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 # Bitcoin ABC file system
 
 **Contents**
 
 - [Data directory location](#data-directory-location)
 
 - [Data directory layout](#data-directory-layout)
 
 - [Multi-wallet environment](#multi-wallet-environment)
 
 - [GUI settings](#gui-settings)
 
 - [Legacy subdirectories and files](#legacy-subdirectories-and-files)
 
 - [Notes](#notes)
 
 ## Data directory location
 
 The data directory is the default location where the Bitcoin ABC files are stored.
 
 1. The default data directory paths for supported platforms are:
 
 Platform | Data directory path
 ---------|--------------------
 Linux    | `$HOME/.bitcoin/`
 macOS    | `$HOME/Library/Application Support/Bitcoin/`
 Windows  | `%APPDATA%\Bitcoin\` <sup>[\[1\]](#note1)</sup>
 
 2. The non-default data directory path can be specified by `-datadir` option.
 
 3. All content of the data directory, except for `bitcoin.conf` file, is chain-specific. This means the actual data directory paths for non-mainnet cases differ:
 
 Chain option        | Data directory path
 --------------------|--------------------
 no option (mainnet) | *path_to_datadir*`/`
 `-testnet`          | *path_to_datadir*`/testnet3/`
 `-regtest`          | *path_to_datadir*`/regtest/`
 
 ## Data directory layout
 
 Subdirectory       | File(s)               | Description
 -------------------|-----------------------|------------
 `blocks/`          |                       | Blocks directory; can be specified by `-blocksdir` option (except for `blocks/index/`)
 `blocks/index/`    | LevelDB database      | Block index; `-blocksdir` option does not affect this path
 `blocks/`          | `blkNNNNN.dat`<sup>[\[2\]](#note2)</sup> | Actual Bitcoin blocks (in network format, dumped in raw on disk, 128 MiB per file)
 `blocks/`          | `revNNNNN.dat`<sup>[\[2\]](#note2)</sup> | Block undo data (custom format)
 `chainstate/`      | LevelDB database      | Blockchain state (a compact representation of all currently unspent transaction outputs and some metadata about the transactions they are from)
 `indexes/txindex/` | LevelDB database      | Transaction index; *optional*, used if `-txindex=1`
 `indexes/blockfilter/basic/db/` | LevelDB database      | Blockfilter index LevelDB database for the basic filtertype; *optional*, used if `-blockfilterindex=basic`
 `indexes/blockfilter/basic/`    | `fltrNNNNN.dat`<sup>[\[2\]](#note2)</sup> | Blockfilter index filters for the basic filtertype; *optional*, used if `-blockfilterindex=basic`
 `wallets/`         |                       | [Contains wallets](#multi-wallet-environment); can be specified by `-walletdir` option; if `wallets/` subdirectory does not exist, a wallet resides in the data directory
 `./`               | `banlist.dat`         | Stores the IPs/subnets of banned nodes
 `./`               | `bitcoin.conf`        | User-defined [configuration settings](bitcoin-conf.md) for `bitcoind` or `bitcoin-qt`. File is not written to by the software and must be created manually. Path can be specified by `-conf` option
 `./`               | `bitcoind.pid`        | Stores the process ID (PID) of `bitcoind` or `bitcoin-qt` while running; created at start and deleted on shutdown; can be specified by `-pid` option
 `./`               | `debug.log`           | Contains debug information and general logging generated by `bitcoind` or `bitcoin-qt`; can be specified by `-debuglogfile` option
 `./`               | `guisettings.ini.bak` | Backup of former [GUI settings](#gui-settings) after `-resetguisettings` option is used
 `./`               | `mempool.dat`         | Dump of the mempool's transactions
 `./`               | `onion_private_key`   | Cached Tor hidden service private key for `-listenonion` option
 `./`               | `peers.dat`           | Peer IP address database (custom format)
 `./`               | `settings.json`       | Read-write settings set through GUI or RPC interfaces, augmenting manual settings from [bitcoin.conf](bitcoin-conf.md). File is created automatically if read-write settings storage is not disabled with `-nosettings` option. Path can be specified with `-settings` option
 `./`               | `.cookie`             | Session RPC authentication cookie; if used, created at start and deleted on shutdown; can be specified by `-rpccookiefile` option
 `./`               | `.lock`               | Data directory lock file
 
