diff --git a/Annex/Drop.hs b/Annex/Drop.hs
index b7543ec794..416ce49e94 100644
--- a/Annex/Drop.hs
+++ b/Annex/Drop.hs
@@ -27,8 +27,8 @@ import qualified Data.Set as S
 
 type Reason = String
 
-{- Drop a key from local and/or remote when allowed by the preferred content
- - and numcopies settings.
+{- Drop a key from local and/or remote when allowed by the preferred content,
+ - required content, and numcopies settings.
  -
  - Skips trying to drop from remotes that are appendonly, since those drops
  - would presumably fail. Also skips dropping from exporttree/importtree remotes,
@@ -103,16 +103,13 @@ handleDropsFrom locs rs reason fromhere key afile si preverified runner = do
 			dropr fs r n >>= go fs rest
 		| otherwise = pure n
 
-	checkdrop fs n u a
-		| null fs = check $ -- no associated files; unused content
-			wantDrop True u (Just key) (AssociatedFile Nothing)
-		| otherwise = check $
-			allM (wantDrop True u (Just key) . AssociatedFile . Just) fs
-		where
-			check c = ifM c
-				( dodrop n u a
-				, return n
-				)
+	checkdrop fs n u a =
+		let afs = map (AssociatedFile . Just) fs
+		    pcc = Command.Drop.PreferredContentChecked True
+		in ifM (wantDrop True u (Just key) afile (Just afs))
+			( dodrop n u (a pcc)
+			, return n
+			)
 
 	dodrop n@(have, numcopies, mincopies, _untrusted) u a = 
 		ifM (safely $ runner $ a numcopies mincopies)
@@ -130,12 +127,12 @@ handleDropsFrom locs rs reason fromhere key afile si preverified runner = do
 			, return n
 			)
 
-	dropl fs n = checkdrop fs n Nothing $ \numcopies mincopies ->
+	dropl fs n = checkdrop fs n Nothing $ \pcc numcopies mincopies ->
 		stopUnless (inAnnex key) $
-			Command.Drop.startLocal afile ai si numcopies mincopies key preverified
+			Command.Drop.startLocal pcc afile ai si numcopies mincopies key preverified
 
-	dropr fs r n  = checkdrop fs n (Just $ Remote.uuid r) $ \numcopies mincopies ->
-		Command.Drop.startRemote afile ai si numcopies mincopies key r
+	dropr fs r n  = checkdrop fs n (Just $ Remote.uuid r) $ \pcc numcopies mincopies ->
+		Command.Drop.startRemote pcc afile ai si numcopies mincopies key r
 
 	ai = mkActionItem (key, afile)
 
diff --git a/Annex/Init.hs b/Annex/Init.hs
index cea69f8237..c1834085d1 100644
--- a/Annex/Init.hs
+++ b/Annex/Init.hs
@@ -134,8 +134,8 @@ initialize' mversion = checkInitializeAllowed $ do
 		else deconfigureSmudgeFilter
 	unlessM isBareRepo $ do
 		when supportunlocked $ do
-			showSideAction "scanning for unlocked files"
-			scanUnlockedFiles
+			showSideAction "scanning for annexed files"
+			scanAnnexedFiles
 		hookWrite postCheckoutHook
 		hookWrite postMergeHook
 	AdjustedBranch.checkAdjustedClone >>= \case
diff --git a/Annex/Wanted.hs b/Annex/Wanted.hs
index 021fe5cafb..cb2a8e3401 100644
--- a/Annex/Wanted.hs
+++ b/Annex/Wanted.hs
@@ -1,6 +1,6 @@
 {- git-annex checking whether content is wanted
  -
- - Copyright 2012 Joey Hess <id@joeyh.name>
+ - Copyright 2012-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -10,6 +10,10 @@ module Annex.Wanted where
 import Annex.Common
 import Logs.PreferredContent
 import Annex.UUID
+import Annex.CatFile
+import Git.FilePath
+import qualified Database.Keys
+import Types.FileMatcher
 
 import qualified Data.Set as S
 
@@ -17,13 +21,55 @@ import qualified Data.Set as S
 wantGet :: Bool -> Maybe Key -> AssociatedFile -> Annex Bool
 wantGet d key file = isPreferredContent Nothing S.empty key file d
 
-{- Check if a file is preferred content for a remote. -}
+{- Check if a file is preferred content for a repository. -}
 wantSend :: Bool -> Maybe Key -> AssociatedFile -> UUID -> Annex Bool
 wantSend d key file to = isPreferredContent (Just to) S.empty key file d
 
