diff --git a/compact_history.py b/compact_history.py
index 9186c64..0a4574b 100755
--- a/compact_history.py
+++ b/compact_history.py
@@ -48,7 +48,7 @@ async def compact_history():
     environ['DAEMON_URL'] = ''   # Avoid Env erroring out
     env = Env()
     db = DB(env)
-    await db.open_for_sync()
+    await db.open_for_compacting()
 
     assert not db.first_sync
     history = db.history
diff --git a/contrib/query.py b/contrib/query.py
index 60f1696..56dbda8 100755
--- a/contrib/query.py
+++ b/contrib/query.py
@@ -62,7 +62,7 @@ async def query(args):
     db = DB(env)
     coin = env.coin
 
-    await db._open_dbs(False)
+    await db.open_for_serving()
 
     if not args.scripts:
         await print_stats(db.hist_db, db.utxo_db)
diff --git a/electrumx/lib/server_base.py b/electrumx/lib/server_base.py
index 06557b5..db7a213 100644
--- a/electrumx/lib/server_base.py
+++ b/electrumx/lib/server_base.py
@@ -100,6 +100,9 @@ class ServerBase(object):
             self.logger.info('shutting down')
             server_task.cancel()
 
+        # Prevent some silly logs
+        await asyncio.sleep(0.01)
+
         self.logger.info('shutdown complete')
 
     def run(self):
diff --git a/electrumx/server/block_processor.py b/electrumx/server/block_processor.py
index 97f93a0..c9bde57 100644
--- a/electrumx/server/block_processor.py
+++ b/electrumx/server/block_processor.py
@@ -20,8 +20,8 @@ from aiorpcx import TaskGroup, run_in_thread
 import electrumx
 from electrumx.server.daemon import DaemonError
 from electrumx.lib.hash import hash_to_hex_str, HASHX_LEN
-from electrumx.lib.util import chunks, formatted_time, class_logger
-import electrumx.server.db
+from electrumx.lib.util import chunks, class_logger
+from electrumx.server.db import FlushData
 
 
 class Prefetcher(object):
@@ -142,26 +142,26 @@ class ChainError(Exception):
     '''Raised on error processing blocks.'''
 
 
-class BlockProcessor(electrumx.server.db.DB):
+class BlockProcessor(object):
     '''Process blocks and update the DB state to match.
 
     Employ a prefetcher to prefetch blocks in batches for processing.
     Coordinate backing up in case of chain reorganisations.
     '''
 
-    def __init__(self, env, daemon, notifications):
-        super().__init__(env)
-
+    def __init__(self, env, db, daemon, notifications):
+        self.env = env
+        self.db = db
         self.daemon = daemon
         self.notifications = notifications
 
+        self.coin = env.coin
         self.blocks_event = asyncio.Event()
         self.prefetcher = Prefetcher(daemon, env.coin, self.blocks_event)
+        self.logger = class_logger(__name__, self.__class__.__name__)
 
         # Meta
-        self.cache_MB = env.cache_MB
         self.next_cache_check = 0
-        self.last_flush = time.time()
         self.touched = set()
         self.reorg_count = 0
 
@@ -189,17 +189,6 @@ class BlockProcessor(electrumx.server.db.DB):
                 return await run_in_thread(func, *args)
         return await asyncio.shield(run_in_thread_locked())
 
-    async def _maybe_flush(self):
-        # If caught up, flush everything as client queries are
-        # performed on the DB.
-        if self._caught_up_event.is_set():
-            await self.flush(True)
-        elif time.time() > self.next_cache_check:
-            flush_arg = self.check_cache_size()
-            if flush_arg is not None:
-                await self.flush(flush_arg)
-            self.next_cache_check = time.time() + 30
-
     async def check_and_advance_blocks(self, raw_blocks):
         '''Process the list of raw blocks passed.  Detects and handles
         reorgs.
@@ -217,7 +206,7 @@ class BlockProcessor(electrumx.server.db.DB):
             start = time.time()
             await self.run_in_thread_with_lock(self.advance_blocks, blocks)
             await self._maybe_flush()
-            if not self.first_sync:
+            if not self.db.first_sync:
                 s = '' if len(blocks) == 1 else 's'
                 self.logger.info('processed {:,d} block{} in {:.1f}s'
                                  .format(len(blocks), s,
@@ -257,13 +246,19 @@ class BlockProcessor(electrumx.server.db.DB):
             except Exception:
                 return await self.daemon.raw_blocks(hex_hashes)
 
+        def flush_backup():
+            # self.touched can include other addresses which is
+            # harmless, but remove None.
+            self.touched.discard(None)
+            self.db.flush_backup(self.flush_data(), self.touched)
+
         start, last, hashes = await self.reorg_hashes(count)
         # Reverse and convert to hex strings.
         hashes = [hash_to_hex_str(hash) for hash in reversed(hashes)]
         for hex_hashes in chunks(hashes, 50):
             raw_blocks = await get_raw_blocks(last, hex_hashes)
             await self.run_in_thread_with_lock(self.backup_blocks, raw_blocks)
-            await self.backup_flush()
+            await self.run_in_thread_with_lock(flush_backup)
             last -= len(raw_blocks)
         await self.prefetcher.reset_height(self.height)
 
