READ THE CODE

Inside Bitcoin

The code behind the network, with room to understand it.

Stable release v32.1bitcoin/bitcoin · f0de87f04062 ↗3,020 source files · 47.5 MB stored on the server440 file guides

Release published 2026-10-28 · Snapshot fetched 2026-11-01. Latest stable release at fetch time; development branches and Git history are excluded. Only the selected code window and visible folders load here.

7,884 bytes · SHA-256 8000d20f46f02740… · View upstream ↗

1## Files
2
3The implementation of leveldb is similar in spirit to the representation of a
4single [Bigtable tablet (section 5.3)](http://research.google.com/archive/bigtable.html).
5However the organization of the files that make up the representation is
6somewhat different and is explained below.
7
8Each database is represented by a set of files stored in a directory. There are
9several different types of files as documented below:
10
11### Log files
12
13A log file (*.log) stores a sequence of recent updates. Each update is appended
14to the current log file. When the log file reaches a pre-determined size
15(approximately 4MB by default), it is converted to a sorted table (see below)
16and a new log file is created for future updates.
17
18A copy of the current log file is kept in an in-memory structure (the
19`memtable`). This copy is consulted on every read so that read operations
20reflect all logged updates.
21
22## Sorted tables
23
24A sorted table (*.ldb) stores a sequence of entries sorted by key. Each entry is
25either a value for the key, or a deletion marker for the key. (Deletion markers
26are kept around to hide obsolete values present in older sorted tables).
27
28The set of sorted tables are organized into a sequence of levels. The sorted
29table generated from a log file is placed in a special **young** level (also
30called level-0). When the number of young files exceeds a certain threshold
31(currently four), all of the young files are merged together with all of the
32overlapping level-1 files to produce a sequence of new level-1 files (we create
33a new level-1 file for every 2MB of data.)
34
35Files in the young level may contain overlapping keys. However files in other
36levels have distinct non-overlapping key ranges. Consider level number L where
37L >= 1. When the combined size of files in level-L exceeds (10^L) MB (i.e., 10MB
38for level-1, 100MB for level-2, ...), one file in level-L, and all of the
39overlapping files in level-(L+1) are merged to form a set of new files for
40level-(L+1). These merges have the effect of gradually migrating new updates
41from the young level to the largest level using only bulk reads and writes
42(i.e., minimizing expensive seeks).
43
44### Manifest
45
46A MANIFEST file lists the set of sorted tables that make up each level, the
47corresponding key ranges, and other important metadata. A new MANIFEST file
48(with a new number embedded in the file name) is created whenever the database
49is reopened. The MANIFEST file is formatted as a log, and changes made to the
50serving state (as files are added or removed) are appended to this log.
51
52### Current
53
54CURRENT is a simple text file that contains the name of the latest MANIFEST
55file.
56
57### Info logs
58
59Informational messages are printed to files named LOG and LOG.old.
60
61### Others
62
63Other files used for miscellaneous purposes may also be present (LOCK, *.dbtmp).
64
65## Level 0
66
67When the log file grows above a certain size (4MB by default):
68Create a brand new memtable and log file and direct future updates here.
69
70In the background:
71
721. Write the contents of the previous memtable to an sstable.
732. Discard the memtable.
743. Delete the old log file and the old memtable.
754. Add the new sstable to the young (level-0) level.
76
77## Compactions
78
79When the size of level L exceeds its limit, we compact it in a background
80thread. The compaction picks a file from level L and all overlapping files from
81the next level L+1. Note that if a level-L file overlaps only part of a
82level-(L+1) file, the entire file at level-(L+1) is used as an input to the
83compaction and will be discarded after the compaction.  Aside: because level-0
84is special (files in it may overlap each other), we treat compactions from
85level-0 to level-1 specially: a level-0 compaction may pick more than one