-{- Check if a file can be dropped, maybe from a remote.
- - Don't drop files that are preferred content. -}
-wantDrop :: Bool -> Maybe UUID -> Maybe Key -> AssociatedFile -> Annex Bool
-wantDrop d from key file = do
-	u <- maybe getUUID (return . id) from
-	not <$> isPreferredContent (Just u) (S.singleton u) key file d
+{- Check if a file is not preferred or required content, and can be
+ - dropped. When a UUID is provided, checks for that repository.
+ -
+ - The AssociatedFile is the one that the user requested to drop.
+ - There may be other files that use the same key, and preferred content
+ - may match some of those and not others. If any are preferred content,
+ - that will prevent dropping. When the other associated files are known,
+ - they can be provided, otherwise this looks them up.
+ -}
+wantDrop :: Bool -> Maybe UUID -> Maybe Key -> AssociatedFile -> (Maybe [AssociatedFile]) -> Annex Bool
+wantDrop d from key file others =
+	isNothing <$> checkDrop isPreferredContent d from key file others
+
+{- Generalization of wantDrop that can also be used with isRequiredContent.
+ -
+ - When the content should not be dropped, returns Just the file that
+ - the checker matches.
+ -}
+checkDrop :: (Maybe UUID -> AssumeNotPresent -> Maybe Key -> AssociatedFile -> Bool -> Annex Bool) -> Bool -> Maybe UUID -> Maybe Key -> AssociatedFile -> (Maybe [AssociatedFile]) -> Annex (Maybe AssociatedFile)
+checkDrop checker d from key file others = do
+	u <- maybe getUUID (pure . id) from
+	let s = S.singleton u
+	let checker' f = checker (Just u) s key f d
+	ifM (checker' file)
+		( return (Just file)
+		, do
+			others' <- case others of
+				Just afs -> pure (filter (/= file) afs)
+				Nothing -> case key of
+					Just k ->
+						mapM (\f -> AssociatedFile . Just <$> fromRepo (fromTopFilePath f))
+							=<< Database.Keys.getAssociatedFiles k
+					Nothing -> pure []
+			l <- filterM checker' others'
+			if null l
+				then return Nothing
+				else checkassociated l
+		)
+  where
+	-- Some associated files that are in the keys database may no
+	-- longer correspond to files in the repository, and should
+	-- not prevent dropping.
+	checkassociated [] = return Nothing
+	checkassociated (af@(AssociatedFile (Just f)):fs) =
+		catKeyFile f >>= \case
+			Just k | Just k == key -> return (Just af)
+			_ -> checkassociated fs
+	checkassociated (AssociatedFile Nothing:fs) = checkassociated fs
diff --git a/Annex/WorkTree.hs b/Annex/WorkTree.hs
index 42abde34aa..226a00d1df 100644
--- a/Annex/WorkTree.hs
+++ b/Annex/WorkTree.hs
@@ -66,19 +66,19 @@ whenAnnexed a file = ifAnnexed file (a file) (return Nothing)
 ifAnnexed :: RawFilePath -> (Key -> Annex a) -> Annex a -> Annex a
 ifAnnexed file yes no = maybe no yes =<< lookupKey file
 
-{- Find all unlocked files and update the keys database for them. 
+{- Find all annexed files and update the keys database for them.
  - 
  - This is expensive, and so normally the associated files are updated
  - incrementally when changes are noticed. So, this only needs to be done
- - when initializing/upgrading repository.
+ - when initializing/upgrading a repository.
  -
- - Also, the content for the unlocked file may already be present as
+ - Also, the content for an unlocked file may already be present as
  - an annex object. If so, populate the pointer file with it.
  - But if worktree file does not have a pointer file's content, it is left
  - as-is.
  -}
-scanUnlockedFiles :: Annex ()
-scanUnlockedFiles = whenM (inRepo Git.Ref.headExists <&&> not <$> isBareRepo) $ do
+scanAnnexedFiles :: Annex ()
+scanAnnexedFiles = whenM (inRepo Git.Ref.headExists <&&> not <$> isBareRepo) $ do
 	dropold <- liftIO $ newMVar $ 
 		Database.Keys.runWriter $
 			liftIO . Database.Keys.SQL.dropAllAssociatedFiles
@@ -87,9 +87,10 @@ scanUnlockedFiles = whenM (inRepo Git.Ref.headExists <&&> not <$> isBareRepo) $
 		(Git.LsTree.LsTreeLong False)
 		Git.Ref.headRef
 	forM_ l $ \i -> 
-		when (isregfile i) $
-			maybe noop (add dropold i)
-				=<< catKey (Git.LsTree.sha i)
+		maybe noop (add dropold i)
+			=<< catKey'
+				(Git.LsTree.sha i)
+				(fromMaybe 0 (Git.LsTree.size i))				
 	liftIO $ void cleanup
   where
 	isregfile i = case Git.Types.toTreeItemType (Git.LsTree.mode i) of
@@ -101,7 +102,7 @@ scanUnlockedFiles = whenM (inRepo Git.Ref.headExists <&&> not <$> isBareRepo) $
 		let tf = Git.LsTree.file i
 		Database.Keys.runWriter $
 			liftIO . Database.Keys.SQL.addAssociatedFileFast k tf
-		whenM (inAnnex k) $ do
+		whenM (pure (isregfile i) <&&> inAnnex k) $ do
 			f <- fromRepo $ fromTopFilePath tf
 			liftIO (isPointerFile f) >>= \case
 				Just k' | k' == k -> do
diff --git a/CHANGELOG b/CHANGELOG
index b00b6ae1a6..1e5b360e07 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,16 @@
 git-annex (8.20210429) UNRELEASED; urgency=medium
 
