R&M Data Center Handbook

www.datacenter.rdm.com
DAS – Direct Attached Storage
Direct Attached Storage (DAS) or Server Attached Storage is a term for hard disks in a separate housing and that
are connected to a single host.
The usual interfaces for this implementation are SCSI (Small Computer System Interface) and SAS (Serial
Attached SCSI). The parallel SCSI interface with its final standard Ultra-320 SCSI was the forerunner to SAS.
However, it represented the physical boundaries of SCSI, since the signal propagation delay of individual bits on
the parallel bus were too different. The clock rate on the bus had to be limited so that the slowest and fastest bit
could still be evaluated at the bit sample time. However, this process conflicted with the goal of continuously
increasing bus performance. As a result, a new interface was designed, one that was serial and therefore offered
greater performance reserves. Since the Serial ATA (S-ATA) serial interface had already been introduced into
desktop PCs a few years earlier, it also made sense to keep SCSI’s successor compatible with S-ATA to a great
extent, in order to reduce development and manufacturing costs through its reusability.
Nevertheless, all block-oriented transmission protocols can be used for direct (point to point) connections.
NAS – Network Attached Storage
Network Attached Storage (NAS) describes an easy to manage file server. NAS is generally used to provide
independent storage capacity in a computer network without great effort. NAS uses the existing Ethernet network
with a TCP/IP protocol like NFS (Network File System) or CIFS (Common Internet File System), so computers that
are connected to the network can access the data media. They often operate purely as file servers.
An NAS generally provides many more functions than just assigning computer storage over the network. In
contrast to Direct Attached Storage, an NAS is therefore always either an independent computer (host) or a virtual
computer (Virtual Storage Appliance, VSA for short) with its own operating system, and is integrated into the
network as such. Many systems therefore also possess RAID functions to prevent data loss arising from defects.
NAS systems can manage large volumes of data for company uses. Extensive volumes of data are also made
quickly accessible to users through the use of high-performance hard disks and caches. Professional NAS
solutions are well-suited for consolidating file services in companies. NAS solutions are high-performance,
redundant and therefore fail-safe, and represent an alternative to traditional Windows/Linux/Unix file servers.
Due to the hardware they use and their ease of administration, NAS solutions are significantly cheaper to implement
than comparable SAN solutions. However, this is at the expense of performance.
SAN – Storage Area Networks
A Storage Area Network (SAN) in a
data processing context denotes a
network used to connect hard disk
subsystems and tape libraries to server
systems.
LAN aggregation
& core switch
SANs are designed for serial, continuous,
high-speed transfers of large
volumes of data (up to 16 Gbit/s). They
are currently based on the implementation
of Fibre Channel standards
for high-availability, high-performance
installations. Servers are connected
into the FC-SAN using Host Bus
Adapters (HBA).
LAN
access
switch
Ethernet
Ethernet
FC
FC
FC SAN
switch
The adjacent graphic shows a typical
network configuration in which the SAN
network is being run as a separate
network by means of Fibre Channel.
With the continued development of
FCoE (Fibre Channel over Ethernet),
this network will merge with Ethernetbased
LAN in the future.
Server
FO
Copper
FC Storage
FC and the migration to FCoE is shown in sections 3.7.2 and 3.8.3 as well as 3.8.4.
R&M Data Center Handbook V2.0 © 08/2011 Reichle & De-Massari AG Page 83 of 156