This does not affect wallet forward or backward compatibility. There are no known problems when downgrading from 0.11.x to 0.10.x.

If you want to be able to downgrade smoothly, make a backup of your entire data directory. Without this your node will need start syncing (or importing from bootstrap.dat) anew afterwards. It is possible that the data from a completely synchronised 0.10 node may be usable in older versions as-is, but this is not supported and may break as soon as the older version attempts to reindex.

The block index database will now hold headers for which no block is stored on disk, which earlier versions won’t support.

Blocks will be stored on disk out of order (in the order they are received, really), which makes it incompatible with some tools or other programs. Reindexing using earlier versions will also not work anymore as a result of this.

Because release 0.10.0 and later makes use of headers-first synchronization and parallel block download (see further), the block files and databases are not backwards-compatible with pre-0.10 versions of Bitcoin Core or other software:

If you are running an older version, shut it down. Wait until it has completely shut down (which might take a few minutes for older versions), then run the installer (on Windows) or just copy over /Applications/Bitcoin-Qt (on Mac) or bitcoind/bitcoin-qt (on Linux).

The other is to restrict the relaying of free transactions with limitfreerelay . This option sets the number of kB/minute at which free transactions (with enough priority) will be accepted. It defaults to 15. Reducing this number reduces the speed at which the mempool can grow due to free transactions.

One is to increase the minimum transaction relay fee minrelaytxfee , which defaults to 0.00001. This will cause transactions with fewer BTC/kB fee to be rejected, and thus fewer transactions entering the mempool.

If this growth of the mempool causes problematic memory use on your node, it is possible to change a few configuration options to work around this. The growth of the mempool can be monitored with the RPC command getmempoolinfo .

At the time of this release, the P2P network is being flooded with low-fee transactions. This causes a ballooning of the mempool size.

Notable changes

Block file pruning

This release supports running a fully validating node without maintaining a copy of the raw block and undo data on disk. To recap, there are four types of data related to the blockchain in the bitcoin system: the raw blocks as received over the network (blk???.dat), the undo data (rev???.dat), the block index and the UTXO set (both LevelDB databases). The databases are built from the raw data.

Block pruning allows Bitcoin Core to delete the raw block and undo data once it’s been validated and used to build the databases. At that point, the raw data is used only to relay blocks to other nodes, to handle reorganizations, to look up old transactions (if -txindex is enabled or via the RPC/REST interfaces), or for rescanning the wallet. The block index continues to hold the metadata about all blocks in the blockchain.

The user specifies how much space to allot for block & undo files. The minimum allowed is 550MB. Note that this is in addition to whatever is required for the block index and UTXO databases. The minimum was chosen so that Bitcoin Core will be able to maintain at least 288 blocks on disk (two days worth of blocks at 10 minutes per block). In rare instances it is possible that the amount of space used will exceed the pruning target in order to keep the required last 288 blocks on disk.

Block pruning works during initial sync in the same way as during steady state, by deleting block files “as you go” whenever disk space is allocated. Thus, if the user specifies 550MB, once that level is reached the program will begin deleting the oldest block and undo files, while continuing to download the blockchain.

For now, block pruning disables block relay. In the future, nodes with block pruning will at a minimum relay “new” blocks, meaning blocks that extend their active chain.

Block pruning is currently incompatible with running a wallet due to the fact that block data is used for rescanning the wallet and importing keys or addresses (which require a rescan.) However, running the wallet with block pruning will be supported in the near future, subject to those limitations.

Block pruning is also incompatible with -txindex and will automatically disable it.

Once you have pruned blocks, going back to unpruned state requires re-downloading the entire blockchain. To do this, re-start the node with -reindex. Note also that any problem that would cause a user to reindex (e.g., disk corruption) will cause a pruned node to redownload the entire blockchain. Finally, note that when a pruned node reindexes, it will delete any blk???.dat and rev???.dat files in the data directory prior to restarting the download.

To enable block pruning on the command line:

-prune=N : where N is the number of MB to allot for raw block & undo data.

Modified RPC calls:

getblockchaininfo now includes whether we are in pruned mode or not.

now includes whether we are in pruned mode or not. getblock will check if the block’s data has been pruned and if so, return an error.

will check if the block’s data has been pruned and if so, return an error. getrawtransaction will no longer be able to locate a transaction that has a UTXO but where its block file has been pruned.

Pruning is disabled by default.

Big endian support

Experimental support for big-endian CPU architectures was added in this release. All little-endian specific code was replaced with endian-neutral constructs. This has been tested on at least MIPS and PPC hosts. The build system will automatically detect the endianness of the target.

Memory usage optimization

There have been many changes in this release to reduce the default memory usage of a node, among which:

Accurate UTXO cache size accounting (#6102); this makes the option -dbcache precise where this grossly underestimated memory usage before

precise where this grossly underestimated memory usage before Reduce size of per-peer data structure (#6064 and others); this increases the number of connections that can be supported with the same amount of memory

Reduce the number of threads (#5964, #5679); lowers the amount of (esp. virtual) memory needed

Fee estimation changes

This release improves the algorithm used for fee estimation. Previously, -1 was returned when there was insufficient data to give an estimate. Now, -1 will also be returned when there is no fee or priority high enough for the desired confirmation target. In those cases, it can help to ask for an estimate for a higher target number of blocks. It is not uncommon for there to be no fee or priority high enough to be reliably (85%) included in the next block and for this reason, the default for -txconfirmtarget=n has changed from 1 to 2.

Privacy: Disable wallet transaction broadcast

This release adds an option -walletbroadcast=0 to prevent automatic transaction broadcast and rebroadcast (#5951). This option allows separating transaction submission from the node functionality.

Making use of this, third-party scripts can be written to take care of transaction (re)broadcast:

Send the transaction as normal, either through RPC or the GUI

Retrieve the transaction data through RPC using gettransaction (NOT getrawtransaction ). The hex field of the result will contain the raw hexadecimal representation of the transaction

(NOT ). The field of the result will contain the raw hexadecimal representation of the transaction The transaction can then be broadcasted through arbitrary mechanisms supported by the script

One such application is selective Tor usage, where the node runs on the normal internet but transactions are broadcasted over Tor.

For an example script see bitcoin-submittx.

Privacy: Stream isolation for Tor

This release adds functionality to create a new circuit for every peer connection, when the software is used with Tor. The new option, -proxyrandomize , is on by default.

When enabled, every outgoing connection will (potentially) go through a different exit node. That significantly reduces the chance to get unlucky and pick a single exit node that is either malicious, or widely banned from the P2P network. This improves connection reliability as well as privacy, especially for the initial connections.