A storage area network is a dedicated network that provides access to consolidated, block
level data storage.
SANs are primarily used to enhance storage devices, such as disk arrays, tape libraries,
and optical jukeboxes, accessible to servers so that the devices appear like locally attached
devices to the operating system.
A SAN typically has its own network of storage devices that are generally not accessible
through the local area network by other devices.
The cost and complexity of SANs dropped in the early 2000s to levels allowing wider adoption
across both enterprise and small to medium sized business environments.
A SAN does not provide file abstraction, only block-level operations.
However, file systems built on top of SANs do provide file-level access, and are known
as SAN filesystems or shared disk file systems.
Storage Historically, data centers first created "islands"
of SCSI disk arrays as direct-attached storage, each dedicated to an application, and visible
as a number of "virtual hard drives".
Essentially, a SAN consolidates such storage islands together using a high-speed network.
Operating systems maintain their own file systems on their own dedicated, non-shared
LUNs, as though they were local to themselves.
If multiple systems were simply to attempt to share a LUN, these would interfere with
each other and quickly corrupt the data.
Any planned sharing of data on different computers within a LUN requires advanced solutions,
such as SAN file systems or clustered computing.
Despite such issues, SANs help to increase storage capacity utilization, since multiple
servers consolidate their private storage space onto the disk arrays.
Common uses of a SAN include provision of transactionally accessed data that require
high-speed block-level access to the hard drives such as email servers, databases, and
high usage file servers.
SAN compared to NAS Network-attached storage was designed before
the emergence of SAN as a solution to the limitations of the traditionally used Direct
Attached Storage, in which individual storage devices such as disk drives are connected
directly to each individual computer and not shared.
In both a NAS and SAN solution the various computers in a network, such as individual
user's desktop computers and dedicated servers running applications, can share a more centralized
collection of storage devices via a network connection through the LAN.
Concentrating the storage on one or more NAS servers or in a SAN instead of placing storage
devices on each application server allows application server configurations to be optimized
for running their applications instead of also storing all the related data and moves
the storage management task to the NAS or SAN system.
Both NAS and SAN have the potential to reduce the amount of excess storage that must be
purchased and provisioned as spare space.
In a DAS-only architecture, each computer must be provisioned with enough excess storage
to ensure that the computer does not run out of space at an untimely moment.
In a DAS architecture the spare storage on one computer cannot be utilized by another.
With a NAS or SAN architecture, where storage is shared across the needs of multiple computers,
one normally provisions a pool of shared spare storage that will serve the peak needs of
the connected computers, which typically is less than the total amount of spare storage
that would be needed if individual storage devices were dedicated to each computer.
In a NAS solution the storage devices are directly connected to a "NAS-Server" that
makes the storage available at a file-level to the other computers across the LAN.
In a SAN solution the storage is made available via a server or other dedicated piece of hardware
at a lower "block-level", leaving file system concerns to the "client" side.
SAN protocols include Fibre Channel, iSCSI, ATA over Ethernet and HyperSCSI.
One way to loosely conceptualize the difference between a NAS and a SAN is that NAS appears
to the client OS as a file server whereas a disk available through a SAN still appears
to the client OS as a disk, visible in disk and volume management utilities, and available
to be formatted with a file system and mounted.
One drawback to both the NAS and SAN architecture is that the connection between the various
CPU's and the storage units are no longer dedicated high-speed busses tailored to the
needs of storage access, instead the CPUs use the LAN to communicate, potentially creating
bandwidth bottlenecks.
While it is possible to use the NAS or SAN approach to eliminate all storage at user
or application computers, typically those computers still have some local Direct Attached
Storage for the operating system, various program files and related temporary files
used for a variety of purposes, including caching content locally.
To understand their differences, a comparison of DAS, NAS and SAN architectures may be helpful.
SAN-NAS hybrid
Despite their differences, SAN and NAS are not mutually exclusive, and may be combined
as a SAN-NAS hybrid, offering both file-level protocols and block-level protocols from the
same system.
An example of this is Openfiler, a free software product running on Linux-based systems.
A shared disk file system can also be run on top of a SAN to provide filesystem services.
Benefits Sharing storage usually simplifies storage
administration and adds flexibility since cables and storage devices do not have to
be physically moved to shift storage from one server to another.
Other benefits include the ability to allow servers to boot from the SAN itself.
This allows for a quick and easy replacement of faulty servers since the SAN can be reconfigured
so that a replacement server can use the LUN of the faulty server.
While this area of technology is still new, many view it as being the future of the enterprise
datacenter.
SANs also tend to enable more effective disaster recovery processes.
A SAN could span a distant location containing a secondary storage array.
This enables storage replication either implemented by disk array controllers, by server software,
or by specialized SAN devices.
Since IP WANs are often the least costly method of long-distance transport, the Fibre Channel
over IP and iSCSI protocols have been developed to allow SAN extension over IP networks.
The traditional physical SCSI layer could only support a few meters of distance - not
nearly enough to ensure business continuance in a disaster.
The economic consolidation of disk arrays has accelerated the advancement of several
features including I/O caching, snapshotting, and volume cloning.
Network types Most storage networks use the SCSI protocol
for communication between servers and disk drive devices.