@@ -280,7 +275,7 @@ class BlockProcessor(electrumx.server.db.DB):
         self.logger.info(f'chain was reorganised replacing {count:,d} '
                          f'block{s} at heights {start:,d}-{last:,d}')
 
-        return start, last, await self.fs_block_hashes(start, count)
+        return start, last, await self.db.fs_block_hashes(start, count)
 
     async def calc_reorg_range(self, count):
         '''Calculate the reorg range'''
@@ -298,7 +293,7 @@ class BlockProcessor(electrumx.server.db.DB):
             start = self.height - 1
             count = 1
             while start > 0:
-                hashes = await self.fs_block_hashes(start, count)
+                hashes = await self.db.fs_block_hashes(start, count)
                 hex_hashes = [hash_to_hex_str(hash) for hash in hashes]
                 d_hex_hashes = await self.daemon.block_hex_hashes(start, count)
                 n = diff_pos(hex_hashes, d_hex_hashes)
@@ -314,135 +309,40 @@ class BlockProcessor(electrumx.server.db.DB):
 
         return start, count
 
-    def flush_state(self, batch):
-        '''Flush chain state to the batch.'''
-        now = time.time()
-        self.wall_time += now - self.last_flush
-        self.last_flush = now
-        self.last_flush_tx_count = self.tx_count
-        self.write_utxo_state(batch)
-
-    def assert_flushed(self):
-        '''Asserts state is fully flushed.'''
-        assert self.tx_count == self.fs_tx_count == self.db_tx_count
-        assert self.height == self.fs_height == self.db_height
-        assert not self.undo_infos
-        assert not self.utxo_cache
-        assert not self.db_deletes
-        self.history.assert_flushed()
+    def estimate_txs_remaining(self):
+        # Try to estimate how many txs there are to go
+        daemon_height = self.daemon.cached_height()
+        coin = self.coin
+        tail_count = daemon_height - max(self.height, coin.TX_COUNT_HEIGHT)
+        # Damp the initial enthusiasm
+        realism = max(2.0 - 0.9 * self.height / coin.TX_COUNT_HEIGHT, 1.0)
+        return (tail_count * coin.TX_PER_BLOCK +
+                max(coin.TX_COUNT - self.tx_count, 0)) * realism
+
+    # - Flushing
+    def flush_data(self):
+        '''The data for a flush.  The lock must be taken.'''
+        assert self.state_lock.locked()
+        return FlushData(self.height, self.tx_count, self.headers,
+                         self.tx_hashes, self.undo_infos, self.utxo_cache,
+                         self.db_deletes, self.tip)
 
     async def flush(self, flush_utxos):
-        if self.height == self.db_height:
-            self.assert_flushed()
-        else:
-            await self.run_in_thread_with_lock(self._flush_body, flush_utxos)
-
-    def _flush_body(self, flush_utxos):
-        '''Flush out cached state.
-
-        History is always flushed.  UTXOs are flushed if flush_utxos.'''
-        flush_start = time.time()
-        last_flush = self.last_flush
-        tx_diff = self.tx_count - self.last_flush_tx_count
-
-        # Flush to file system
-        self.fs_flush()
-        fs_end = time.time()
-        if self.utxo_db.for_sync:
-            self.logger.info('flushed to FS in {:.1f}s'
-                             .format(fs_end - flush_start))
-
-        # History next - it's fast and frees memory
-        hashX_count = self.history.flush()
-        if self.utxo_db.for_sync:
-            self.logger.info('flushed history in {:.1f}s for {:,d} addrs'
-                             .format(time.time() - fs_end, hashX_count))
-
-        # Flush state last as it reads the wall time.
-        with self.utxo_db.write_batch() as batch:
-            if flush_utxos:
-                self.flush_utxos(batch)
-            self.flush_state(batch)
-
-        # Update and put the wall time again - otherwise we drop the
-        # time it took to commit the batch
-        self.flush_state(self.utxo_db)
-
-        self.logger.info('flush #{:,d} took {:.1f}s.  Height {:,d} txs: {:,d}'
-                         .format(self.history.flush_count,
-                                 self.last_flush - flush_start,
-                                 self.height, self.tx_count))
-
-        # Catch-up stats
-        if self.utxo_db.for_sync:
-            tx_per_sec = int(self.tx_count / self.wall_time)
-            this_tx_per_sec = 1 + int(tx_diff / (self.last_flush - last_flush))
-            self.logger.info('tx/sec since genesis: {:,d}, '
-                             'since last flush: {:,d}'
-                             .format(tx_per_sec, this_tx_per_sec))
-
-            daemon_height = self.daemon.cached_height()
-            if self.height > self.coin.TX_COUNT_HEIGHT:
-                tx_est = (daemon_height - self.height) * self.coin.TX_PER_BLOCK
-            else:
-                tx_est = ((daemon_height - self.coin.TX_COUNT_HEIGHT)
-                          * self.coin.TX_PER_BLOCK
-                          + (self.coin.TX_COUNT - self.tx_count))
-
-            # Damp the enthusiasm
-            realism = 2.0 - 0.9 * self.height / self.coin.TX_COUNT_HEIGHT
-            tx_est *= max(realism, 1.0)
-
-            self.logger.info('sync time: {}  ETA: {}'
-                             .format(formatted_time(self.wall_time),
-                                     formatted_time(tx_est / this_tx_per_sec)))
-
-    def fs_flush(self):
-        '''Flush the things stored on the filesystem.'''
-        assert self.fs_height + len(self.headers) == self.height
-        assert self.tx_count == self.tx_counts[-1] if self.tx_counts else 0
-
-        self.fs_update(self.fs_height, self.headers, self.tx_hashes)
-        self.fs_height = self.height
-        self.fs_tx_count = self.tx_count
-        self.tx_hashes = []
-        self.headers = []
-
-    async def backup_flush(self):
-        assert self.height < self.db_height
-        assert not self.headers
-        assert not self.tx_hashes
-        self.history.assert_flushed()
-        await self.run_in_thread_with_lock(self._backup_flush_body)
-
-    def _backup_flush_body(self):
-        '''Like flush() but when backing up.  All UTXOs are flushed.
-
-        hashXs - sequence of hashXs which were touched by backing
-        up.  Searched for history entries to remove after the backup
-        height.
-        '''
-        flush_start = time.time()
-
-        self.backup_fs(self.height, self.tx_count)
-
-        # Backup history.  self.touched can include other addresses
-        # which is harmless, but remove None.
-        self.touched.discard(None)
-        nremoves = self.history.backup(self.touched, self.tx_count)
-        self.logger.info('backing up removed {:,d} history entries'
-                         .format(nremoves))
+        def flush():
+            self.db.flush_dbs(self.flush_data(), flush_utxos,
+                              self.estimate_txs_remaining)
+        await self.run_in_thread_with_lock(flush)
 
