In Network Attached Storage (NAS) systems, the le system is divided into two components, a client and a server, which reside on different nodes. The client component of thele system sends I/O requests from the application to the le system server over a network such as Gigabit Ethernet (GE).
Network Attached Storage, compared to server attached storage on the other hand is a dedicated file server optimized to do just one function only and do it well - file serving. NAS is a system independent, shareable storage that is connected directly to the network and is accessible directly by any number of heterogeneous clients or other servers. NAS file servers are essentially stripped down servers specifically designed for file serving and offloading file management services from the more expensive application servers.
NAS systems provide a richer, typed, variable-size (file), hierarchical interface (including create or delete file, open or close file, read or write file subset, get or set file attribute, create or delete directory, insert or remove link between directory and file or directory, and lookup file name). NAS systems internally interface with nonvolatile magnetic media, usually through a SAN-like interface to ensure data reliability. From the perspective of a datapath abstraction, there is little functional
difference between the interfaces of a NAS system and those of a traditional local file system.
By locating the same les in a single location, NAS helps solve the problem of multiple different copies and versions of the same le and data residing in different local le systems. However, NAS exposes a new set of problems not present with a local le system. These problems include network round-trip latency,
possibility of dropped or duplicate non-idempotent I/O requests, increased overhead at the server head and limited scalability as the server increasingly becomes a bottleneck and nally, increased exposure to failures, since the NAS head is a single point of failure. Usually, the problems with performance and failures may be alleviated to some extent by using Non-Volatile memory (NVRAM) to store some transactions or components of transactions at the server.
In NAS, you can add storage at random without disrupting the network. When the storage was on the server as in SAS, the administrator had to take down the system, install or upgrade the drives and bring the system back up again. That created a lot of unacceptable downtime. NAS is being installed increasingly now to mitigate the downtime associated with SAS. NAS is making inroads into the marketplace at different price, performance and size levels. As business operations become more global and around the clock, more and more demanding 24x7 uptime. Feeding this frenzy of 24x7 uptime are the obiquitos internet using email messaging and around the clock customer information browsing demanding richer and richer content from text to images to audio/video clips, virtual private nets for ecommerce and data warehousing and ERP applications on the intranet.
NAS architectures generally sport a light proprietary OS kernel and file system able to operate autonomous of other applications and are thus devoid of all overhead from extraneous drivers prevalent in SAS architecture. The NAS operating system is fully compatible with server operating systems such as NT, Unix, Netware etc.
Generally called a Network Appliance, NAS devices are relatively easy to set up turning painful storage upgrades into simple plugand-play devices requiring no server downtime to set up. After plugging a NAS server onto a network and assigning an IP address, setting up control lists and user permissions and voila, all is done. This is because the NAS server boards integrate the Ethernet connection, the SCSI (or Fibre Channel) controller-to-disk connections, the operating system and boot up software all on one simple card. Much as NAS devices have built-in security features, administrators generally choose to rely on existing robust security features of their networks. One of the main benefits of NAS is that it allows clients to directly access data without burdening the application servers.
Availability Factors motivating rise of NAS servers:
Fault Resiliency -Majority of data has becoming mission critical to run a business and so must be made secure and reliable
Need for 99.9% availability (8 Hours per year of downtime). Some applications require even higher data availability such as 99.99% (1 hour of downtime per
year) and recovery from failure from hardware, software and application switchover within 30 seconds.
Ease of remote vaulting for data recovery
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