In what scenarios can the blockchain size decrease for bitcoin?

2

1

I am running a private bitcoin network for which I changed the target time between two blocks to 12 seconds and the difficulty adjustment to 25 blocks interval. I ran the network for about 4 hours with 50 nodes. In one of the node's logs I observed that the blockchain height increased up to a maximum of 181 and then started decreasing, all the way to 38. what could be an explanation for such a strange behaviour.

Please refer to the log below:

2015-11-04 01:58:47 receive version message: /Satoshi:0.11.99/: version 70011, blocks=181, us=0.0.0.0:0, peer=2, peeraddr=127.0.0.1:44117
2015-11-04 01:58:47 UpdateTip: new best=0000005265ca4ce01ad0d06f45cf475bf303de3d64e942c5cf1177e00f346c78  height=180  log2_work=37.083283  tx=30941  date=2015-11-04 01:53:17 progress=1.000000  cache=0.0MiB(1tx)
2015-11-04 01:58:47 UpdateTip: new best=00000052a34cedf3c5ddbeb46d36644654523db855c4cce984d2623e840dd219  height=179  log2_work=37.082953  tx=30940  date=2015-11-04 01:53:10 progress=1.000000  cache=0.0MiB(2tx)
2015-11-04 01:58:47 UpdateTip: new best=00000030fd7652affb883f05fe0c98e7fe3fbc3cfd74808e061ed05ec61c22e6  height=178  log2_work=37.082623  tx=30939  date=2015-11-04 01:52:55 progress=1.000000  cache=0.0MiB(3tx)
2015-11-04 01:58:47 AddToWallet c32bcbd8102c602a5e71ee717232e204435f331dce6fbfb9eb5d552698faa95b
2015-11-04 01:58:47 AddToWallet 1c91517aeadd12bcbcfdf4a1423b671d405543ae9abfbd87078969ce1971663f
2015-11-04 01:58:47 AddToWallet b11f9c2e3b1ab3d3983da63783bb95903d89405243d0716ea88272a9261b7a33

bawejakunal

Posted 2015-11-04T03:19:46.880

Reputation: 467

That's really strange. Can you pastebin the full log, including where the blocks are added/removed? – Nick ODell – 2015-11-04T03:29:12.487

Agreed, can you show us where the blockchain height decreased to 38? It's not in the log you've posted. – Jimmy Song – 2015-11-04T03:37:21.970

@NickODell http://codeshare.io/lp1G6

– bawejakunal – 2015-11-04T04:55:26.870

1Best chain is selected by the most cumulative difficulty, not height. Your modifications have essentially ensured frequent forks and reorganisations. The difficulty adjustments are long in Bitcoin are a long period partly as attack resistance, but also to avoid noisy chain movements. It's completely legal for the height of the chain to drastically reduce. – Anonymous – 2015-11-04T06:22:48.997

@Bitcoin But the log bawejakunal posted shows that the total work goes down when the client changes the blockchain. Before: height=181 log2_work=37.083613 After: height=38 log2_work=36.832912 The 38 block chain is entirely contained within the 181 block chain. – Nick ODell – 2015-11-04T07:32:55.483

@NickODell I think there's other misguided tinkering going on in that case which we aren't seeing. It's difficult to say without knowing the whole patch set. – Anonymous – 2015-11-04T08:29:55.860

@bawejakunal, I don't see anything in the link you provided. Can you try pastebin? – Jimmy Song – 2015-11-04T15:55:37.963

@JimmySong Reuploaded: https://gist.github.com/nickodell/1d8f29faad7a566c08fd

– Nick ODell – 2015-11-04T21:20:08.147

Answers

2

It looks like this part of the code is what was adding all those UpdateTip messages:

https://github.com/bitcoin/bitcoin/blob/8fe30fb4d130532d4a0e4c9d143f03e1b85a749e/src/main.cpp#L2234

Essentially, bitcoind thinks another chain with more cumulative work exists on your 4-node network, though from the logs, it's hard to tell how much lower it goes. You'll have to run it for longer in order to find out.

Jimmy Song

Posted 2015-11-04T03:19:46.880

Reputation: 7 067

Oh so you mean this is the case of switching between two competing blockchains ? – bawejakunal – 2015-11-05T02:33:50.107

1Yes. This could happen if, for example, one of your nodes wasn't receiving blocks, only sending them. – Jimmy Song – 2015-11-05T15:16:51.837

Asked: 2015-11-04T03:19:46.880

Viewed: 189 times

Active: 2015-11-06T14:52:33.333