Red Hat CLUSTER SUITE FOR ENTERPRISE LINUX 5.1 Instrukcja Użytkownika Pobierz pdf (Strona 18)

Introduction to Linux Clustering

7.1.2 NAS – NETWORK ATTACHED STORAGE

A NAS is a hardware device consisting of a RAID array of hard drives (like a SAN).

However, instead of using fibre channel, it connects to a standard ethernet network and

supports protocols like iSCSI or ATA over ethernet.

It still appears as a block device which is configured to appear as a real disk on the server

Advantages

● Can be a lot cheaper than a SAN

● No physical limit to the number of nodes possible. (instead, limited by the number

and speed of the ethernet interfaces and network)

● Commodity servers work out of the box.

● With high-speed networks like 10gig ethernet, performance can surpass fibre channel

SANs.

Disadvantages

● Network outage can cause a failure which can cause outages preventing access to the

storage. However, the risk of this can be reduced, by running the NAS traffic on a

separate network to the normal network. (dedicated hardware, etc.)

● Some network storage protocols like iSCSI introduce overheads – for example iSCSI

is a TCP/IP protocol, which adds overhead of TCP windowing, headers, etc. Other

protocols like ATA over Ethernet add less overhead and should definitely be a factor

to consider when evaluating NAS.

Redundancy

For proper redundancy, a second NAS should be purchased and made to mirror the primary

NAS. They can either be set to have a floating IP addrress, or both of them can have iSCSI

exports which are then multipathed on the nodes (so the node can choose which one to use).

If you require additional redundancy for the nodes, a second ethernet card can be installed

into the nodes and both cards can be put into bonded mode.

In bonded mode, both interfaces work together to provide one virtual interface – if an

interface fails, the other one will continue to work.

With many servers coming with dual ethernet interfaces out of the box, there is usually little

need for further hardware investment for the nodes.

Ideal Use

From a high-level view, a SAN and a NAS is a very similar device, so make your choices

based on what will give you the best value for your investment based on your needs.

A budget minded organisation may find a NAS over 100mbit/1000mbit ethernet provides the

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