Storage is consuming an increasing proportion of IT spend. Bryan
Betts offers some pointers on how to choose between the warring
factions claiming the best solution
The battle between storage area networks (Sans) and network
attached storage (Nas) has become a phoney war. With Nas suppliers
adopting San technology and San developers looking to broaden their
market appeal, some are hoping that a peace treaty cannot be far
away. However, all is not yet quiet on the storage front -
skirmishes continue in the demilitarised zone. In addition, the
arrival of fresh battalions under the Internet small computer
systems interface (iSCSI) banner could significantly disrupt the
balance of power.
For many, any truce would be a temporary slowdown in their quest to
convert the world to their way of thinking. To San proponents, Nas
is just another component of the storage network, while for some
Nas developers, the main use of San technology is to build bigger
and more capable Nas systems.
"I don't believe that the two will ever converge fully," says Jim
Selby, Nas product manager with Microtest. "The underlying
technologies are just too different. I think it is just a marketing
ploy by certain suppliers to make it seem that they cover both
options. Nas and San are very different animals and the IT manager
needs to consider both, as each has good and bad points."
The most fundamental difference between the two is that Nas
provides file-level access to storage, whereas Sans operate at
block level. Block access is the way most storage is accessed at
the drive level: it is how a Nas server talks to a direct-attached
SCSI drive and it is well suited to storage-specific applications
such as back-up and storage management.
Block access also suits record-based applications such as
databases, where moving the whole database across the network to
change a single record would be counter-productive. File-level
access is preferable for smaller files, and for file and data
sharing.
However, Sans are an enabling technology, whereas Nas is more of an
application of technology. Developers are starting to see how the
two can complement one another. Nas specialist Network Appliance
uses Fibre Channel storage within its systems and recently added
the ability to back up its file servers to a shared tape library
via Fibre Channel and a San switch.
This is just a first step, according to Network Appliance's chief
executive officer, Dan Warmenhoven. "[We were] already using Fibre
Channel discs," he says. "The next step is to pool that storage for
management purposes by integrating Brocade switches into our
systems - we will put Fibre Channel loops behind our filers."
Warmenhoven quotes figures from analyst firm IDC, which value the
storage market for 2001 at $39bn (£26bn) and show it growing 12% a
year. "Deduct proprietary storage and you have $32bn left, growing
at 14.7%," he says. "Then discount server internal storage and that
leaves $25bn, growing 19.5% - that's the NT and Unix external
storage.
"In every commercial application, everything is external. Nobody
buys a server with storage in. The classic approach was to use
direct-attached SCSI, but that's declining and what's taking over
is networked storage, which splits into two categories, Nas and
San," says Warmenhoven.
In his view, these two technologies are going to converge over time
because they address two different problems, one physical and the
other logical.
"For Nas, I use the logical analogy of a Web URL converting to an
IP address," he explains. "San addresses the physical issue of
connecting the server to the storage. Ultimately, I believe all our
customers are going to do both."
Warmenhoven makes the point that while Nas can outperform
direct-attached storage, because it off-loads the processing load
associated with storage, this requires dedicated machine-room
networks of gigabit class, segmented from the other networks.
"That's a San," he says. "The only issue is semantics."
Users seem to agree. "The world was relatively homogeneous, but now
there is a second technology arising in the market, in Nas," says
Erhard Schmidt of Dresdner Bank Global IT Services, which provides
storage services, among other things, to companies within the
Dresdner Bank group.
"I think it is complementary to San, there are good reasons to
implement both - the service provider has to deliver the best
solution for the client. The challenge for the San guys is to find
the places for synergy, for example, the back-up process from Nas
into the San, a common free-space pool, and management," explains
Schmidt.
As the two converge, the Nas server begins to look different. It no
longer needs to contain its own storage, so it can become an
appliance embedded within the San, and sure enough, these first
devices have appeared, called Nas head-ends or IP gateways.
One of the first was Raidtec's SNAZ-FC, which uses existing
Linux-based Nas technology to provide Ethernet access to Fibre
Channel storage.
"It provides cheap file-level access to a San storage pool," says
Raidtec's sales manager Orla Donohoe. "SNAZ-FC also allows Nas to
be managed centrally and it lets any server or client PC access
files stored on the San, even if that computer doesn't have a Fibre
Channel connection."
Others working in this area include start-up company BlueArc, which
uses a San behind a specialised Nas box to give better performance
and support more storage. It aims to operate at wire speed, with
Gigabit Ethernet on one side and Gigabit Fibre Channel on the
other.
