Storage Basics: Choosing
a RAID Controller
May 7, 2004
The last time we looked
at server room components, we examined NAS
and SANs. This week, letís turn our focus to another storage corner of the
data center ó RAID controllers.
The RAID controller is
best described as a device in which servers and storage intersect. The
controller can be internal to the server, in which case it is a card or chip,
or external, in which case it is an independent enclosure, such as a NAS
(network-attached storage) (define).
In either case, the RAID controller manages the physical storage units in a
RAID system and delivers them to the server in logical units (e.g., six physical
disks may be used to ensure that one drive stays correctly backed up, but the
server sees only one drive).
While a RAID controller
is almost never purchased separately from the RAID itself, the controller is a
vital piece of the puzzle and therefore not as much a commodity purchase as the
A RAID (redundant array
of independent disks) system is simply a collection of disk drives that employs
two or more drives in combination for fault tolerance and performance. RAID
drives vary in robustness from Level 0 (data striping without redundancy) to
Level 5 (data striping at the byte level with stripe error correction
Lots of Options
Like the controllers that
manage them, RAID devices are internal or external to the server, and itís here
where the waters grow murky. Enterprises have five routes they can explore when
choosing a controller and a device. The first choice to be made, though, is
whether to go with an internal or external solution.
controller-based storage, "the RAID controller and the disk technology are
all outside of a host-based server, and normally housed in high availability
enclosures," says Gartner research vice president Roger Cox. The second
type is "host-based storage," also known as direct-attached storage
(DAS), which breaks down further into two categories, also internal and
external. In simplest terms, host-based storage can be internal or external,
while non-host-based storage exists autonomously and does not require a host
The following table
breaks out the basic storage options.
According to Gartner data, host-based storage accounts for 34 percent of the
overall market for external storage, with the remaining 66 percent going to
"fabric-attached" (network) storage. Cox expects this share to grow
from 66 percent to 77 percent by 2007
Internal host-based RAID
technology is divided into two camps: Serial ATA
(SATA) and SCSI.
Choosing between them involves a trade off of speed vs. price, as SCSI is
faster, but more expensive.
"From a price
perspective, the delta between SCSI and SATA drives is over 5x for a comparably
sized drive," states Barbara Murphy, vice president of marketing for
Applied Micro Circuits Corp.'s storage business unit.
AMCC, which completed the
purchase of RAID management tools vendor 3ware last
month, claims 3ware's recently unveiled Escalade 9000 SATA RAID controllers
bring SATA speeds closer to enterprise level, at least for certain
applications. "For applications that have a high degree of sequential I/O
(disk backup, streaming video, image capture)," Murphy says, "the
3ware 9000 series controllers with SATA drives will outperform any other SCSI
or SATA controller on the market."
Frugality has its limits
though. "We would not suggest that you put a Serial ATA disk in a
situation where it's a highly transaction-oriented, high-performance, critical
application environment," cautions Cox. However, "as people get more
experienced with Serial ATA technology, you're going to hear 'Whoa, holy cow,
you know, I can buy this stuff a lot cheaper.'"
This may be bad news for
vendors, as SCSI and Fibre Channel drives carry a higher gross margin than do
ATA drives, according to Cox, who adds that they "would have to sell a lot
more units in order to keep their revenue up and, more importantly, to keep
their gross margin up."
Another player in the
SATA market is Intel. In early 2003, Intel began selling a four-port SRCS14L
controller. The product tightly integrates the RAID controller with the server
architecture. "So instead of just saying, 'Go and shop for the motherboard
and for the RAID card that suits your needs, and then, it's up to you to make
sure that it all works together,' we say, 'If it's on our list as a validated
combination, then you can be very confident that it's a robust solution,'"
says Steve Fingerhut, Intel product line manager for volume products marketing.
The adoption of the
speedier SATA II standard may bring
another ball to SATA's court. "SATA will [occupy] a stronger market
position as SATA II features arrive," says Luca Bert, director of product
& programs management, RAID storage adapters for LSI Logic. "With
[features] such as native command queuing, enclosure management, and port
multiplier support, SATA II will be a viable solution for an enterprise-level
server, especially in the cost sensitive segment."
The other type of
host-based RAID isn't really RAID at all. External host-based offerings are
commonly described as "just a bunch of disks," or JBOD enclosures. A JBOD system
is not configured as a RAID setup, and therefore we have elected to not to
cover any specific JBOD offerings for this piece. For those interested in
learning more, the DF2000J from LSI
Logic and theFS4100
Fibre to SATA JBOD from Adaptec are two JBOD products worthy of
Meanwhile, on the
external controller-based front, iSCSI, a flexibly routed storage technology,
is gaining momentum and garnering market share. Fibre Channel currently
dominates this space, however. To date, Network Appliance is the lone player in
the iSCSI RAID arena.
"The problem that
iSCSI currently is encountering is that Fibre Channel's not standing
still," Cox says. "They're bringing the price differential down on
Fibre Channel host adapters and switches to a point where iSCSI's losing some
of its value proposition in terms of cost." Cox sees value in iSCSI
despite its slower speed, as "to me the primary value proposition of
iSCSI, until it gets to be 10 Gigabit, will be interoperability."
controller-based product banking on this interoperability is Digi-Data's STORM family of RAID
controllers. STORM Digi-Data CEO Bill Tomeo pins STORM's modular
architecture as its main strength. "The STORM controllers house eight
completely independent channels in one box," Tomeo says. "Each 1U box
has eight independent Fibre Channels on it, so what we've done is eliminate the
catastrophic failures that can occur in a back plane." Controllers
typically have controller boards that share a common back plane, which presents
a problem should a catastrophic failure occur, he explains.
SAN and NAS offerings
also fall into the external controller-based category. For more information
about SANs and NAS, check out last
With five types of RAID
controllers and countless vendors peddling their wares, what's an enterprise to
do? Bert sums up customer considerations best: Ultimately, "the choices of
ATA/SATA, SCSI, or Fibre Channel solutions will depend on the desired trade off
between performance, connectivity, reliability, scalability, capacity, and