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I sent some bitcoins to an address and then looked on blockexplorer and it was there in about 10 minutes, which is what one should expect.

I did another send to a previously unused address and it took hours for the network to acknowledge it.

I contacted the help desk at Mt Gox and they said this happens fairly often. My question: Is this a known phenomena and if so what causes it?

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4 Answers

Four factors affect this:

1) The size of the transaction. Smaller transactions get a lower priority. Abnormally small transactions look like spam.

2) The transaction fee. Abnormally low transaction fees may mean the transaction isn't even relayed. Miners have an incentive to include transactions with higher fees.

3) Luck. Some miners don't include any transactions. Some don't include any transactions without fees, even if the fee would normally be zero. So it comes down to who mines the next block or blocks. Also, 10 minutes is just the average. You could create a transaction and then by sheer bad luck the next block isn't mined for 20 minutes.

4) Security. The client 'dribbles' out its own transactions rather than broadcasting them to try to conceal the origin.

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20 minutes is still pretty good. I have waited more than one hour for a block. I also would like to add that blockexplorer only includes transactions with one confirmation. As an alternative, you can use blockchain.info that lists unconfirmed transactions –  nmat Dec 10 '11 at 8:44
    
I looked at the floating transactions in the website below and most of them were 20 bitcoins or more. Only one was less than 1 bitcoin. So small size doesn't seem to be a factor here. So that crosses out factor #1 and #2. Factor #4 is not the case since I sent one transaction. And Factor #3, Luck, may be a factor in differences between 10 minutes and 30 minutes, but my transaction took about 6 hours to be confirmed. So we are still in want of a good reason why a 50 bitcoin transaction would take 6 hours. –  shoeless joe Dec 10 '11 at 16:08
    
@shoeless joe Since you are sending the bitcoins through MtGox, they might not send the transaction instantaneously. It would be different if you were using the client... And the "size of the transaction" is in bytes, not in bitcoins. –  nmat Dec 10 '11 at 16:46
    
@Shoeless joe: That is extremely unusual. Was the client modified to use a different transaction fee? Was the transaction in the list of pending transactions for the full 6 hours? –  David Schwartz Dec 10 '11 at 22:39
    
There was no modifications. I sent 5 transactions one right after the other. The second one took 6 hours and the rest took 15 minutes. I didn't know to look on the list of pending transactions. –  shoeless joe Dec 11 '11 at 17:05
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This is caused by various characteristics of the transaction (the details of which I do not know), which make the Bitcoin client consider it low priority and hence only includes it in a mined block if it is sufficiently old.

Note also that http://bitcoincharts.com/bitcoin/ has a list of known floating transactions, you don't need to wait for it to appear in Block Explorer.

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Perhaps your transaction took longer because the Bitcoin Days Destroyed is different..

I've seen this metric described as as a technique to throttle spammy transactions, where older coins get faster treatment than the same amount of coins that were just transferred to a wallet.

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The age of the coins comes into play if they were really recently received hours to a day or two at most. Anything older has no bearing on "spamminess". –  Stephen Gornick Sep 17 '12 at 7:25
    
@StephenGornick Thanks! I'm looking for an actual metric or algorithm referring to this. Do you what this is, or where in the source code I can find it? –  makerofthings7 Sep 17 '12 at 12:32
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Simple (and please do correct me if I am wrong):

MtGox send their bitcoin network transactions via Eligius with no fees.

So you are at the mercy of that one pool finding a block and ensuring your transaction is included in that block.

Therefore transactions sent from MtGox do not fairly represent the experience of the majority of transactions made via the bitcoin network (which is your transaction should make it in to the blockchain in 10 minutes or less, on average).

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Bitcoin is a peer-to-peer network. Any node that receives a transaction will generally relay it to peers. Also, most transactions in a block don't have fees, no matter which pool has solved the block. –  Stephen Gornick Sep 17 '12 at 7:28
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