-        with self.utxo_db.write_batch() as batch:
-            # Flush state last as it reads the wall time.
-            self.flush_utxos(batch)
-            self.flush_state(batch)
-
-        self.logger.info('backup flush #{:,d} took {:.1f}s.  '
-                         'Height {:,d} txs: {:,d}'
-                         .format(self.history.flush_count,
-                                 self.last_flush - flush_start,
-                                 self.height, self.tx_count))
+    async def _maybe_flush(self):
+        # If caught up, flush everything as client queries are
+        # performed on the DB.
+        if self._caught_up_event.is_set():
+            await self.flush(True)
+        elif time.time() > self.next_cache_check:
+            flush_arg = self.check_cache_size()
+            if flush_arg is not None:
+                await self.flush(flush_arg)
+            self.next_cache_check = time.time() + 30
 
     def check_cache_size(self):
         '''Flush a cache if it gets too big.'''
@@ -451,10 +351,10 @@ class BlockProcessor(electrumx.server.db.DB):
         one_MB = 1000*1000
         utxo_cache_size = len(self.utxo_cache) * 205
         db_deletes_size = len(self.db_deletes) * 57
-        hist_cache_size = self.history.unflushed_memsize()
+        hist_cache_size = self.db.history.unflushed_memsize()
         # Roughly ntxs * 32 + nblocks * 42
-        tx_hash_size = ((self.tx_count - self.fs_tx_count) * 32
-                        + (self.height - self.fs_height) * 42)
+        tx_hash_size = ((self.tx_count - self.db.fs_tx_count) * 32
+                        + (self.height - self.db.fs_height) * 42)
         utxo_MB = (db_deletes_size + utxo_cache_size) // one_MB
         hist_MB = (hist_cache_size + tx_hash_size) // one_MB
 
@@ -465,8 +365,9 @@ class BlockProcessor(electrumx.server.db.DB):
 
         # Flush history if it takes up over 20% of cache memory.
         # Flush UTXOs once they take up 80% of cache memory.
-        if utxo_MB + hist_MB >= self.cache_MB or hist_MB >= self.cache_MB // 5:
-            return utxo_MB >= self.cache_MB * 4 // 5
+        cache_MB = self.env.cache_MB
+        if utxo_MB + hist_MB >= cache_MB or hist_MB >= cache_MB // 5:
+            return utxo_MB >= cache_MB * 4 // 5
         return None
 
     def advance_blocks(self, blocks):
@@ -474,7 +375,7 @@ class BlockProcessor(electrumx.server.db.DB):
 
         It is already verified they correctly connect onto our tip.
         '''
-        min_height = self.min_undo_height(self.daemon.cached_height())
+        min_height = self.db.min_undo_height(self.daemon.cached_height())
         height = self.height
 
         for block in blocks:
@@ -482,7 +383,7 @@ class BlockProcessor(electrumx.server.db.DB):
             undo_info = self.advance_txs(block.transactions)
             if height >= min_height:
                 self.undo_infos.append((undo_info, height))
-                self.write_raw_block(block.raw, height)
+                self.db.write_raw_block(block.raw, height)
 
         headers = [block.header for block in blocks]
         self.height = height
@@ -529,10 +430,10 @@ class BlockProcessor(electrumx.server.db.DB):
             update_touched(hashXs)
             tx_num += 1
 
