STORAGE AREA NETWORK (SAN)
A Brief Documentary by
Soumyajit Basu
Student Of
SYMBIOSIS
INSTITUTE
OF COMPUTER
STUDIES
AND RESEARCH
(SICSR)
AFFILIATED UNDER
ABSTRACT
Data is an integral part of any technology.
Today’s concern is totally based on how to manage storage resources in order to
accommodate more data for future business implementations and how more
improvements can be made in order to provide consistent and better availability
of data. Storage Area Networks enable new ways of moving, storing and accessing
data that promote more reliable and cost effective enterprise wide information
processing. It helps in load balancing among different storage devices to
improve better availability and backing up of data.
STORAGE AREA NETWORK
Storage Area
Network
represents a cluster of storage devices interacting with each other and the
system over the network. What makes a SAN different from normal storage is “Universal
Storage Connectivity”. This triggers the storage of data by negotiating
between devices and ideally helps in sharing of data.
For example in the above figure we see
all of the servers are physically connected to the storage devices. If Server F needs to access the data of Server E there’s no need to copy the
data because Server F can access the
devices on which the Server E stored
the data. Only that Server F needs
the permission to access the data of Server
E.
There are a few beneficial implications
of using the Universal Storage Connectivity or SAN.
·
There’s no need
to schedule or check data transfer between pairs of servers.
·
There’s no need
to purchase and maintain extra storage to temporarily stage one server’s data
onto another.
2.1 What makes Storage Area Network good?
If SAN has to provide I/O backbone for
information service operation then it needs to have few qualities.
·
The SAN must be highly available since failure in providing retrieval of data is not
permissible. A good SAN implementation will have a built-in operation to avoid
any kind of failure imaginable.
·
The I/O performance of a SAN must be scalable as the I/O performance of SAN must grow as the
number of interconnected devices grows for ease of business purposes.
2.2 Why connect to a
Storage Area Network?
·
For providing Universal Connectivity.
·
Higher Availability of data, without
compromising data consistency.
·
High performance.
·
Reduced cost of
providing information services that contribute positively to overall enterprise
goals.
·
If a storage
cluster can be formed over the network then it reduces the overall cost by avoiding temporary storage which is
required to temporarily store data produced by one computer and used by other.
2.3 Architecture of Storage Area Network
In systems that include Storage Area
Network the disks are connected to RAID controllers, to storage
servers. External RAID controllers organize disks into arrays and present the
volumes to the client. External RAID controllers may directly connect to SANs,
to storage servers or directly to application or database servers.
In case of SAN
the RAID controllers divide the storage area into blocks of data. This is
called Stripping and after stripping this chunks of data are then
written onto the disk. But before Stripping is done Mirroring needs to be
done to maintain availability of data so that if any error occurs while retrieving
data onto the disk the system can rollback in order to avail the data for retrieving.
When the storage is manipulated by RAID
controllers the volumes are usually called virtual disks, LUNs
(Logical Units) or logical disks.
The
infrastructure of storage area includes switches, hubs, routers and bridges.
Their main purpose is to route data between different storages across the
network. The switches multiply the bandwidth among the network.
2.4 What are Storage
Appliance devices? What are the different kinds of Storage Area Network
Appliance?
A Virtual
Storage Appliance or a Virtual SAN Appliance is a
software bundle that allows a storage manager to turn the unused storage
capacity of a virtual server within a network into a Storage Area Network
(SAN). Some storage hardware vendors are trying to encapsulate the bundle with
the firmware itself. More recently
this feature is being added at hardware level for optimized utilisation of
memory.
2.4.1 In-band SAN Appliance
In-band SAN Appliance
or symmetric
virtualization actually comes in between the hosts and
the storage. All I/O request and their data pass through the device. Hosts
perform I/O to the virtualization device and do not interact directly with the
actual storage device. The virtualization device in turn interacts with the
storage device for the host.