 ## Multi-wallet environment
 
 Wallets are Berkeley DB (BDB) databases:
 
 Subdirectory | File(s)           | Description
 -------------|-------------------|------------
 `database/`  | BDB logging files | Part of BDB environment; created at start and deleted on shutdown; a user *must keep it as safe* as personal wallet `wallet.dat`
 `./`         | `db.log`          | BDB error file
 `./`         | `wallet.dat`      | Personal wallet (BDB) with keys and transactions
 `./`         | `.walletlock`     | Wallet lock file
 
 1. Each user-defined wallet named "wallet_name" resides in `wallets/wallet_name/` subdirectory.
 
 2. The default (unnamed) wallet resides in `wallets/` subdirectory; if the latter does not exist, the wallet resides in the data directory.
 
 3. A wallet database path can be specified by `-wallet` option.
 
+4. `wallet.dat` files must not be shared across different node instances, as that can result in key-reuse and double-spends due the lack of synchronization between instances.
+
+5. Any copy or backup of the wallet should be done through a `backupwallet` call in order to update and lock the wallet, preventing any file corruption caused by updates during the copy.
+
 ## GUI settings
 
 `bitcoin-qt` uses [`QSettings`](https://doc.qt.io/qt-5/qsettings.html) class; this implies platform-specific [locations where application settings are stored](https://doc.qt.io/qt-5/qsettings.html#locations-where-application-settings-are-stored).
 
 ## Legacy subdirectories and files
 
 These subdirectories and files are no longer used by the Bitcoin ABC:
 
 Path           | Description | Repository notes
 ---------------|-------------|-----------------
 `blktree/`     | Blockchain index; replaced by `blocks/index/` in [0.8.0](https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.8.0.md#improvements) | [PR #2231](https://github.com/bitcoin/bitcoin/pull/2231), [`8fdc94cc`](https://github.com/bitcoin/bitcoin/commit/8fdc94cc8f0341e96b1edb3a5b56811c0b20bd15)
 `coins/`       | Unspent transaction output database; replaced by `chainstate/` in 0.8.0 | [PR #2231](https://github.com/bitcoin/bitcoin/pull/2231), [`8fdc94cc`](https://github.com/bitcoin/bitcoin/commit/8fdc94cc8f0341e96b1edb3a5b56811c0b20bd15)
 `blkindex.dat` | Blockchain index BDB database; replaced by {`chainstate/`, `blocks/index/`, `blocks/revNNNNN.dat`<sup>[\[2\]](#note2)</sup>} in 0.8.0 | [PR #1677](https://github.com/bitcoin/bitcoin/pull/1677)
 `blk000?.dat`  | Block data (custom format, 2 GiB per file); replaced by `blocks/blkNNNNN.dat`<sup>[\[2\]](#note2)</sup> in 0.8.0 | [PR #1677](https://github.com/bitcoin/bitcoin/pull/1677)
 `addr.dat`     | Peer IP address BDB database; replaced by `peers.dat` in [0.7.0](https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.7.0.md) | [PR #1198](https://github.com/bitcoin/bitcoin/pull/1198), [`928d3a01`](https://github.com/bitcoin/bitcoin/commit/928d3a011cc66c7f907c4d053f674ea77dc611cc)
 
 ## Notes
 
 <a name="note1">1</a>. The `/` (slash, U+002F) is used as the platform-independent path component separator in this paper.
 
 <a name="note2">2</a>. `NNNNN` matches `[0-9]{5}` regex.