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SAS: The demise of FC as we know it?
Vivekanand Venugopal writes that SAS offers superior
value and better availability than Fiber Channel
Vivekanand Venugopal |
It’s a debate that has raged on for years. On one side
of the ring are the Serial Attached SCSI (SAS) proponents with their strong
views of it being an evolutionary step up from SCSI and the replacement for
the well-entrenched Fiber Channel (FC) which is the other contender. Meanwhile,
the FC side maintains the dominant view that the technology still outperforms
other technologies.
Over the past decade, the market has been singing FC’s tune. The technology
has gained a strong reputation and market share, especially in the enterprise
storage space. Although it was not supposed to be a replacement for the slower
SCSI, it soon became a symbol of fast performance and high availability. Any
serious enterprise storage system supports FC; anything else is looked upon
with disenchantment and not as a serious enterprise solution.
However, the topology of the market has changed since then. What was thought
to be a theoretical debate is now threatening to topple FC’s ten-year reign
as the preferred standard for enterprise storage. Moreover, this is for a good
reason—the concern over the global economy.
Although performance and availability remain as major concerns when designing
a storage system, cost is becoming an influential driver. In fact, it is today’s
dogged concern about cost, especially during a worldwide economic recovery,
which is driving renewed interest in SAS as worthy competitor of FC.
Although there are many reasons for SAS’ growing importance most are centered
on three important ones.
More bang for the buck
When placed side by side with a 3 Gbps SAS design, a 4 Gbps FC version will
provide better throughput on a single link with a single I/O. However, the performance
of most storage systems is seldom based on a single I/O; instead the entire
system will need to be gauged. This is where SAS outshines its rival.
For example, if any storage product uses a SAS design, the total bandwidth for
disk access eclipses any competitive modular storage system with a FC backend.
The solution also has more backend links, allowing more concurrent I/Os.
Loop arbitration wait times do not affect the product that supports SAS-based
switch architectures. In comparison, FC-AL (Fiber Channel-Arbitrated Loop) based
systems share a common loop and suffer from wait times.
In addition, SAS always promises full duplex. Full duplex allows any device
to transmit and receive I/Os simultaneously. In short, full duplex is always
preferred. FC, however, offers half or full duplex depending upon the system
architecture.
Better availability
In today’s market, availability is a huge concern. Systems are now running
24 hours a day, 365 days a year; failures have become a luxury that no business
can afford. This has led to many customers shelling out huge amounts of cash
for high availability.
SAS not only provides a less expensive option, it handles failures more efficiently
than FC. It uses component level reporting for failures. Essentially, this means
that if a component on a SAS link fails, it will be mapped out and the failure
broadcasted and reported by the system-monitoring tool. For an administrator,
this saves time and effort to find out where the failure has occurred.
In comparison, FC does not report at the component level. Instead, it reports
any failures at loop level. This simply means that failures will take a longer
time to be corrected, reducing availability rates. In addition, SAS allows for
greater redundancy to be built in.
Co-existence with SATA
Serial ATA (SATA) is an evolutionary step for ATA drives, akin to what SAS is
for SCSI drives. Both SATA and SAS utilize serial technology (hence the name)
so that data is transferred faster than ATA and SCSI, which use slower parallel-based
technology.
SATA extends the ATA roadmap and will continue to be the mainstay technology
for disk interface where cost is a primary concern, such as desktop PCs and
sub-entry servers. SAS extends SCSI’s roadmap and will be seen as a disk
interface technology for enterprise storage systems.
Now with SAS and SATA drive connectors, administrators gain the freedom to intermix
them or the choice to install drives according to business need and budget.
Basically, the ICO chip encapsulates the SATA commands within SAS packets, using
SATA Tunneling Protocol of the SAS specification. This allows the administrator
to use SATA disks and later use SAS drives when they are really needed.
In which case, the product will support both SAS and SATA drive connectors and
allows the administrator to intermix both types in the 15-disk enclosure, simplifying
cabling and forgoing the costly purchase of unique disk trays.
Although there are connector dongles or even FATA that allows FC and SATA drives
to be installed in the same tray, these require conversion of the I/O from FC
to SATA. This results in additional cost and performance degrading overhead
that SAS easily overcomes.
Possible outcome
It is still premature to say whether SAS will totally displace FC. However,
with SAS already as reliable and as fast as FC, its eventual success will depend
solely on companies taking the first steps to offer SAS solutions and customers
purchasing them.
Obviously, loosening the ten-year hold of FC will not be easy. It will take
time for customers to see FC and SAS on the same level playing field. Nevertheless,
when this occurs, customers will use their wallets to vote. By playing the cost-effective
performance card, SAS will be a better position to win the debate.
The author is VP & GM, India, Hitachi Data Systems
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