Matthew Toseland ?????:
> On Sunday 14 February 2010 04:29:31 Evan Daniel wrote:
>> On Sat, Feb 13, 2010 at 7:00 PM, xor <xor at gmx.li> wrote:
>>> First of all, I like your calculations very much and I wonder why nobody
>>> calculated this before FEC was implemented. If I understood this correctly
>>> then a 700mib file with block success rate p=0.58 will have a 48% total
>>> success chance. This sucks...
>> Thanks :)
>>
>> That's correct. However, in some ways it isn't *quite* that bad.
>> There's a reason I picked p=0.58: it's nice and dramatic. But, the
>> curve is fairly steep there :) At p=0.60, it's 92% success rate. At
>> p=0.61, it's 97%.
>>
>> There are two ways to look at the success rate. One option is to pick
>> some block success rate and look at the resulting file success rates
>> for a variety of files. By this metric, the interleaved coding is a
>> stunning improvement. The other approach is to pick a target file
>> success rate, and see what block success rates are required to get it
>> for different scenarios. So at 700M, we need p=0.61 to get 97% with
>> simple segments. With interleaved coding, we need p=0.56.
>>
>> The latter approach is actually probably more directly meaningful.
>> The improvement from interleaved coding is significant, and useful,
>> but modest. What it says is that instead of being able to lose 39% of
>> blocks and still expect to recover the file, we can now expect to
>> recover the file after losing 44%. If we assume that individual
>> blocks are well modeled as having a half-life to disappearance, then
>> it takes 1.17 times longer for the file to become inaccessible if we
>> use the interleaved coding.
>
> However, in practice, it appears that many files do stall at close to 100%.
> This could be a client layer bug, but it could be that we have lost a segment.
Isn't it exactly the expected behaviour when the last segment is significantly
smaller than the rest?
- Volodya
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