86level-0 file in case some of these files overlap each other.
87
88A compaction merges the contents of the picked files to produce a sequence of
89level-(L+1) files. We switch to producing a new level-(L+1) file after the
90current output file has reached the target file size (2MB). We also switch to a
91new output file when the key range of the current output file has grown enough
92to overlap more than ten level-(L+2) files.  This last rule ensures that a later
93compaction of a level-(L+1) file will not pick up too much data from
94level-(L+2).
95
96The old files are discarded and the new files are added to the serving state.
97
98Compactions for a particular level rotate through the key space. In more detail,
99for each level L, we remember the ending key of the last compaction at level L.
100The next compaction for level L will pick the first file that starts after this
101key (wrapping around to the beginning of the key space if there is no such
102file).
103
104Compactions drop overwritten values. They also drop deletion markers if there
105are no higher numbered levels that contain a file whose range overlaps the
106current key.
107
108### Timing
109
110Level-0 compactions will read up to four 1MB files from level-0, and at worst
111all the level-1 files (10MB). I.e., we will read 14MB and write 14MB.
112
113Other than the special level-0 compactions, we will pick one 2MB file from level
114L. In the worst case, this will overlap ~ 12 files from level L+1 (10 because
115level-(L+1) is ten times the size of level-L, and another two at the boundaries
116since the file ranges at level-L will usually not be aligned with the file
117ranges at level-L+1). The compaction will therefore read 26MB and write 26MB.
118Assuming a disk IO rate of 100MB/s (ballpark range for modern drives), the worst
119compaction cost will be approximately 0.5 second.
120
121If we throttle the background writing to something small, say 10% of the full
122100MB/s speed, a compaction may take up to 5 seconds. If the user is writing at
12310MB/s, we might build up lots of level-0 files (~50 to hold the 5*10MB). This
124may significantly increase the cost of reads due to the overhead of merging more
125files together on every read.
126
127Solution 1: To reduce this problem, we might want to increase the log switching
128threshold when the number of level-0 files is large. Though the downside is that
129the larger this threshold, the more memory we will need to hold the
130corresponding memtable.
131
132Solution 2: We might want to decrease write rate artificially when the number of
133level-0 files goes up.
134
135Solution 3: We work on reducing the cost of very wide merges. Perhaps most of
136the level-0 files will have their blocks sitting uncompressed in the cache and
137we will only need to worry about the O(N) complexity in the merging iterator.
138
139### Number of files
140
141Instead of always making 2MB files, we could make larger files for larger levels
142to reduce the total file count, though at the expense of more bursty
143compactions.  Alternatively, we could shard the set of files into multiple
144directories.
145
146An experiment on an ext3 filesystem on Feb 04, 2011 shows the following timings
147to do 100K file opens in directories with varying number of files:
148
149
150| Files in directory | Microseconds to open a file |
151|-------------------:|----------------------------:|
152|               1000 |                           9 |
153|              10000 |                          10 |
154|             100000 |                          16 |
155
156So maybe even the sharding is not necessary on modern filesystems?
157
158## Recovery
159
160* Read CURRENT to find name of the latest committed MANIFEST
161* Read the named MANIFEST file
162* Clean up stale files
163* We could open all sstables here, but it is probably better to be lazy...
164* Convert log chunk to a new level-0 sstable
165* Start directing new writes to a new log file with recovered sequence#
166
167## Garbage collection of files
168
169`DeleteObsoleteFiles()` is called at the end of every compaction and at the end
170of recovery. It finds the names of all files in the database. It deletes all log
171files that are not the current log file. It deletes all table files that are not
172referenced from some level and are not the output of an active compaction.
173