IBM, too, has jumped in, with its 300G IP gateway. "Our customers
say that where Nas fits, we want to put Nas, but we want Sans
technology in the datacentre," says Daniel Sazbon, IBM's storage
networking solutions manager.
He says customers often have islands of San and Nas which need to
be integrated to the level where the same staff can manage both.
"The main difference is that Fibre Channel Sans scale better,"
Sazbon adds. "In the Nas environment, you sometimes see 3Tbyte
that's big for Nas, but you could see 80Tbyte in a San."
Others argue that it is important not to assume that everything
which uses Fibre Channel is a San, or vice versa. The iSCSI
standard has not yet been ratified, but the first iSCSI products
are already appearing, demonstrating that it is indeed possible to
build a San on Ethernet.
"Sans are not a technology - they can be built from a variety of
hardware, using a number of protocols," says Nigel Williams,
vice-president of software operations at software supplier Legato
Systems. "Sans are networks that transport storage and back-up
traffic separately from the rest of the network." In his view, even
a standard Ethernet segment that is dedicated to connecting Nas
servers to a back-up device could legitimately be regarded as a
San.
Most Nas suppliers are turning to Fibre Channel for this
application though, because they need something that is tuned for
storage and iSCSI is not ready yet. The question is whether this
San gets connected to anything else or is used for in-box
connectivity.
For example, Auspex is using Fibre Channel to connect its
high-capacity Nas servers to tape libraries and to interconnect
servers for synchronous data replication, yet the company's
European marketing director Paul Sleep expects the Nas servers to
remain as discrete entities.
"We have requirements for performance improvements, which can only
be met by using San technology," he explains. "We are moving to
Fibre Channel as it doesn't have the inherent latency issues of
Ethernet, but I don't think we'll logically separate storage and
servers. There will be applications for a Nas head, but some users
like it all centralised."
It seems certain that stand-alone appliances will remain popular at
the low-end of the Nas market as well. The ability to add gigabytes
of low-cost shared storage at a department or workgroup level is
too attractive, even if it does not readily submit to centralised
management and pooling.
That is changing though, as companies such as FalconStor introduce
virtualisation techniques which can incorporate both San and
Nas-hosted storage. Other storage virtualisation schemes rely on
embedding intelligence within a San, in the form of a specialist
server which can assume control of all storage and then parcel it
out to the application servers as required.
FalconStor's approach adds a layer of Linux-based IP storage
servers which provide a uniform interface to any storage, whether
attached by Fibre Channel, parallel SCSI, iSCSI or normal Ethernet,
turning it into a logical device which the application servers can
then use.
FalconStor marketing vice-president Wayne Lam says, "The one thing
we identified was virtualisation - the ability to isolate the real
hardware technology from the logical view that the application
server sees. We disassociate the local storage into a storage farm,
hosted behind our servers."
The merged architecture still assumes that most storage will be on
separate network segments for better performance, but it can be
delivered either in block form, using a virtual SCSI driver that
runs on the application server and sees the San storage as a local
drive, or in file form via a Nas approach.
As San and Nas converge, this difference in the delivery method is
perhaps the only remaining solid technical difference between the
two. However, it is perfectly possible to envisage the same network
delivering both block and file traffic, even to the same
application server.
One result of this convergence is that suppliers and industry
groups are starting to use the all-encompassing term "storage
networks". The Storage Networking Industry Association (SNIA) has
even come up with a single model which can be used to describe any
storage set-up.
Key to the SNIA single storage model is the concept of block
aggregation and where it takes place. Block aggregation covers a
host of technologies, but it takes real storage and presents it in
a logical form that a server can use, such as blocks or
file-space.
"For example, block aggregation is what a Raid controller does when
it mirrors - it's the structure laid on the block map as presented
by the drive," says SNIA representative
Paul Massiglia. Operating systems can also do block aggregation and
file servers such as Nas boxes are a specialist form too.
San virtualisation is another form of it, but Massiglia, whose
full-time job is technical director at Veritas Software, prefers
not to use this term. "The SNIA technical council is trying to
avoid the virtualisation word, because so many people are doing so
many different virtualisations - the term has been co-opted," he
says.
So, will we end up with a single storage infrastructure that does
both Nas and San? "Absolutely," says Rupert Beeby, technical
manager at storage integrator Sagitta Performance Systems. "But
San's advantage will remain high performance - Nas will always
offer lower performance, especially for very large files. And while
the technologies will converge, customer requirements will remain
separate and there will always be some who want a Nas appliance."