Understanding Storage Technology – Part 2: The Alphabet Soup of Storage

By - January 9, 2014

In a previous blog entry, we looked at the evolution of today’s storage technologies.  Too often, however, the dizzying array of acronyms makes understanding storage difficult.  What’s the difference between SAN and NAS?  What is DAS?  What are iSCSI, NFS, CIFS, FC, and FCoE?  These are the topics we will cover today.

As we break down our alphabet soup of storage terms, the first thing is to separate the technologies.  Storage technology today is going to either be a storage area network (SAN), network attached storage (NAS), or direct attached storage (DAS).  The first two, SAN and NAS, probably look very similar; almost the same words in their names, just in a different order.  DAS, on the other hand, is a bit different.  We will start by looking at DAS. 

DAS – Direct Attached Storage

What differentiates DAS from SAN and NAS is that DAS typically connects storage to a single computer through the computer’s host bus adapter (HBA) port.  The DAS device has a one-to-one relationship with the system it serves.  It may help to think of DAS like a common external hard drive available at the local electronics store; it plugs into the computer and provides storage, however it typically cannot be simultaneously plugged into another computer.

The protocols used by DAS are similar to those protocols used by computers to connect and communicate with internal storage (hard drives, CD/DVD drives, etc.).  Typical DAS protocols used by DAS are Advanced Technology Attachment (ATA), Serial Advanced Technology Attachment (SATA), External Serial Advanced Technology Attachment (eSATA), Small Computer System Interface (SCSI), Serial Attached SCSI (SAS), and Fiber Channel (FC).

With DAS technology, the attached computer or server is usually responsible for the file system and file management of the data on the DAS device.  The DAS appliance simply stores the data blocks on its disk (or array of disks), and has no knowledge or control of the file and folder structure, or permissions and attributes of the data. 

SAN – Storage Area Network

A SAN consists of a storage device; a dedicated, isolated network (sometimes referred to as a “fabric”); and one or more host bus adapters (HBAs) in the computers or servers connected to the SAN.

The network fabric that provides storage connectivity between the SAN device and the connected computers can come in a few different varieties.  Most commonly an IP-based copper Ethernet network is used.  This is very similar to the network fabric that connects a desktop computer to the company file server or the Internet.  The center of this type of fabric is usually one or more Ethernet switches running at port speeds of 1 gigabit/second (Gb/s) or faster.  In some cases these are dedicated SAN-only switches.  In other cases an Ethernet switch is shared with other general network traffic and a few ports are logically segregated for SAN traffic into a virtual local area network (VLAN).  

Another type of SAN fabric is fiber optic.  With a fiber-based SAN, a dedicated fiber channel switch is used to provide the connectivity between the storage device and the connected computers.  Because this scenario requires the introduction of a specialized switch and less-common fiber cables, this type of SAN fabric is less common.

The three primary protocols used in SAN communications are carry-over protocols from DAS technology.  The choice of which protocol to use is largely driven by the fabric type in place.  Fiber Channel (FC) protocol is the typical choice in a fiber channel network.  Internet Small Computer System Interface (iSCSI) is the usual protocol for copper-based 1 Gb/s Ethernet fabrics.  The third option, Fiber Channel over Ethernet (FCoE), combines the behavior of FC but uses faster Ethernet (typically 10 Gb/s or faster) networks.  The physical connection to the FCoE network can be with copper or optical cables.

Just like DAS, with SAN, the attached computers are responsible for the file system and file management of the data on the SAN device.  The SAN appliance simply stores the data blocks on its array of disks and has no knowledge or control of the file and folder structure, or permissions and attributes of the data. 

NAS – Network Attached Storage

Where SAN required a dedicated storage network, and DAS required a direct connection to the served computer, NAS connects to an existing network.  NAS typically uses a common Ethernet network and can provide storage services to any authorized devices on that network.

Two primary NAS protocols are used in most environments.  The choice of protocol depends largely on the type of computer or server connecting to the storage.  Network File System (NFS) protocol usually used by servers to access storage in a NAS environment.  Common Internet File System (CIFS), also sometimes called Server Message Block (SMB), is usually used for desktops, especially those running Microsoft Windows.

Unlike DAS and SAN, NAS is a file-level storage technology.  This means the NAS appliance maintains and controls the files, folder structures, permission, and attributes of the data it holds.  A typical NAS deployment integrates the NAS appliance with a user database, such as Active Directory, so file permissions can be assigned based on established users and groups.  With Active Directory integration, most Windows New Technology File System (NTFS) permissions can be set on the files contained on a NAS device.

To help decipher our alphabet soup, here are the most common acronyms used when discussing storage technologies:

ATA ‑ Advanced Technology Attachment
CIFS ‑ Common Internet File System
DAS ‑ Direct Attached Storage
eSATA ‑ External Serial Advanced Technology Attachment
FC ‑ Fiber Channel
FCoE ‑ Fiber Channel over Ethernet
Gb/s ‑ Gigabits per second
HBA ‑ Host Bus Adapter
iSCSI ‑ Internet Small Computer System Interface
NAS ‑ Network Attached Storage
NFS ‑ Network File System
NTFS ‑ New Technology File System
SAN ‑ Storage Area Network
SAS ‑ Serial Attached SCSI
SATA ‑ Serial Advanced Technology Attachment
SCSI ‑ Small Computer System Interface
SMB ‑ Server Message Block
VLAN ‑ Virtual Local Area Network

Finally, when speaking to people about storage, it is common for a person to point to their new, shiny appliance and say, “that’s our new SAN,” or, “all the data is on that NAS.”  The problem with this is that SAN, NAS, and DAS are technologies.  They are not devices.  It is a little like cooking or baking.  It probably would not make sense to point to your oven and say, “that’s my baking.”  While the oven is a critical part of baking, the concept (or technology) of baking involves much more.  But if you have made the mistake of calling a device “a SAN” or “a NAS,” don’t worry.  This is a common mistake for even seasoned IT veterans. 

One reason this distinction is so important is that many devices can perform more than just a single function.  Just like your oven at home may also have a stove top on it, thereby providing both baking and cooking services.  Many storage devices will provide both SAN and NAS services; sometimes concurrently.  Other devices can serve both DAS and NAS functions.

In our next blog entry, we will look at the important metrics that help to determine which storage device is right.  We’ll discover that proper sizing of a device goes way beyond just adding up terabytes of capacity.

In the meantime, if you are considering a storage project or if you are making any updates or changes to your datacenter, it is a great time to think about modern storage technologies.  RSM has information technology consultants that specialize in storage evaluation, sizing, design, implementation, administration, and optimization. For additional information about our offerings check out our website. To talk with one of our storage consultants, call 800-274-3978, or send us an email.  We look forward to talking with you.

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