src/wallet/receive.cpp

src/wallet/coinselection.cpp

src/consensus/validation.h

src/primitives/transaction.cpp

src/script/descriptor.cpp

src/bitcoin-cli.cpp

src/rpc/blockchain.cpp

src/rpc/rawtransaction.cpp

src/init.cpp

src/crypto/sha256.cpp

src/consensus/merkle.h

src/consensus/tx_check.h

src/consensus/tx_verify.h

src/policy/feerate.h

src/policy/feerate.cpp

src/primitives/transaction_identifier.h

src/chainparamsbase.h

src/chainparamsbase.cpp

src/util/moneystr.cpp

src/crypto/ripemd160.h

src/crypto/sha1.h

src/crypto/sha512.h

src/crypto/hmac_sha256.h

src/crypto/hmac_sha512.h

src/crypto/hmac_sha256.cpp

src/crypto/hmac_sha512.cpp

src/crypto/hkdf_sha256_32.h

src/crypto/hkdf_sha256_32.cpp

src/support/cleanse.h

src/support/cleanse.cpp

src/crypto/siphash.cpp

src/crypto/muhash.h

src/crypto/chacha20.h

src/crypto/poly1305.h

src/crypto/chacha20poly1305.h

src/crypto/chacha20poly1305.cpp

src/util/chaintype.cpp

src/util/obfuscation.h

src/util/byte_units.h

src/util/overflow.h

src/blockencodings.cpp

src/net_permissions.cpp

src/policy/packages.cpp

src/policy/rbf.cpp

src/policy/truc_policy.cpp

src/policy/ephemeral_policy.cpp

src/policy/settings.cpp

src/policy/policy.cpp

src/node/transaction.cpp

src/node/coin.cpp

src/rpc/client.cpp

src/rpc/external_signer.cpp

src/rpc/request.cpp

src/rpc/server_util.cpp

src/rpc/fees.cpp

src/rpc/output_script.cpp

src/rpc/rawtransaction_util.cpp

src/rpc/rawtransaction_util.h

src/rpc/register.h

src/rpc/request.h

src/rpc/server_util.h

src/rpc/protocol.h

src/rpc/server.h

src/rpc/signmessage.cpp

src/rpc/txoutproof.cpp

src/rpc/util.cpp

src/rpc/util.h

src/wallet/rpc/addresses.cpp

src/wallet/rpc/backup.cpp

src/wallet/rpc/coins.cpp

src/wallet/rpc/encrypt.cpp

src/wallet/rpc/signmessage.cpp

src/wallet/rpc/util.cpp

src/wallet/rpc/util.h

src/wallet/coincontrol.cpp

src/wallet/coincontrol.h

src/wallet/context.cpp

src/wallet/context.h

src/wallet/crypter.cpp

src/wallet/crypter.h

src/wallet/db.cpp

src/wallet/export.cpp

src/wallet/external_signer_scriptpubkeyman.cpp

src/wallet/rpc/spend.cpp

src/wallet/rpc/transactions.cpp

src/wallet/rpc/wallet.cpp

src/wallet/feebumper.cpp

src/wallet/feebumper.h

src/index/base.h

src/index/base.cpp

src/index/txindex.h

src/index/txindex.cpp

src/index/txindex_key.h

src/index/txospenderindex.h

src/index/txospenderindex.cpp

src/index/blockfilterindex.h

src/index/blockfilterindex.cpp

src/index/coinstatsindex.h

src/index/coinstatsindex.cpp

src/index/db_key.h

src/index/disktxpos.h

src/kernel/chain.cpp

src/kernel/chain.h

src/kernel/checks.cpp

src/kernel/checks.h

src/kernel/context.cpp

src/kernel/context.h

src/kernel/cs_main.cpp

src/kernel/cs_main.h

src/kernel/disconnected_transactions.cpp

src/kernel/disconnected_transactions.h

src/kernel/mempool_removal_reason.cpp

src/kernel/mempool_removal_reason.h

src/kernel/coinstats.cpp

src/kernel/coinstats.h

src/kernel/chainparams.cpp

src/kernel/chainparams.h

src/wallet/sqlite.cpp

src/wallet/sqlite.h

src/wallet/transaction.cpp

src/wallet/transaction.h

src/wallet/dump.cpp

src/wallet/dump.h

src/wallet/init.cpp

src/wallet/walletutil.cpp

src/wallet/walletutil.h

src/wallet/wallettool.cpp

src/wallet/wallettool.h

src/wallet/export.h

src/wallet/migrate.cpp

src/wallet/migrate.h

src/wallet/db.h

src/wallet/walletdb.cpp

src/wallet/interfaces.cpp

src/wallet/scriptpubkeyman.cpp

src/wallet/scriptpubkeyman.h

src/kernel/bitcoinkernel.cpp

src/kernel/bitcoinkernel.h

src/kernel/bitcoinkernel_wrapper.h

src/node/chainstate.cpp

src/node/chainstate.h

src/node/blockstorage.cpp

src/node/blockstorage.h

src/node/mempool_persist.cpp

src/node/mempool_persist.h

src/node/utxo_snapshot.cpp

src/node/utxo_snapshot.h

src/node/kernel_notifications.cpp

src/node/kernel_notifications.h

src/node/blockmanager_args.cpp

src/kernel/blockmanager_opts.h

src/util/expected.cpp

src/policy/settings.h

src/util/types.h

src/kernel/messagestartchars.h

src/kernel/warning.h

src/util/exception.h

src/common/license_info.h

src/node/coins_view_args.h

