IP SAN Data Storage for Media Applications

Introduction

Digitization of data is one of the key drivers of the media and entertainment industry in India according

to a recent FICCI-KPMG report. Replacing the over 50 year old analog technology, digital transmission

and storage of media bring improved communication, security, quality and flexibility to the table. While,

traditionally the media firms have looked to DAS or Fibre Channel (FC) based SAN for storage of digital

media for various reasons, IP SAN technologies are fast becoming cost-effective and competitive

technology to replace them.

Quality of Service

Media applications are bandwidth-intensive technologies that place high demands on the storage

subsystem in comparison to traditional file serving and online transaction processing (OLTP) storage.

While OLTP loads are typically random loads that are drive limited and do not saturate the links, media

applications such as CCTV and media broadcasting applications have one or more audio and video

streams that demand specific data transfer rates that must be met to satisfy Quality of Service (QoS)

requirements.

In the case of audio/video streaming applications, QoS refers to the need that the user must receive the

audio/video without interruption and loss of quality by lost frames. In order to keep the media stream

continuous, often, some type of buffering is implemented to eliminate the effects of kinks in the line.

The user’s experience of uninterrupted reception can be guaranteed only if the buffer is maintained to

be non-empty at any time. One way of ensuring continuous playback is by dropping intermittent frames

thereby reducing the frame rate. However, this would result in loss of audio/video quality. With the

modern day demands of high definition video and audio, this would not be an acceptable solution.

Another way to ensure a continuous playback in such a scenario is to have a bigger buffer thereby

mitigating the effect of slower links. Thus it becomes clear that the rate of the incoming streams (link)

play a big role providing the required quality of service. This depends on both the link available and the

ability of the storage to deliver the required throughput (data transfer rate).

Likewise, for CCTV type of applications when different parts of a surveillance area are monitored using

many recording cameras and monitors, there are a number of recording and playback streams

happening simultaneously to and from the storage. These parallel streams place high demands on

throughputs from the storage.

Storage approaches

Traditionally, such setups have a local storage attached to the cameras, thereby avoiding any link delays.

However, such distributed Direct Attached Storage (DAS) are inefficient means of allocating, using and

scaling storage. Storage consolidation with SAN and NAS storage solutions address these concerns of

flexibility and scalability by centralizing the storage for all recording and playback streams. However, this

would mean that the bandwidth of the links play a critical role in deciding the throughputs achievable.

Thus far, Fibre Channel based SAN with their 4Gbps throughputs have been favored over IP SAN and

NAS solutions with traditionally much lower throughputs. However, the costs of setting up a Fibre

Channel network remain expensive and include an elaborate setup of equipment not commonly

available in enterprises including Fibre channel adapters and switches.

Ethernet based IP SAN solutions work with the existing LAN networks available in the enterprises,

thereby increasing the ease of installation and reducing the acquisition costs. However, until recently, IP

SAN solutions using iSCSI have been confined to file serving or OLTP type of applications that typically do

not need high throughputs. But, when it comes to bandwidth-intensive applications such as media

applications the throughput assumes critical significance. With gigabit Ethernet (GigE) ports used for

commercial iSCSI storage solutions, data transfer rates of 1Gbps or about 125 MB/s (half-duplex) or over

200 MB/s (full-duplex) can be achieved. But, this is significantly lower than the rates achievable from

fibre channel links. For these reasons, and reasons of relative newness of iSCSI, Fibre Channel based

storage solutions have been preferred thus far for media applications. However, of late, with the influx

of several key technologies and performance improvements, IP SAN solutions have gained acceptance in

the media industry.

Benefits of iSCSI

iSCSI built on top of the erstwhile SCSI (Small Computer System Interface) works by transmitting SCSI

packets using TCP and IP protocols that offer services such as guaranteed delivery, flow control and

addressing. Since TCP/IP are the ubiquitous transport and network protocol layers of Ethernet networks

and the Internet, iSCSI storage solutions can co-exist with existing networks in the enterprise making

them easy to setup and administer and without the need for specialized hardware lowers the overall

storage costs.

Today, a number of vendors offer a spectrum of robust and scalable and iSCSI based storage solutions

that include advanced SAN feature sets that mirror the feature sets of FC SAN solutions. Several

advanced features such as Thin Provisioning, Snapshots, Synchronous and asynchronous replication, HA

Clusters and Clustering, CDP etc are available in iSCSI based storage solutions as well. In fact, certain

features such as asynchronous replication that needs to operate over long distances benefit from iSCSI

storage that work with the omnipresent Ethernet networks when compared to distance limited FC

setups. Storage solutions such as AMI’s StorTrends iTX have in-built WAN acceleration techniques that

help in speeding up the data replication throughputs to near-bandwidth level.

