docs: major overhaul (#19395)

docs/_install-and-build/Backup--restore.md

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+---
+title: Backup & restore
+---
+**DO NOT FORGET YOUR PASSWORD** and **BACKUP YOUR KEYSTORE**
+
+# Backup & restore
+
+## Data directory
+
+Everything `geth` persists gets written inside its data directory (except for the PoW Ethash DAG, see note below).
+The default data directory locations are platform specific:
+
+* Mac: `~/Library/Ethereum`
+* Linux: `~/.ethereum`
+* Windows: `%APPDATA%\Ethereum`
+
+Accounts are stored in the `keystore` subdirectory. The contents of this directories should be transportable between nodes, platforms, implementations (C++, Go, Python).
+
+To configure the location of the data directory, the `--datadir` parameter can be specified. See [CLI Options](../interface/Command-Line-Options) for more details.
+
+_**Note:** The [Ethash DAG](../how-to/Mining#ethash-dag) is stored at `~/.ethash` (Mac/Linux) or `%APPDATA%\Ethash` (Windows) so that it can be reused by all clients. You can store this in a different location by using a symbolic link._
+
+## Upgrades
+
+Sometimes the internal database formats need updating (for example, when upgrade from before 0.9.20). This can be run with the following command (geth should not be otherwise running):
+
+```
+geth upgradedb
+```
+
+## Cleanup
+
+Geth's blockchain and state databases can be removed with:
+
+```
+geth removedb
+```
+
+This is useful for deleting an old chain and sync'ing to a new one. It only affects data directories that can be re-created on synchronisation and does not touch the keystore.
+
+## Blockchain import/export
+
+Export the blockchain in binary format with:
+```
+geth export <filename>
+```
+
+Or if you want to back up portions of the chain over time, a first and last block can be specified. For example, to back up the first epoch:
+
+```
+geth export <filename> 0 29999
+```
+
+Note that when backing up a partial chain, the file will be appended rather than truncated.
+
+Import binary-format blockchain exports with:
+```
+geth import <filename>
+```
+
+_See https://github.com/ethereum/wiki/wiki/Blockchain-import-export for more info_
+
+
+And finally: **DO NOT FORGET YOUR PASSWORD** and **BACKUP YOUR KEYSTORE**