-        self.history.add_unflushed(hashXs_by_tx, self.tx_count)
+        self.db.history.add_unflushed(hashXs_by_tx, self.tx_count)
 
         self.tx_count = tx_num
-        self.tx_counts.append(tx_num)
+        self.db.tx_counts.append(tx_num)
 
         return undo_info
 
@@ -542,7 +443,7 @@ class BlockProcessor(electrumx.server.db.DB):
         The blocks should be in order of decreasing height, starting at.
         self.height.  A flush is performed once the blocks are backed up.
         '''
-        self.assert_flushed()
+        self.db.assert_flushed(self.flush_data())
         assert self.height >= len(raw_blocks)
 
         coin = self.coin
@@ -558,14 +459,14 @@ class BlockProcessor(electrumx.server.db.DB):
             self.tip = coin.header_prevhash(block.header)
             self.backup_txs(block.transactions)
             self.height -= 1
-            self.tx_counts.pop()
+            self.db.tx_counts.pop()
 
         self.logger.info('backed up to height {:,d}'.format(self.height))
 
     def backup_txs(self, txs):
         # Prevout values, in order down the block (coinbase first if present)
         # undo_info is in reverse block order
-        undo_info = self.read_undo_info(self.height)
+        undo_info = self.db.read_undo_info(self.height)
         if undo_info is None:
             raise ChainError('no undo information found for height {:,d}'
                              .format(self.height))
@@ -673,14 +574,14 @@ class BlockProcessor(electrumx.server.db.DB):
         # Value: hashX
         prefix = b'h' + tx_hash[:4] + idx_packed
         candidates = {db_key: hashX for db_key, hashX
-                      in self.utxo_db.iterator(prefix=prefix)}
+                      in self.db.utxo_db.iterator(prefix=prefix)}
 
         for hdb_key, hashX in candidates.items():
             tx_num_packed = hdb_key[-4:]
 
             if len(candidates) > 1:
                 tx_num, = unpack('<I', tx_num_packed)
-                hash, height = self.fs_tx_hash(tx_num)
+                hash, height = self.db.fs_tx_hash(tx_num)
                 if hash != tx_hash:
                     assert hash is not None  # Should always be found
                     continue
@@ -688,7 +589,7 @@ class BlockProcessor(electrumx.server.db.DB):
             # Key: b'u' + address_hashX + tx_idx + tx_num
             # Value: the UTXO value as a 64-bit unsigned integer
             udb_key = b'u' + hashX + hdb_key[-6:]
-            utxo_value_packed = self.utxo_db.get(udb_key)
+            utxo_value_packed = self.db.utxo_db.get(udb_key)
             if utxo_value_packed:
                 # Remove both entries for this UTXO
                 self.db_deletes.append(hdb_key)
@@ -698,48 +599,6 @@ class BlockProcessor(electrumx.server.db.DB):
         raise ChainError('UTXO {} / {:,d} not found in "h" table'
                          .format(hash_to_hex_str(tx_hash), tx_idx))
 
-    def flush_utxos(self, batch):
-        '''Flush the cached DB writes and UTXO set to the batch.'''
-        # Care is needed because the writes generated by flushing the
-        # UTXO state may have keys in common with our write cache or
-        # may be in the DB already.
-        flush_start = time.time()
-        delete_count = len(self.db_deletes) // 2
-        utxo_cache_len = len(self.utxo_cache)
-
-        # Spends
-        batch_delete = batch.delete
-        for key in sorted(self.db_deletes):
-            batch_delete(key)
-        self.db_deletes = []
-
-        # New UTXOs
-        batch_put = batch.put
-        for cache_key, cache_value in self.utxo_cache.items():
-            # suffix = tx_idx + tx_num
-            hashX = cache_value[:-12]
-            suffix = cache_key[-2:] + cache_value[-12:-8]
-            batch_put(b'h' + cache_key[:4] + suffix, hashX)
-            batch_put(b'u' + hashX + suffix, cache_value[-8:])
-        self.utxo_cache = {}
-
-        # New undo information
-        self.flush_undo_infos(batch_put, self.undo_infos)
-        self.undo_infos = []
-
-        if self.utxo_db.for_sync:
-            self.logger.info('flushed {:,d} blocks with {:,d} txs, {:,d} UTXO '
-                             'adds, {:,d} spends in {:.1f}s, committing...'
-                             .format(self.height - self.db_height,
-                                     self.tx_count - self.db_tx_count,
-                                     utxo_cache_len, delete_count,
-                                     time.time() - flush_start))
-
-        self.utxo_flush_count = self.history.flush_count
-        self.db_tx_count = self.tx_count
-        self.db_height = self.height
-        self.db_tip = self.tip
-
     async def _process_prefetched_blocks(self):
         '''Loop forever processing blocks as they arrive.'''
         while True:
@@ -759,8 +618,8 @@ class BlockProcessor(electrumx.server.db.DB):
     async def _first_caught_up(self):
         self.logger.info(f'caught up to height {self.height}')
         # Flush everything but with first_sync->False state.
-        first_sync = self.first_sync
-        self.first_sync = False
+        first_sync = self.db.first_sync
+        self.db.first_sync = False
         await self.flush(True)
         if first_sync:
             self.logger.info(f'{electrumx.version} synced to '
@@ -768,22 +627,13 @@ class BlockProcessor(electrumx.server.db.DB):
         # Initialise the notification framework
         await self.notifications.on_block(set(), self.height)
         # Reopen for serving
-        await self.open_for_serving()
+        await self.db.open_for_serving()
 
