Choosing the right shared storage solution: Part 1 of 3: Bandwidth & Connections

Shared Storage – the least sexy of the post production technologies. Shared Storage is not the hot chick in class. She’s the quiet librarian with glasses that you end up falling for when the unreliable hot chick loses her sheen. She’s got everything you need: reliability and stability, and flashing lights. (Maybe the flashing lights thing is just me.)

In part 1 of 3 part posting, I’m going to examine shared storage for video – SAN & NAS.

Many factors go into choosing the correct shared storage solution:

  1. Bandwidth & Connections
  2. Drives: Size, Spindles & Protection (RAID)
  3. Management, Permissions & Support

In this post, we’ll examine #1: Connections and Bandwidth.

Before we jump into the geeky goodness, let’s make sure we’re all following the same nomenclature.

  1. SAN: Storage Area Network. Based on Fibre topology.
  2. NAS: Network Attached Storage. Based on Ethernet topology.
  3. Currently, there exists 2 main methods of connecting many computers to the same storage: Ethernet and Fibre. A good rule of thumb is if it’s Fibre, it’s a SAN, and if it’s Ethernet, it’s a NAS. While there are a few exceptions, this guideline is true a majority of the time. I humbly await the flames…

    The endgame is that BOTH (when configured and deployed correctly) can be run long distances, are somewhat flexible, and most importantly, can be used for mass storage in the video realm. The big difference to the end user is bandwidth.

    Bandwidth – also known as throughput – is the amount of constant data your computer can sustain to and from storage, and is paramount for video playback. Too little bandwidth – or poor sustained bandwidth – causes your video to stutter or downright refuse to play.

    When it comes to bandwidth, the video industry is actually in an excellent place right now. Acquisition codecs, like h.264 (Canon 5D, 7D), XDCAM, and P2, take up a small amount of bandwidth, while still delivering decent quality. Let’s look at Standard Definition (SD) video from 10 years ago. Many broadcast networks would play material to air that had been onlined at 1:1 uncompressed or frequently 2:1. The video datarate of SD at 1:1 is 22 megabytes (MB) per second. The current broadcast quality HD Avid codec (DNxHD 145) is actually less, clocking in at about 18MB/s. Heck, even DNxHD 220x (the least amount of compression before going uncompressed in the Avid realm) is only about 27 MB/s. Thus, we can take advantage of the drops in storage pricing, advancement of computing power and still use the same bandwidth we were using back in the SD days.

    Below is a quick list of current Ethernet and Fibre transmission speeds.

    Connection Throughput 20% Real World
    4GB Fibre 425MB/s ~340 MB/s
    8GB Fibre 850 MB/s ~680MB/s
    1 Gig Ethernet 125 MB/s ~100 MB/s
    10Gig Ethernet 1250 MB/s ~1000 MB/s

    The smart reader has noticed that real world throughput differs from theoretical throughput. For example, Ethernet typically has more overhead (read: less bandwidth) than Fibre, but neither typically perform as fast as the theoretical speed. Kind of like that bag of life-shortening potato chips you picked up at the grocery store. It’s a bag of chips, yes, but is it ever really filled to the top?

    Personally, I normally ballpark 10-30% off of theoretical for a more real world ballpark estimation. ALL shared storage solutions are a little bit different. However, for sake of discussion, and to be safe when planning your facility, let’s say 20%.

    In addition, all of these connections inherently have the ability to be bonded, that is, the aggregate speed of multiple connections yield more throughput. This, of course, is based on the SAN or NAS, the switch, and the Fibre or Ethernet card in your system supporting it. Never assume any of them handle aggregation– verify. If they do, then you now have an opportunity to potentially double your available bandwidth. This is a very good thing.

    Now that we know how wide our highway is, let’s see how many cars we can fit on it. Here is a short list of some of the most popular codecs at their highest data rates.

    Codec (at 1080i/60) Speed (Mb/s & MB/s)
    AVCHD 24 Mb/s (3 MB/s)
    HDV 25Mb/s (3.125 MB/s)
    Canon7D (h.264) (1080p) 48 Mb/s (6MB/s)
    XDCAM HD 50 50Mb/s (6.25 MB/s)
    AVC-Intra (100) 100 Mb/s (12.5 MB/s)
    DVCPro HD 115 Mb/s (14.4 MB/s)
    DNxHD 145 145 Mb/s (18.125 MB/s)
    ProRes 422 (HD) 167 Mb/s (20.82 MB/s)
    REDCODE 42 336 Mb/s (42 MB/s)
    Uncompressed 10-bit RGB 1990.24Mb/s (248.78 MB/s)

    For you non-math majors (don’t worry, I’m not one, either) you’ll notice some big number discrepancy. Here’s why:

    Apparently, not everyone got the memo on standardizing measuring practices. So much so, that codec data rates are typically measured in MegaBITS (often written as Mb/s, Mbit/s, or Mbps), while storage is usually calculated in MegaBYTES (MB/s), GigaBYTES (GB/s), TeraBYTES, (TB/s) etc. This makes calculating storage somewhat of a headache. A tip? Divide the codec data rate by 8. (A byte contains 8 bits). Thus, a codec running at 48 Mb/s = 6MB/s. Genius.

    So, let’s now match up our cars (codecs) against our highway (throughput). Below we see codecs vs bandwidth (remember, bandwidth calculated at 20% less than theoretical max, which is a better approximation of real-world performance):

    Codecs and Bandwidth Comparison

    Codecs and Bandwidth Comparison

    So, what does this mean?

    Clearly, most codecs can easily fit through the pipeline 1GigE provides. Fibre becomes the obvious solution when uncompressed codecs (or similiar) come into play. Thus, for most video-centric applications, an Ethernet connected solution – single or bonded (even better) – can handle the job.

    I would add that not all Ethernet solutions are created equal. Sustained throughput is the name of the game, and off-the-shelf or I.T. centric solutions almost never handle video properly. A Pinto can get onto a race track, doesn’t mean it performs well, ya know?

    In closing for the 1st entry in the 3 part series, (1 for 3 may be good in baseball, but not in Post, and certainly not when choosing a shared storage system) I cannot stress enough that if you’re considering a SAN or NAS, consult a Video Shared Storage specialist. SAN and NAS solutions are not at the point where they are plug and play.

    Yet.

    Be on the look out for part 2: Drives: Size, Spindles & Protection (RAID).

    Special Thanks: David Sallak of Isilon, Pat Howley of Key Code Media.

1 Comment

  1. Chuck:

    thanks for making these concepts easy to grasp. Most articles that try to explain this subject completely leave out the Mb/MB dilemma. Looking forward to the next two!

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