Re: [Discuss-gnuradio] UHD USRP Source for B2x0 overflows File Sink

[Murphy, John]

Marcus et al,

Had to drop this to do some work on another project yesterday, but
still want to pursue this just a little further if you don't mind,
because the numbers you are giving all look to me like it should be
able to be made to work.

You found my SDD sequential sustained write speed of 69 MBytes/sec.
If I attempt to save data at 14 MSamp/sec to disk with complex 16-bit
integers I believe there is an average long-term rate of 56 MBytes/sec
going to the disk.

So I am not understanding - it seems to me like I have plenty of
sustained throughput overhead to make this work, with the right
buffering to take up the temporary slack.

With 16 GBytes of RAM (the system is using some, but still) I would
expect that I can buffer up something like 4 minutes of data at the
required 56 MBytes/sec rate - seems like with the proper setup there
should be plenty of capability to ride through whatever other kernel
operations etc are momentarily stalling the disk writes.

Thanks, I appreciate you taking the time and list bandwidth to help me
understand this,
- John

Date: Thu, 07 May 2015 17:35:57 -0400
From: "Marcus D. Leech" <address@hidden>

    The basic problem is that if the long-term-average offered-load on your
    write medium (your SSD in this case) is higher than it can sustain, it
    doesn't
       matter how much buffering you add in front of it, eventually, the
    piper has to be paid.   Buffering is useful to meeting short-term
    short-falls in
       throughput capacity.  They *cannot* help if offered load, on average,
    exceeds the long-term capacity of the resource.  Now, having said that,
       if you only need to record for a short time, consider adding more
    RAM, and creating a ramdisk, then stage the ramdisk out to your hard disk.
       But this ONLY WORKS if you don't need to record continuously,
    otherwise, you're back to the buffering issue....
    But at 8Msps, and 4-bytes per sample, that's 32Mbyte/second, you have
    about 30 seconds/gigabyte.

> On Thu, May 7, 2015 at 3:43 PM,  <address@hidden> wrote:

>> I looked at their blurb on that drive, and its *sustained* rate comes out to
>> about 69Mbyte/second.  Sure, it'll take bursts at screaming-fast rates,
>> because, like the Linux kernel, it has a whacking great write-behind buffer.