     async def _first_open_dbs(self):
-        await self.open_for_sync()
-        # An incomplete compaction needs to be cancelled otherwise
-        # restarting it will corrupt the history
-        self.history.cancel_compaction()
-        # These are our state as we move ahead of DB state
-        self.fs_height = self.db_height
-        self.fs_tx_count = self.db_tx_count
-        self.height = self.db_height
-        self.tip = self.db_tip
-        self.tx_count = self.db_tx_count
-        self.last_flush_tx_count = self.tx_count
-        if self.utxo_db.for_sync:
-            self.logger.info(f'flushing DB cache at {self.cache_MB:,d} MB')
+        await self.db.open_for_sync()
+        self.height = self.db.db_height
+        self.tip = self.db.db_tip
+        self.tx_count = self.db.db_tx_count
 
     # --- External API
 
diff --git a/electrumx/server/chain_state.py b/electrumx/server/chain_state.py
index 8e33830..135d42a 100644
--- a/electrumx/server/chain_state.py
+++ b/electrumx/server/chain_state.py
@@ -14,18 +14,18 @@ class ChainState(object):
     blocks, transaction history, UTXOs and the mempool.
     '''
 
-    def __init__(self, env, daemon, bp):
+    def __init__(self, env, db, daemon, bp):
         self._env = env
+        self._db = db
         self._daemon = daemon
-        self._bp = bp
 
         # External interface pass-throughs for session.py
-        self.force_chain_reorg = self._bp.force_chain_reorg
-        self.tx_branch_and_root = self._bp.merkle.branch_and_root
-        self.read_headers = self._bp.read_headers
-        self.all_utxos = self._bp.all_utxos
-        self.limited_history = self._bp.limited_history
-        self.header_branch_and_root = self._bp.header_branch_and_root
+        self.force_chain_reorg = bp.force_chain_reorg
+        self.tx_branch_and_root = db.merkle.branch_and_root
+        self.read_headers = db.read_headers
+        self.all_utxos = db.all_utxos
+        self.limited_history = db.limited_history
+        self.header_branch_and_root = db.header_branch_and_root
 
     async def broadcast_transaction(self, raw_tx):
         return await self._daemon.sendrawtransaction([raw_tx])
@@ -34,7 +34,7 @@ class ChainState(object):
         return await getattr(self._daemon, method)(*args)
 
     def db_height(self):
-        return self._bp.db_height
+        return self._db.db_height
 
     def get_info(self):
         '''Chain state info for LocalRPC and logs.'''
@@ -57,7 +57,7 @@ class ChainState(object):
 
     async def query(self, args, limit):
         coin = self._env.coin
-        db = self._bp
+        db = self._db
         lines = []
 
         def arg_to_hashX(arg):
diff --git a/electrumx/server/controller.py b/electrumx/server/controller.py
index e3115c6..665d39c 100644
--- a/electrumx/server/controller.py
+++ b/electrumx/server/controller.py
@@ -13,6 +13,7 @@ import electrumx
 from electrumx.lib.server_base import ServerBase
 from electrumx.lib.util import version_string
 from electrumx.server.chain_state import ChainState
+from electrumx.server.db import DB
 from electrumx.server.mempool import MemPool
 from electrumx.server.session import SessionManager
 
@@ -93,10 +94,11 @@ class Controller(ServerBase):
 
         notifications = Notifications()
         daemon = env.coin.DAEMON(env)
+        db = DB(env)
         BlockProcessor = env.coin.BLOCK_PROCESSOR
-        bp = BlockProcessor(env, daemon, notifications)
-        mempool = MemPool(env.coin, daemon, notifications, bp.lookup_utxos)
-        chain_state = ChainState(env, daemon, bp)
+        bp = BlockProcessor(env, db, daemon, notifications)
+        mempool = MemPool(env.coin, daemon, notifications, db.lookup_utxos)
+        chain_state = ChainState(env, db, daemon, bp)
         session_mgr = SessionManager(env, chain_state, mempool,
                                      notifications, shutdown_event)
 
@@ -108,7 +110,7 @@ class Controller(ServerBase):
             await group.spawn(session_mgr.serve(serve_externally_event))
             await group.spawn(bp.fetch_and_process_blocks(caught_up_event))
             await caught_up_event.wait()
-            await group.spawn(bp.populate_header_merkle_cache())
+            await group.spawn(db.populate_header_merkle_cache())
             await group.spawn(mempool.keep_synchronized(synchronized_event))
             await synchronized_event.wait()
             serve_externally_event.set()
diff --git a/electrumx/server/db.py b/electrumx/server/db.py
index b23f87c..6e82a52 100644
--- a/electrumx/server/db.py
+++ b/electrumx/server/db.py
@@ -18,17 +18,30 @@ from collections import namedtuple
 from glob import glob
 from struct import pack, unpack
 
+import attr
 from aiorpcx import run_in_thread
 
 import electrumx.lib.util as util
 from electrumx.lib.hash import hash_to_hex_str, HASHX_LEN
 from electrumx.lib.merkle import Merkle, MerkleCache
+from electrumx.lib.util import formatted_time
 from electrumx.server.storage import db_class
 from electrumx.server.history import History
 