Thus, concerns about reliability and stability of iSCSI solutions have been answered. This is apparent

from an Enterprise Strategy Group (ESG) survey couple of years ago that found that 50% of iSCSI

applications are Tier-1 application primary storage and 36% of the implementations had used iSCSI for

their application primary data. These numbers are on the rise and iSCSI will continue to take a bigger

share of the exploding storage market.

iSCSI Performance – Numbers Matter

An (ESG) Lab report from 5 years ago has indicated that the difference in FC and iSCSI performance is

very minimal (about 5-16% depending on the application including bandwidth-hungry applications such

as media applications) and a carefully crafted iSCSI setup can outperform FC setups. An eye-opening

statistic in this ESG Lab Report was that a large amount of FC infrastructure deployments were in

production environments that were still 1 Gb. This goes to show that the performance level offered by

GigE based iSCSI storage solutions would have been sufficient for these deployments. Since the lab

report was published both FC and iSCSI have had several improvements made towards improving their

performance. Several key performance improvement techniques can be done to improve the

performance of iSCSI storage solutions. These include –

Advanced Caching techniques

Caching is a critical component in storage systems. A carefully designed caching architecture would go a

long way in improving the performance of iSCSI storage solutions. By selecting a kernel based iSCSI stack

that integrates with the caching module in such a way that it ensures that there is zero memory copy

involved thereby placing less demands on the system resources such as CPU and memory and ensure

that the system performs with enough steam left to handle much more loads. iSCSI storage solutions,

such as the StorTrends storage servers from American Megatrends Inc (AMI), also tune the caching

granularities based on the underlying block sizes, which includes the RAID stripe size, thereby helping to

automatically tuning the disk spindles to yield the best possible I/O performance. In addition, for media

applications whose IO load patterns are sequential in nature reading blocks ahead using Read-ahead

Caching mode would minimize the number of disk IOs, thereby drawing the best performance from the

system.

Multipathing and Network Teaming

Storage solutions such as AMI’s StorTrends iTX support multiple connections to the same target from an

initiator, meaning multiple GigE links can communicate with the same target. Thus using MPIO

(Multipath IO) and network aggregation and virtualization techniques where multiple NICs are

abstracted as a single interface having higher bandwidth will ensure that the throughput achieved for

sequential loads of a media application can match or exceed the throughputs of FC by balancing the

loads across multiple links and concurrent transport utilization.

Initiator CPU tuning

One of the precepts for favoring iSCSI over FC is the minimal requirements on the host side and the ease

of setup. In order to do this, software iSCSI initiators are often used to connect to the iSCSI target and

provide a virtual block device on the host side. However, one of the concerns raised by critics of iSCSI is

that this would mean higher CPU utilization in the host causing applications to slow down.

iSCSI was designed to work over Ethernet, which was devised for small packets in the range of 1.5 KB.

The general line of thinking is that since more packets mean more interrupts and, by extension, more

CPU overhead, iSCSI would seem to be at a disadvantage here. By offering the support for jumbo frame

sizes of up to 9KB in size, iSCSI storage servers adequately address this issue, allowing for enhanced I/O

performance and lower CPU utilization. Conversely, if the CPU becomes an issue on the server side, then

an iSCSI Host Bus Adapter can also be used to increase performance.

10 GbE and FCoE

Despite the vast performance improvements offered by iSCSI today the hard core critics of iSCSI remain

unmoved. iSCSI addresses these concerns and steps to the next level with the arrival of 10 Gigabit

Ethernet exceeding bandwidths offered by FC. Offering 10 times the bandwidth offered by the current 1

Gbps offerings, the throughputs of iSCSI solutions grow manifold satisfying even the most bandwidth-

hungry media applications.

While the gap to the performance has been bridged by the iSCSI industry, now there is a growing need

for both these technologies to coexist. Often, enterprises, who have already adopted Fibre Channel,

now seek to use iSCSI but do not want to get rid of their already spent money and time on build the FC

setup. Fiber Channel over Ethernet (FCoE), an encapsulation of Fiber Channel frames over Ethernet, is

aimed at bridging this gap.

Conclusion Media applications are bandwidth-hungry applications that place high demands on the Quality of

Service obtained from the underlying storage. Initially favoring DAS and FC SAN solutions, now these

applications have started moving towards iSCSI based storage solutions. ISCSI based storage solutions

have now stabilized and offer any and every service that FC offers and have started eating in to the

market space that was previously dominated by FC based solutions.

For More Information about IP SAN Storage Solutions , http://www.amiindia.co.in or

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