disks and enlarging or reducing these (Figure.3).

• This virtualisation within the volume manager makes it possible for system administrators to quickly react to changed storage requirements of applications such as databases and file systems.

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Figure.3 The volume manager aggregates physical hard disks into virtual hard disks, which it can break back down into smaller virtual hard disks. In the illustration one virtual hard disk is used directly from a database, the others are shared between two file systems.

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Network File systems and file servers

• Network file systems are the natural extension of local file systems.

• End users and applications can access directories and files that are physically located on a different computer – the file server – over a network file system.

• File servers are so important in modern IT environments that preconfigured file servers, called Network Attached Storage (NAS), have emerged as a separate product category

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Basic principle

• The metaphor of directories and files for the management of data is so easy to understand that it was for a long time the prevailing model for the access of data over networks.

• So-called network file systems give end users and applications access to data stored on a different computer (Figure.1).

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Figure.1 Network file systems make local files and directories available over the LAN. Several end users can thus work on common files

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Basic principle

• The first widespread network file system was the Network File System (NFS) developed by Sun Microsystems, which is now the standard network file system on all Unix systems.

• Microsoft developed its own network file system – the Common Internet File System (CIFS) – for its Windows operating system and this is incompatible with NFS

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Network Attached Storage (NAS)

• File servers are so important in current IT environments that they have developed into an independent product group in recent years.

• Network Attached Storage (NAS) is the name for preconfigured file servers. They consist of one or more internal servers, preconfigured disk capacity and usually a stripped-down or special operating system

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• NAS servers are usually connected via Ethernet to the LAN, where they provide their disk space as file servers. Web servers represent a further important field of application for NAS servers.

• NAS servers were specially developed for file sharing. This has two advantages: since, by definition, the purpose of NAS servers is known, NAS operating systems can be significantly better optimised than generic operating systems

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Advantages of NAS

• NAS servers provide Plug & Play file systems, i.e. connect – power up – use.

• NAS servers are very scalable

Disadvantage of NAS

• Unclear upgrade path

• Performance bottlenecks

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Shared Disk File systems: Case study

• The greatest performance limitation of NAS servers and self-configured file servers is that each file must pass through the internal buses of the file servers twice before the files arrive at the computer where they are required(Fig.1).

• Even DAFS and its alternatives like NFS over RDMA cannot get around this ‘eye of the needle

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Figure.1 The file server becomes like the eye of the needle: en route between hard disk and client all data passes through the internal buses of the file server twice.

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• With storage networks it is possible for several computers to access a storage device simultaneously. The I/O bottleneck in the file server can be circumvented if all clients fetch the files from the disk directly via the storage network (Figure.2).

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Figure.2 In a shared disk file system all clients can access the disks directly via the storage network (1). LAN data traffic is now only necessary for the synchronisation of the write accesses (2). The data of a shared disk file system can additionally be exported over the LAN in the form of a network file system with NFS or CIFS (3).

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• Problem: today’s file systems consider their storage devices as local. They concentrate upon the caching and the aggregation of I/O operations; they increase performance by reducing the number of disk accesses needed.

• Solution :Shared disk file systems can deal with this problem. Integrated into them are special algorithms that synchronise the simultaneous accesses of several computers to common disks. As a result, shared disk file systems make it possible for several computers to access files simultaneously without causing version conflict.

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Advantage:

• The computers accessing files and the storage devices in question now communicate with each other directly.

• The load on the CPU in the accessing machine is reduced because communication via Fibre Channel places less of a load on the processor than communication via IP and Ethernet.

• The availability of the shared disk file system can be better than that of conventional network file systems

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module 3 advanced storage technology : network attached ... · over a network file system. •file...

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