 
 UTXO = namedtuple("UTXO", "tx_num tx_pos tx_hash height value")
 
+@attr.s(slots=True)
+class FlushData(object):
+    height = attr.ib()
+    tx_count = attr.ib()
+    headers = attr.ib()
+    block_tx_hashes = attr.ib()
+    # The following are flushed to the UTXO DB if undo_infos is not None
+    undo_infos = attr.ib()
+    adds = attr.ib()
+    deletes = attr.ib()
+    tip = attr.ib()
 
 class DB(object):
     '''Simple wrapper of the backend database for querying.
@@ -62,6 +75,7 @@ class DB(object):
         self.history = History()
         self.utxo_db = None
         self.tx_counts = None
+        self.last_flush = time.time()
 
         self.logger.info(f'using {self.env.db_engine} for DB backend')
 
@@ -90,7 +104,7 @@ class DB(object):
         else:
             assert self.db_tx_count == 0
 
-    async def _open_dbs(self, for_sync):
+    async def _open_dbs(self, for_sync, compacting):
         assert self.utxo_db is None
 
         # First UTXO DB
@@ -110,12 +124,16 @@ class DB(object):
 
         # Then history DB
         self.utxo_flush_count = self.history.open_db(self.db_class, for_sync,
-                                                     self.utxo_flush_count)
+                                                     self.utxo_flush_count,
+                                                     compacting)
         self.clear_excess_undo_info()
 
         # Read TX counts (requires meta directory)
         await self._read_tx_counts()
 
+    async def open_for_compacting(self):
+        await self._open_dbs(True, True)
+
     async def open_for_sync(self):
         '''Open the databases to sync to the daemon.
 
@@ -123,7 +141,7 @@ class DB(object):
         synchronization.  When serving clients we want the open files for
         serving network connections.
         '''
-        await self._open_dbs(True)
+        await self._open_dbs(True, False)
 
     async def open_for_serving(self):
         '''Open the databases for serving.  If they are already open they are
@@ -134,13 +152,13 @@ class DB(object):
             self.utxo_db.close()
             self.history.close_db()
             self.utxo_db = None
-        await self._open_dbs(False)
+        await self._open_dbs(False, False)
 
     # Header merkle cache
 
     async def populate_header_merkle_cache(self):
         self.logger.info('populating header merkle cache...')
-        length = max(1, self.height - self.env.reorg_limit)
+        length = max(1, self.db_height - self.env.reorg_limit)
         start = time.time()
         await self.header_mc.initialize(length)
         elapsed = time.time() - start
@@ -149,6 +167,178 @@ class DB(object):
     async def header_branch_and_root(self, length, height):
         return await self.header_mc.branch_and_root(length, height)
 