A mapping layer to other protocols is used to form a network:
ATA over Ethernet, mapping of ATA over Ethernet Fibre Channel Protocol, the most prominent
one, is a mapping of SCSI over Fibre Channel Fibre Channel over Ethernet
ESCON over Fibre Channel, used by mainframe computers
HyperSCSI, mapping of SCSI over Ethernet iFCP or SANoIP mapping of FCP over IP
iSCSI, mapping of SCSI over TCP/IP iSCSI Extensions for RDMA, mapping of iSCSI
over InfiniBand Storage networks may also be built using SAS
and SATA technologies.
SAS evolved from SCSI direct-attached storage.
SATA evolved from IDE direct-attached storage.
SAS and SATA devices can be networked using SAS Expanders.
SAN infrastructure
SANs often use a Fibre Channel fabric topology - an infrastructure specially designed to
handle storage communications.
It provides faster and more reliable access than higher-level protocols used in NAS.
A fabric is similar in concept to a network segment in a local area network.
A typical Fibre Channel SAN fabric is made up of a number of Fibre Channel switches.
Today, all major SAN equipment vendors also offer some form of Fibre Channel routing solution,
and these bring substantial scalability benefits to the SAN architecture by allowing data to
cross between different fabrics without merging them.
These offerings use proprietary protocol elements, and the top-level architectures being promoted
are radically different.
They often enable mapping Fibre Channel traffic over IP or over SONET/SDH.
Compatibility One of the early problems with Fibre Channel
SANs was that the switches and other hardware from different manufacturers were not compatible.
Although the basic storage protocols FCP were always quite standard, some of the higher-level
functions did not interoperate well.
Similarly, many host operating systems would react badly to other operating systems sharing
the same fabric.
Many solutions were pushed to the market before standards were finalized and vendors have
since innovated around the standards.
SANs in media and entertainment Video editing workgroups require very high
data transfer rates and very low latency.
Outside of the enterprise market, this is one area that greatly benefits from SANs.
SANs in Media and Entertainment are often referred to as Serverless SANs due to the
nature of the configuration which places the video workflow clients directly on the SAN
rather than attaching to servers.
Control of data flow is managed by a distributed file system such as StorNext by Quantum.
Per-node bandwidth usage control, sometimes referred to as Quality of Service, is especially
important in video workgroups as it ensures fair and prioritized bandwidth usage across
the network, if there is insufficient open bandwidth available.
Storage virtualization
Storage virtualization is the process of abstracting logical storage from physical storage.
The physical storage resources are aggregated into storage pools, from which the logical
storage is created.
It presents to the user a logical space for data storage and transparently handles the
process of mapping it to the physical location, a concept called location transparency.
This is implemented in modern disk arrays, often using vendor proprietary solutions.
However, the goal of storage virtualization is to group multiple disk arrays from different
vendors, scattered over a network, into a single storage device.
The single storage device can then be managed uniformly.
SAN Storage QoS SAN Storage QoS is the coordination of capacity
and performance in a dedicated storage area network.
This enables the desired storage performance to be calculated and maintained for network
customers accessing the device.
Key factors that affect Storage Area Network QoS(Quality of Service) are:
Bandwidth – The rate of data throughput available on the system.
Latency – The time delay for a read/write operation to execute.
Queue depth – The number of outstanding operations waiting to execute to the underlying
disks.
QoS can be impacted in a SAN storage system by unexpected increase in data traffic from
one network user that can cause performance to decrease for other users on the same network.
This can be known as the “Noisy Neighbor Effect.”
When QoS services are enabled in a SAN storage system, the “Noisy Neighbor Effect” can
be prevented and network storage performance can be accurately predicted.
Using SAN storage QoS is in contrast to using disk over-provisioning in a SAN environment.
Over-provisioning can be used to provide additional capacity to compensate for peak network traffic
loads.
However, where network loads are not predictable, over-provisioning can eventually cause all
bandwidth to be fully consumed and latency to increase significantly resulting in SAN
performance degradation.
See also ATA over Ethernet
Direct-attached storage Disk array
Fibre Channel Fibre Channel over Ethernet
File Area Network iSCSI
iSCSI Extensions for RDMA List of networked storage hardware platforms
List of storage area network management systems Massive array of idle disks
Network-attached storage Redundant array of independent disks
SCSI RDMA Protocol Storage Management Initiative – Specification
— Storage hypervisor
Storage Resource Management Storage virtualization
System area network References
^ "Novel Doc: OES 1 - Direct Attached Storage Solutions".
^ "Storage Architectures graphic".
http:www.evaluatorgroup.comstorage-architectures/.
Evaluator Group.
^ "SAN vs DAS: A Cost Analysis of Storage in the Enterprise".
SAN vs DAS: A Cost Analysis of Storage in the Enterprise.
31 October 2008.
Retrieved 2010-01-28.
^ "TechEncyclopedia: IP Storage".
Retrieved 2007-12-09.
^ "TechEncyclopedia: SANoIP".
Retrieved 2007-12-09.
^ "StorNext Storage Manager - High-speed file sharing, Data Management and Digital Archiving
Software".
Quantum.com.
Retrieved 2013-07-08.
5.
Evaluator Group Storage Architectures Graphic http:www.evaluatorgroup.comstorage-architectures/
External links Storage Area Network Courses
Introduction to Storage Area Networks Exhaustive Introduction into SAN, IBM Redbook
SAN vs. DAS: A Cost Analysis of Storage in the Enterprise
SAS and SATA, solid-state storage lower data center power consumption
SAN NAS Videos Storage Area Network Info