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Author Topic: Possible DVR application- bandwidth question  (Read 3684 times)
jdonth
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« on: April 08, 2009, 07:51:35 AM »

I use a SiliconDust HDHomerun http://www.silicondust.com to receive OTA digital television shows.

I currently have the device connected to a XP machine that receives the UDP stream and writes it to the hard drive.

Would the SheevaPort device have enough bandwidth to support writing up to 40 mb/sec to a USB hosted hard drive or the SDIO port?

Any thoughts or suggestions?

Thanks,
Joe

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karurosu
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« Reply #1 on: April 08, 2009, 08:11:19 AM »

Network should be no problem (it has gigabit ethernet, even with 100 mbps it should work), as for writing to the card, AFAIK writing to SD is fairly quick, but it will probably will be a bit slower than a HDD. However, if you plug in an external hard drive (self powered) then you'll get pretty decent speeds and should work just fine.
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jdonth
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« Reply #2 on: April 08, 2009, 08:51:10 AM »

Karurosu,

Thanks for the suggestions. Due to the speed limitations (and cost of 16gb SD cards) I'm going to focus my development towards a self-powered USB hard drive.


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plugit
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« Reply #3 on: April 08, 2009, 09:07:26 AM »

Not to be a drag, but I'd be very surprised if you were able to squeeze 40 megabytes/second through that USB 2.0 interface. Sure, the spec says you should be able to do 60, but real world, I'm a little skeptical. I'd think 20-25 is more likely (due to USB packet overhead).
« Last Edit: April 08, 2009, 09:11:06 AM by plugit » Logged

jdonth
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« Reply #4 on: April 08, 2009, 09:43:51 AM »


Not to be a drag, but I'd be very surprised if you were able to squeeze 40 megabytes/second through that USB 2.0 interface. Sure, the spec says you should be able to do 60, but real world, I'm a little skeptical. I'd think 20-25 is more likely (due to USB packet overhead).

I appreciate the input. Since the HDHomerun supports two UDP feeds (of ~20mb/s each), could I use the micro-usb and write to two hard drives at 20 mbmb/s each?

Any other thoughts? Otherwise I guess this project idea is a non-starter.

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kilowatt
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« Reply #5 on: April 08, 2009, 10:03:22 AM »

If it's 40 mega bits you should be fine.  The USB 2.0 supports 480 mega bits or 60 mega bytes per second.
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edmc
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« Reply #6 on: April 08, 2009, 10:04:48 AM »

Not to be a drag, but I'd be very surprised if you were able to squeeze 40 megabytes/second through that USB 2.0 interface. Sure, the spec says you should be able to do 60, but real world, I'm a little skeptical. I'd think 20-25 is more likely (due to USB packet overhead).

Ah, here we go again... more 'b' versus 'B' confusion.

USB 2.0 Hi-Speed is rated as 480 mega BITS per second.  ATSC (here in the U.S.) specifies a maximum stream rate of 19.2 mega BITS per second.

With the original poster's HDHomeRun streaming up to two such programs, that's only 2 * 19.2Mb/s or 38.4Mb/s.  Add in a third stream a "media extender" will often be "watching", and you are up to 3 * 19.2Mb/s or 57.6Mb/s (though that last stream is traveling in the opposite direction of course).

Just make sure what ever file system you install on that "drive" is friendly to streams.  I'd suggest a "cluster size" of 64KB.  This will keep even these three streams able to utilize the highest throughput of that "drive" (i.e. random I/O rate using 64KB-consecutive blocks is nearly the sequential/maximum I/O rate of most hard disks).
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karurosu
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« Reply #7 on: April 08, 2009, 10:10:28 AM »

Well, there is ATAoE which allows to connect an ATA device over an ethernet connection, so it should be able to go to 40 mb/sec.
I guess your best option is to test if it works, otherwise wait for OpenRD which has a couple of eSATA ports (and a price tag of 200 usd).
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plugit
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« Reply #8 on: April 08, 2009, 11:27:24 AM »

Ooops, bits, bytes...

OK, let's just say that I don't think you can expect the full USB spec. Smiley

38.4Mb/s? I'd be curious to see whether you can get that much through.
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jdonth
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« Reply #9 on: April 08, 2009, 11:55:13 AM »

Anyone know of a linux test that I can perform to see what kind of thruput I can/will get with a usb hard drive?

Since the critical factor is WRITE speed, most tests like hdparm will be of limited use, right?
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edmc
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« Reply #10 on: April 08, 2009, 12:00:34 PM »

'dd' comes to mind.  This will allow you to test the raw bandwidth of the interface and
streaming performance of the drive (by writing directly to the block device).

I'd then install a file system on the drive (using the aforementioned 64KB cluster size
if your chosen file system supports it), create a file, and do 'dd' to that file.

Finally, I question the "write-only" concern.  As I mentioned, you'd probably want to
be able to (simultaneously, in fact) stream a pre-recorded program "from" the drive
at the same time you are writing "to" it as you record live programs.
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Pandemonium
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« Reply #11 on: April 13, 2009, 08:38:05 AM »

Someone has already done some speed tests:
http://ubuntuforums.org/showthread.php?t=1082489&page=7

Look at the second and third posts.  It looks like USB speeds are roughly ~22M/sec write, ~32M/sec read.
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restamp
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« Reply #12 on: May 10, 2009, 09:47:46 AM »

I have the same idea in mind (HDHomerun + the Sheeva Plug for saving the streaming DTV).  I have not received my Plug yet, so have not had a chance to experiment with it at all.  However, on paper, it looks like a reasonable match.  Basically, the Plug would be doing little real-time processing in this application -- it's simply serving as an I/O engine.  I normally get about 25MB (Megabytes) per second thruput when I dd to a USB hard drive on an Intel-based PC, so I doubt disk bandwidth will be an issue.  I question whether the Sheeva is capable of sustaining anywhere near its Gb ethernet NIC's thruput, but I expect it to be able to handle several channels worth of 20Mb (Megabit) data just fine, even if it's used on a 100BaseT Ethernet network.  (My personal experience is that the HDHomerun box does just fine feeding a PC thru a local switch, but my 100BaseT network tends to drop packets when feeding a PC at the other end of the house through three switches.)

One final comment: you may want to consider which type of file system you employ for saving your programs -- I understand some have better characteristics than others for large-file work.  Thus, say, deleting a previously recorded program while another is streaming may be problematic to the real-time stream.  (See, http://www.mythtv.org/docs/mythtv-HOWTO-3.html )
« Last Edit: May 10, 2009, 09:49:42 AM by restamp » Logged

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