src/node/database_args.h

src/util/batchpriority.h

src/util/ui_change_type.h

src/node/coins_view_args.cpp

src/node/peerman_args.h

src/util/serfloat.h

src/node/blockmanager_args.h

src/node/database_args.cpp

src/util/overloaded.h

src/interfaces/types.h

src/node/chainstatemanager_args.h

src/node/minisketchwrapper.h

src/util/macros.h

src/util/syserror.h

src/common/run_command.h

src/util/bytevectorhash.cpp

src/node/mempool_persist_args.cpp

src/util/exec.h

src/util/rbf.h

src/crypto/hex_base.h

src/node/mempool_persist_args.h

src/interfaces/echo.h

src/util/any.h

src/util/batchpriority.cpp

src/util/threadnames.h

src/node/coin.h

src/node/mempool_args.h

src/util/asmap.h

src/util/bytevectorhash.h

src/node/context.cpp

src/util/insert.h

src/util/readwritefile.h

src/node/peerman_args.cpp

src/node/interface_ui.cpp

src/interfaces/handler.h

src/node/mining_args.h

src/util/hasher.cpp

src/common/init.h

src/kernel/mempool_limits.h

src/common/system.h

src/crypto/sha3.h

src/util/threadinterrupt.cpp

src/node/protocol_version.h

src/util/exception.cpp

src/common/messages.h

src/node/connection_types.cpp

src/util/bip32.h

src/common/license_info.cpp

src/util/stdmutex.h

src/common/run_command.cpp

src/util/check.cpp

src/util/golombrice.h

src/node/types.h

src/node/timeoffsets.h

src/util/readwritefile.cpp

src/util/fastrange.h

src/util/signalinterrupt.h

src/util/task_runner.h

src/util/syserror.cpp

src/common/interfaces.cpp

src/common/types.h

src/util/threadinterrupt.h

src/kernel/chainstatemanager_opts.h

src/node/psbt.h

src/policy/ephemeral_policy.h

src/util/trace.h

src/common/url.cpp

src/kernel/mempool_options.h

src/util/serfloat.cpp

src/kernel/notifications_interface.h

src/node/timeoffsets.cpp

src/node/warnings.cpp

src/util/hash_type.h

src/crypto/hex_base.cpp

src/node/eviction.h

src/util/tokenbucket.h

src/crypto/aes.h

src/common/pcp.h

src/util/feefrac.cpp

src/node/transaction.h

src/util/threadnames.cpp

src/common/signmessage.h

src/util/exec.cpp

src/util/signalinterrupt.cpp

src/clientversion.cpp

src/common/signmessage.cpp

src/compat/assumptions.h

src/compat/byteswap.h

src/deploymentinfo.cpp

src/deploymentinfo.h

src/deploymentstatus.cpp

src/deploymentstatus.h

src/dummywallet.cpp

src/external_signer.h

src/init/bitcoin-gui.cpp

src/init/bitcoin-node.cpp

src/init/bitcoin-qt.cpp

src/init/bitcoin-wallet.cpp

src/init/bitcoind.cpp

src/init/common.h

src/interfaces/init.h

src/interfaces/ipc.h

src/key_io.h

src/logging/categories.h

src/logging/timer.h

src/mapport.h

src/musig.h

src/net_permissions.h

src/netgroup.h

src/netmessagemaker.h

src/node/chainstatemanager_args.cpp

src/node/connection_types.h

src/node/context.h

src/node/interface_ui.h

src/node/mempool_args.cpp

src/node/mining_args.cpp

src/node/minisketchwrapper.cpp

src/node/txreconciliation.h

src/policy/fees/estimator_args.cpp

src/policy/fees/estimator_args.h

src/policy/fees/estimator_man.cpp

src/policy/fees/estimator_man.h

src/policy/packages.h

src/policy/rbf.h

src/policy/truc_policy.h

src/pow.h

src/primitives/block.cpp

src/randomenv.h

src/rest.h

src/script/keyorigin.h

src/script/parsing.cpp

src/script/parsing.h

src/script/script_error.h

src/script/sigcache.cpp

src/script/sigcache.h

src/script/solver.h

src/script/verify_flags.h

src/semaphore_grant.h

src/signet.h

src/span.h

src/support/allocators/secure.h

src/support/allocators/zeroafterfree.h

src/util/tokenpipe.cpp

src/util/translation.h

src/util/vector.h

src/versionbits.h

src/versionbits_impl.h

src/wallet/external_signer_scriptpubkeyman.h

src/wallet/fees.h

src/wallet/load.h

src/wallet/receive.h

src/wallet/rpc/wallet.h

src/wallet/types.h

src/walletinitinterface.h

src/zmq/zmqabstractnotifier.cpp

Independent explanations by MyBitcoinExplorer. Not affiliated with or endorsed by Bitcoin Core. Guides are educational and do not replace the source or its tests.

Bitcoin Core source © its contributors, distributed under the MIT license. Original notices and bundled third-party licenses are retained in the source tree.