Setting up a SAN for video applications
By Nelson Nahum
This tip explains how to set up a SAN with virtualization and share file system in order to create a storage system fine-tuned for video and streamming applications.
Requirements for video storage
In the last few years digitizing video has become more and more popular. Storing the video in digital format opens new possibilities for applications for editing and broadcasting while at the same time presents new challenges on how to store the video content. The needs of advanced streaming media applications are very different from traditional storage requirements. Unlike traditional databases, file systems and Web content, video content must be read/written at a minimum guaranteed rate.
A single NTSC or PAL stream requires an uninterruptible 30 Mb/sec stream, while an HDTV requires 178 Mb/sec. Recording high quality video, saved in MPEG-2 format requires 50 Mb/sec sustained rate. Each NTSC or PAL stream will fill 1 GB of storage in about 30 seconds, whicle each HDTV stream will do it in just five seconds. A video system typically has multiple streams entering into the system and multiple editing stations using the data recorded for editing and finishing.
In other words, a video system requires very high data rates, guaranteed bandwidth, dozens of hosts, the capability to constantly add more and more storage and the capability to share those video files between the different hosts. It also requires high availability of all the components and a quick and easy way to store the content in tapes.
One possible combination of technologies we recommend for this purpose includes a Fibre Channel SAN, asymmetric virtualization and a SAN share file system.
With the FC SAN, it is possible to place a large amount of storage devices, and it allows having dozens of hosts with video editing software accessing the same storage devices at very high bandwidth. Large tape libraries can be added to the same SAN for backing up the data. The FC SAN needs to have a special setup in order to ensure that there is no interruption of the streaming when a new device or host is added or removed. We recommend you isolate every host by zoning every host with all the storage systems, and disable receiving RSCN notifications from the FC fabric. At the HBA level, it is necessary to disable sending RESET to the storage devices at power up.
An FC SAN allows the same device to be accessed by multiple hosts, but because of the high data rates needed, those devices will quickly become the bottleneck.
The solution is to use an asymmetric virtualization product, where a single virtual device could be in fact, a stripe of multiple RAID systems. In this manner assuming that a single RAID device could sustain 80 Mb/sec of data transfer, in a system where a virtual device is composed by the striping of 10 of those RAIDs, the virtual device could sustain 800 Mb/sec.
But in order to provide this bandwidth to dozens of hosts, the virtualization must be asymmetric. That is, the virtualization metadata manager should be out of the data path and not interfering with the data streams. In the case of asymmetric virtualization, the data flows directly between the hosts and the storage devices allowing the use of the full bandwidth of the fabric.
Virtualization also supports expanding virtual devices on the fly. It can take care of the multipathing between the host and the new storage devices, thus providing high availability and load balancing between paths.
In other words, asymmetric virtualization allows creating very high performance/high available virtual devices that can be shared by dozens of hosts and that can be expanded as needed.
SAN share file systems
Finally, once large and high-performance virtual devices can be created and accessed by multiple hosts, a SAN shared file system is needed in order to allow those hosts to share the files while accessing the data at Fibre Channel speeds. The SAN share file system works in a similar way as asymmetric virtualization. There is an out-of-the-data-path metadata server, and the data transfer is performed directly between the clients and the storage devices. The SAN share file system may have a host or a backup server that backs up or moves the video files to an FC-attached tape library while active.
In conclusion, the combination of the technologies of SAN, asymmetric virtualization and SAN share file system can be bundled together in order to create a storage system fine-tuned for the digital video and other streaming applications.
1. How can I extend my SAN connections?
Until recently, extending storage applications over metropolitan and wide areas meant dealing with the contradiction between the performance requirements of SANs and the bottleneck imposed by relatively slow WAN links. To attain gigabit performance, it was necessary to use dedicated dark fiber and Dense Wave Division Multiplexing (DWDM) for distances greater than six miles. Now, however, carriers are offering gigabit and multi-gigabit services for IP transport that can span thousands of miles. Learn more about considerations for SAN extensions in this user-submitted tip.
2. How can I avoid being penny-wise and bandwidth foolish?
When Northern Oklahoma College set up its state-of-the-art media lab three years ago, it tried to save money on the SAN. It ended up buying a peck of trouble. With everyone on the system it would take over 30 minutes to shoot over a 100M Byte file, and a significant number of students in the school's multimedia program had files that were 100M Bytes or larger. The school finally had to start from scratch. Read more about the lessons learned in this case study.
3. What kind of resources are out there on video-on-demand storage?
Here is a bibliography of research papers about a Bell Labs project to develop an advanced server architecture for streaming media.
4. How can I improve performance with a filer?
A NAS installation can significantly increase network performance in organizations that have large files, especially if the NAS is combined with a segmented network. This strategy is commonly used by companies such as architectural firms and design offices where individual work groups need frequent access to large files that are seldom accessed from outside the work group.
About the author: Nelson Nahum is a co-founder of StoreAge Networking Technologies and has been its Chief Technology Officer since the company's inception in April 1999.