What weighs more: one pound of bricks or one pound of feathers?
Which is faster: 2 Gb FC or 1 Gb Ethernet? Hint: Both questions
have the same answer.
In storage, an area that is often misunderstood is
iSCSI performance and how it compares to
FC. Both of these SAN interconnects are
typically measured by
bandwidth with "2 Gb" FC SANs dominating the
storage market today and "1 Gb" Ethernet used for the majority
of iSCSI SANs.
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Which would you say is faster: a 2 Gb FC connection or a 1 Gb
Ethernet connection? It's a trick question -- they are equally
fast. They both transfer data at the speed of light. Bandwidth is
not an issue of speed but size. Consider the following analogy;
think about a four-lane highway versus a two-lane highway. If there
are just a few automobiles traveling on either highway drivers will
be able to go the maximum speed. However, as more drivers travel on
each road, the two-lane highway will experience a bottleneck before
the four-lane highway does.
This is the same with FC and Ethernet. A 2 Gb FC interconnect
has twice the bandwidth (double the number of lanes) of 1 Gb
Ethernet. Bandwidth has an impact on storage performance when large
requests are being processed. In this case, most of the work is
spent transferring the data over the network making bandwidth the
critical path. However, for smaller read and write requests the
storage system spends more time accessing data making the CPU,
cache memory, bus speeds and hard drives more important to overall
application performance.
Unless you have a bandwidth intensive application (e.g.,
streaming media or backup data) the difference in performance will
be minimal. Enterprise Strategy Group (ESG) Lab has tested storage
systems that support iSCSI and FC and the performance difference is
minimal -- ranging between five and 15%.
In fact, an iSCSI storage system can actually outperform a
FC-based product depending on other, more important factors than
bandwidth -- including the number of processors, host ports, cache
memory and disk drives and how wide they can be striped.
The slowest component of the storage performance chain is the
hard disk drives. It takes a hard disk drive much longer, sometimes
several thousands-percent longer, to access data in a storage
system than the electronic components like processors, bus and
memory. The timeline for an
I/O starts with a read/write command being
sent to the hard drive from the application. This is followed by
long, mechanical access times waiting for the drive to move the
actuator, referred to as the seek process. The seek process is
by far the slowest part of storage performance. The actuator
then has to spin to the data that's been requested, which is
another long mechanical process that creates latency. Next, the
data is transferred from the drive to the CPU and a status
handshake is performed to terminate the request. Access time
associated with all disk drives, which includes seek + latency,
is clearly responsible for the majority of the "wait time."
Traditional storage systems are typically limited in the number
of drives across which they can stripe data. Many traditional
storage systems can only stripe up to 16 drives, while more
advanced products can stripe across hundreds of drives. Striping
data across a large number of drives, allows a system to leverage
all of the actuators which work in parallel to make read/write
functions a much more efficient process. Striping data across many
drives increases performance and essentially eliminates the need
for tuning performance and determining hot spots. Naturally, there
is a cost associated with acquiring more hard drives, so a balance
and consideration of price/performance is important.
In ESG Lab head-to-head testing, we configured a storage system
using traditional striping methods and another one using wide
striping. ESG Lab used the same workloads to compare the
performance of the traditionally configured system and that of a
system using a wide stripe group of 48 drives. The stripe group of
48 drives significantly outperformed the traditional method.
A comparison of Iometer results revealed a 44% improvement in
the number of disk I/Os per second when switching from traditional
volumes to a 48-drive wide stripe group. That is an amazing
performance difference, much more than the five to 15% difference
that we found between iSCSI and FC.
Some iSCSI storage systems may not have well-tuned performance
optimized iSCSI target drivers. This is the fault of the storage
vendor and they need to go back to their R&D group and do a
better job. Additionally, ESG Lab has found that using a TCP/IP
offload engine (TOE) on the iSCSI target port within the storage
system can have a measurable positive impact on performance. Some
iSCSI storage systems do not have integrated TOE support.
The architecture of the storage system, the speed and number of
processors, the amount of memory and the intelligence of its
caching algorithms, the speed the disk drives and number of drives
in a stripe group, the number of host ports and the backend
interconnect all play a major role in performance. I recommend that
you evaluate the storage system based on all of the above criteria.
It is the storage system itself that will make a bigger difference.
The speed of iSCSI is not the issue.
About the author: Tony Asaro is the senior analyst for
Enterprise Strategy Group.