+    # Flushing
+    def assert_flushed(self, flush_data):
+        '''Asserts state is fully flushed.'''
+        assert flush_data.tx_count == self.fs_tx_count == self.db_tx_count
+        assert flush_data.height == self.fs_height == self.db_height
+        assert flush_data.tip == self.db_tip
+        assert not flush_data.headers
+        assert not flush_data.block_tx_hashes
+        assert not flush_data.adds
+        assert not flush_data.deletes
+        assert not flush_data.undo_infos
+        self.history.assert_flushed()
+
+    def flush_dbs(self, flush_data, flush_utxos, estimate_txs_remaining):
+        '''Flush out cached state.  History is always flushed; UTXOs are
+        flushed if flush_utxos.'''
+        if flush_data.height == self.db_height:
+            self.assert_flushed(flush_data)
+            return
+
+        start_time = time.time()
+        prior_flush = self.last_flush
+        tx_delta = flush_data.tx_count - self.last_flush_tx_count
+
+        # Flush to file system
+        self.flush_fs(flush_data)
+
+        # Then history
+        self.flush_history()
+
+        # Flush state last as it reads the wall time.
+        with self.utxo_db.write_batch() as batch:
+            if flush_utxos:
+                self.flush_utxo_db(batch, flush_data)
+            self.flush_state(batch)
+
+        # Update and put the wall time again - otherwise we drop the
+        # time it took to commit the batch
+        self.flush_state(self.utxo_db)
+
+        elapsed = self.last_flush - start_time
+        self.logger.info(f'flush #{self.history.flush_count:,d} took '
+                         f'{elapsed:.1f}s.  Height {flush_data.height:,d} '
+                         f'txs: {flush_data.tx_count:,d} ({tx_delta:+,d})')
+
+        # Catch-up stats
+        if self.utxo_db.for_sync:
+            flush_interval = self.last_flush - prior_flush
+            tx_per_sec_gen = int(flush_data.tx_count / self.wall_time)
+            tx_per_sec_last = 1 + int(tx_delta / flush_interval)
+            eta = estimate_txs_remaining() / tx_per_sec_last
+            self.logger.info(f'tx/sec since genesis: {tx_per_sec_gen:,d}, '
+                             f'since last flush: {tx_per_sec_last:,d}')
+            self.logger.info(f'sync time: {formatted_time(self.wall_time)}  '
+                             f'ETA: {formatted_time(eta)}')
+
+    def flush_fs(self, flush_data):
+        '''Write headers, tx counts and block tx hashes to the filesystem.
+
+        The first height to write is self.fs_height + 1.  The FS
+        metadata is all append-only, so in a crash we just pick up
+        again from the height stored in the DB.
+        '''
+        prior_tx_count = (self.tx_counts[self.fs_height]
+                          if self.fs_height >= 0 else 0)
+        assert len(flush_data.block_tx_hashes) == len(flush_data.headers)
+        assert flush_data.height == self.fs_height + len(flush_data.headers)
+        assert flush_data.tx_count == (self.tx_counts[-1] if self.tx_counts
+                                       else 0)
+        assert len(self.tx_counts) == flush_data.height + 1
+        hashes = b''.join(flush_data.block_tx_hashes)
+        flush_data.block_tx_hashes.clear()
+        assert len(hashes) % 32 == 0
+        assert len(hashes) // 32 == flush_data.tx_count - prior_tx_count
+
+        # Write the headers, tx counts, and tx hashes
+        start_time = time.time()
+        height_start = self.fs_height + 1
+        offset = self.header_offset(height_start)
+        self.headers_file.write(offset, b''.join(flush_data.headers))
+        self.fs_update_header_offsets(offset, height_start, flush_data.headers)
+        flush_data.headers.clear()
+
+        offset = height_start * self.tx_counts.itemsize
+        self.tx_counts_file.write(offset,
+                                  self.tx_counts[height_start:].tobytes())
+        offset = prior_tx_count * 32
+        self.hashes_file.write(offset, hashes)
+
+        self.fs_height = flush_data.height
+        self.fs_tx_count = flush_data.tx_count
+
+        if self.utxo_db.for_sync:
+            elapsed = time.time() - start_time
+            self.logger.info(f'flushed filesystem data in {elapsed:.2f}s')
+
+    def flush_history(self):
+        self.history.flush()
+
+    def flush_utxo_db(self, batch, flush_data):
+        '''Flush the cached DB writes and UTXO set to the batch.'''
+        # Care is needed because the writes generated by flushing the
+        # UTXO state may have keys in common with our write cache or
+        # may be in the DB already.
+        start_time = time.time()
+        add_count = len(flush_data.adds)
+        spend_count = len(flush_data.deletes) // 2
+
+        # Spends
+        batch_delete = batch.delete
+        for key in sorted(flush_data.deletes):
+            batch_delete(key)
+        flush_data.deletes.clear()
+
+        # New UTXOs
+        batch_put = batch.put
+        for key, value in flush_data.adds.items():
+            # suffix = tx_idx + tx_num
+            hashX = value[:-12]
+            suffix = key[-2:] + value[-12:-8]
+            batch_put(b'h' + key[:4] + suffix, hashX)
+            batch_put(b'u' + hashX + suffix, value[-8:])
+        flush_data.adds.clear()
+
+        # New undo information
+        self.flush_undo_infos(batch_put, flush_data.undo_infos)
+        flush_data.undo_infos.clear()
+
+        if self.utxo_db.for_sync:
+            block_count = flush_data.height - self.db_height
+            tx_count = flush_data.tx_count - self.db_tx_count
+            elapsed = time.time() - start_time
+            self.logger.info(f'flushed {block_count:,d} blocks with '
+                             f'{tx_count:,d} txs, {add_count:,d} UTXO adds, '
+                             f'{spend_count:,d} spends in '
+                             f'{elapsed:.1f}s, committing...')
+
+        self.utxo_flush_count = self.history.flush_count
+        self.db_height = flush_data.height
+        self.db_tx_count = flush_data.tx_count
+        self.db_tip = flush_data.tip
+
+    def flush_state(self, batch):
+        '''Flush chain state to the batch.'''
+        now = time.time()
+        self.wall_time += now - self.last_flush
+        self.last_flush = now
+        self.last_flush_tx_count = self.fs_tx_count
+        self.write_utxo_state(batch)
+
+    def flush_backup(self, flush_data, touched):
+        '''Like flush_dbs() but when backing up.  All UTXOs are flushed.'''
+        assert not flush_data.headers
+        assert not flush_data.block_tx_hashes
+        assert flush_data.height < self.db_height
+        self.history.assert_flushed()
+
+        start_time = time.time()
+        tx_delta = flush_data.tx_count - self.last_flush_tx_count
+
+        self.backup_fs(flush_data.height, flush_data.tx_count)
+        self.history.backup(touched, flush_data.tx_count)
+        with self.utxo_db.write_batch() as batch:
+            self.flush_utxo_db(batch, flush_data)
+            # Flush state last as it reads the wall time.
+            self.flush_state(batch)
+
+        elapsed = self.last_flush - start_time
+        self.logger.info(f'backup flush #{self.history.flush_count:,d} took '
+                         f'{elapsed:.1f}s.  Height {flush_data.height:,d} '
+                         f'txs: {flush_data.tx_count:,d} ({tx_delta:+,d})')
+
     def fs_update_header_offsets(self, offset_start, height_start, headers):
         if self.coin.STATIC_BLOCK_HEADERS:
             return
@@ -178,36 +368,6 @@ class DB(object):
         # Truncate header_mc: header count is 1 more than the height.
         self.header_mc.truncate(height + 1)
 