2.4.2 Out-band SAN Appliance
Out-band SAN Appliance
or asymmetric
virtualization uses meta-data mapping functions in
order to access the storage device. The Out-band SAN requires additional
software in the host that would first request the location of the actual data.
Therefore an I/O request from the host is intercepted much before the request
is forwarded from the host. A meta-data look up is requested from the meta-data
server which returns the physical location of the data to the host. The
information is then retrieved through an actual I/O request to the storage.
3.0 STORAGE AREA NETWORK ESSENTIALS
All the
operations of the SAN are managed through a set of softwares. The various
software requirements that might be necessary for the proper functioning of
Storage Area Network are essential. It is also important for the administrators
to provide proper protection to data in order to guarantee availability and
consistency of data. For that maintenance and backup of the SAN should be
taken.
3.1 Software Requirements for SAN
·
I/O drivers having unique SAN-related
capabilities.
·
Storage
middleware components such as volume managers to enhance and
manipulate SAN exploiting capabilities.
·
Application
middleware components such as cluster managers that implement the
availability and scaling of the functionality of SAN.
·
Database Managers which would
help in load balancing.
·
Enterprise Backup Managers that is able to
make consistent backups of data with minimal overhead while the data are in use
by applications. This might utilise the concept of hot backup or online backup
which would freeze the database headers stopping the increment of SCN, thereby
causing a pause in incrementing the checkpoint. Each data file is then copied
to the backup destination.
·
SAN managers which would be used to manage
the SAN environment.
3.2 Enterprise Data Protection for SAN
·
Periodic backups of online files
and databases that can be used to recover from application or human errors.
·
Electronic
archives
that can be removed from data centres to a secure repository.
·
Running replicas of data that
can be used to recover after a disaster.
·
Transport copies of data that
can be used to move it from one place to another from where it is used less to
a place where it is used more.
3.3 Backup for SAN
It is important to realize that we are
ultimately responsible for the organization’s data. Any discrepancy or loss of
data can cause a huge harm to the growing business of an organization. So
backup, can be defined as the process of making separable copies of online data.
A backup or copy of a set of data objects reflects the contents of the objects
as they existed as a single instance.
There
are generally two kinds of backups
LOGICAL BACKUP – Logical backups are
used to recover from accidentally deleted or modified data.
PHYSICAL BACKUP – Physical backups is
the process of copying configuration files and the database. A physical backup
is mainly responsible from recovery in case of a media failure.
Backups may be:
·
Kept at the data
centre.
·
Moved to
alternate sites.
·
Made
unalterable.
3.4 Enterprise Backup Architecture
There can be several
backup architectures using which we can back up the data.
·
Backup
Clients. The term backup client refers to both a computer
with data to backup and a software component which reads data from online and
feeds it to a backup server.
·
Backup
Servers. It denotes either a computer or a software
component that receives data from the backup client and writes the data onto
the backup media.
·
Backup
storage units. Basically comprises of magnetic tapes,
optical disk drives controlled by a media server.
·
Backup
media. The media in which the data is written.
4.0 Pros and Cons of
Using SAN
The pros of using
Storage Area Network are as follows.
·
Device
Sharing. The ability of two or more servers to access a
storage device.
·
Volume
Sharing. Volume managers keep a track of volume state for a
set of disks. The volume manager maintains co-ordination between two servers by
co-ordinating the meta data updates so that updates from two servers do not
overwrite each other and appear to all servers sharing the volume.
·
File
System Sharing. The ability of two or more servers to share
access to a file system on the device.
·
Database
Sharing. It is common for two or more instance of a
database manager running on separate servers to access the same database for
availability and performance reasons.
·
File
Sharing. File sharing allows processing of files by
applications running on any cluster’s server.
The cons of using Storage Area Network
may be listed as follows.
·
Cost Consumption. The cost for
setting up the infrastructure of SAN is way too costly and is quite expensive
for smaller organizations to implement it.
·
Fragmentation. Generally a
pool of storage must be carved into smaller slices of fragment which is
dedicated to a server. This can often lead to wasted disk space.