-    def fs_update(self, fs_height, headers, block_tx_hashes):
-        '''Write headers, the tx_count array and block tx hashes to disk.
-
-        Their first height is fs_height.  No recorded DB state is
-        updated.  These arrays are all append only, so in a crash we
-        just pick up again from the DB height.
-        '''
-        blocks_done = len(headers)
-        height_start = fs_height + 1
-        new_height = fs_height + blocks_done
-        prior_tx_count = (self.tx_counts[fs_height] if fs_height >= 0 else 0)
-        cur_tx_count = self.tx_counts[-1] if self.tx_counts else 0
-        txs_done = cur_tx_count - prior_tx_count
-
-        assert len(block_tx_hashes) == blocks_done
-        assert len(self.tx_counts) == new_height + 1
-        hashes = b''.join(block_tx_hashes)
-        assert len(hashes) % 32 == 0
-        assert len(hashes) // 32 == txs_done
-
-        # Write the headers, tx counts, and tx hashes
-        offset = self.header_offset(height_start)
-        self.headers_file.write(offset, b''.join(headers))
-        self.fs_update_header_offsets(offset, height_start, headers)
-        offset = height_start * self.tx_counts.itemsize
-        self.tx_counts_file.write(offset,
-                                  self.tx_counts[height_start:].tobytes())
-        offset = prior_tx_count * 32
-        self.hashes_file.write(offset, hashes)
-
     async def read_headers(self, start_height, count):
         '''Requires start_height >= 0, count >= 0.  Reads as many headers as
         are available starting at start_height up to count.  This
@@ -379,6 +539,11 @@ class DB(object):
             self.wall_time = state['wall_time']
             self.first_sync = state['first_sync']
 
+        # These are our state as we move ahead of DB state
+        self.fs_height = self.db_height
+        self.fs_tx_count = self.db_tx_count
+        self.last_flush_tx_count = self.fs_tx_count
+
         # Log some stats
         self.logger.info('DB version: {:d}'.format(self.db_version))
         self.logger.info('coin: {}'.format(self.coin.NAME))
@@ -386,6 +551,8 @@ class DB(object):
         self.logger.info('height: {:,d}'.format(self.db_height))
         self.logger.info('tip: {}'.format(hash_to_hex_str(self.db_tip)))
         self.logger.info('tx count: {:,d}'.format(self.db_tx_count))
+        if self.utxo_db.for_sync:
+            self.logger.info(f'flushing DB cache at {self.env.cache_MB:,d} MB')
         if self.first_sync:
             self.logger.info('sync time so far: {}'
                              .format(util.formatted_time(self.wall_time)))
diff --git a/electrumx/server/history.py b/electrumx/server/history.py
index b525af8..b42ca6c 100644
--- a/electrumx/server/history.py
+++ b/electrumx/server/history.py
@@ -11,6 +11,7 @@
 import array
 import ast
 import bisect
+import time
 from collections import defaultdict
 from functools import partial
 from struct import pack, unpack
@@ -31,10 +32,14 @@ class History(object):
         self.unflushed_count = 0
         self.db = None
 
-    def open_db(self, db_class, for_sync, utxo_flush_count):
+    def open_db(self, db_class, for_sync, utxo_flush_count, compacting):
         self.db = db_class('hist', for_sync)
         self.read_state()
         self.clear_excess(utxo_flush_count)
+        # An incomplete compaction needs to be cancelled otherwise
+        # restarting it will corrupt the history
+        if not compacting:
+            self._cancel_compaction()
         return self.flush_count
 
     def close_db(self):
@@ -80,7 +85,7 @@ class History(object):
             if flush_id > utxo_flush_count:
                 keys.append(key)
 
-        self.logger.info('deleting {:,d} history entries'.format(len(keys)))
+        self.logger.info(f'deleting {len(keys):,d} history entries')
 
         self.flush_count = utxo_flush_count
         with self.db.write_batch() as batch:
@@ -119,6 +124,7 @@ class History(object):
         assert not self.unflushed
 
     def flush(self):
+        start_time = time.time()
         self.flush_count += 1
         flush_id = pack('>H', self.flush_count)
         unflushed = self.unflushed
@@ -132,7 +138,11 @@ class History(object):
         count = len(unflushed)
         unflushed.clear()
         self.unflushed_count = 0
-        return count
+
+        if self.db.for_sync:
+            elapsed = time.time() - start_time
+            self.logger.info(f'flushed history in {elapsed:.1f}s '
+                             f'for {count:,d} addrs')
 
     def backup(self, hashXs, tx_count):
         # Not certain this is needed, but it doesn't hurt
@@ -161,7 +171,7 @@ class History(object):
                     batch.put(key, value)
             self.write_state(batch)
 
-        return nremoves
+        self.logger.info(f'backing up removed {nremoves:,d} history entries')
 
     def get_txnums(self, hashX, limit=1000):
         '''Generator that returns an unpruned, sorted list of tx_nums in the
@@ -307,7 +317,7 @@ class History(object):
                                  100 * cursor / 65536))
         return write_size
 
-    def cancel_compaction(self):
+    def _cancel_compaction(self):
         if self.comp_cursor != -1:
             self.logger.warning('cancelling in-progress history compaction')
             self.comp_flush_count = -1