+  * When two files have the same content, and a required content expression
+    matches one but not the other, dropping the latter file will fail as it
+    would also remove the content of the required file.
+  * drop --auto: When two files have the same content, and a preferred content
+    expression matches one but not the other, do not drop the content.
+  * sync --content, assistant: When two unlocked files have the same
+    content, and a preferred content expression matches one but not the
+    other, do not drop the content. (This was already the case for locked
+    files.)
+  * sync --content, assistant: Fix an edge case where a file that is not
+    preferred content did not get dropped.
   * filter-branch: New command, useful to produce a filtered version of the
     git-annex branch, eg when splitting a repository.
   * fromkey: Create an unlocked file when used in an adjusted branch
diff --git a/Command/Drop.hs b/Command/Drop.hs
index 26a16fdd37..f30b6f4c08 100644
--- a/Command/Drop.hs
+++ b/Command/Drop.hs
@@ -21,8 +21,6 @@ import Annex.Content
 import Annex.Wanted
 import Annex.Notification
 
-import qualified Data.Set as S
-
 cmd :: Command
 cmd = withGlobalOptions [jobsOption, jsonOptions, annexedMatchingOptions] $
 	command "drop" SectionCommon
@@ -86,33 +84,34 @@ start o from si file key = start' o from key afile ai si
 start' :: DropOptions -> Maybe Remote -> Key -> AssociatedFile -> ActionItem -> SeekInput -> CommandStart
 start' o from key afile ai si = 
 	checkDropAuto (autoMode o) from afile key $ \numcopies mincopies ->
-		stopUnless want $
+		stopUnless wantdrop $
 			case from of
-				Nothing -> startLocal afile ai si numcopies mincopies key []
-				Just remote -> startRemote afile ai si numcopies mincopies key remote
+				Nothing -> startLocal pcc afile ai si numcopies mincopies key []
+				Just remote -> startRemote pcc afile ai si numcopies mincopies key remote
   where
-	want
-		| autoMode o = wantDrop False (Remote.uuid <$> from) (Just key) afile
+	wantdrop
+		| autoMode o = wantDrop False (Remote.uuid <$> from) (Just key) afile Nothing
 		| otherwise = return True
+	pcc = PreferredContentChecked (autoMode o)
 
 startKeys :: DropOptions -> Maybe Remote -> (SeekInput, Key, ActionItem) -> CommandStart
 startKeys o from (si, key, ai) = start' o from key (AssociatedFile Nothing) ai si
 
-startLocal :: AssociatedFile -> ActionItem -> SeekInput -> NumCopies -> MinCopies -> Key -> [VerifiedCopy] -> CommandStart
-startLocal afile ai si numcopies mincopies key preverified =
+startLocal :: PreferredContentChecked -> AssociatedFile -> ActionItem -> SeekInput -> NumCopies -> MinCopies -> Key -> [VerifiedCopy] -> CommandStart
+startLocal pcc afile ai si numcopies mincopies key preverified =
 	starting "drop" (OnlyActionOn key ai) si $
-		performLocal key afile numcopies mincopies preverified
+		performLocal pcc key afile numcopies mincopies preverified
 
-startRemote :: AssociatedFile -> ActionItem -> SeekInput -> NumCopies -> MinCopies -> Key -> Remote -> CommandStart
-startRemote afile ai si numcopies mincopies key remote = 
+startRemote :: PreferredContentChecked -> AssociatedFile -> ActionItem -> SeekInput -> NumCopies -> MinCopies -> Key -> Remote -> CommandStart
+startRemote pcc afile ai si numcopies mincopies key remote = 
 	starting ("drop " ++ Remote.name remote) (OnlyActionOn key ai) si $
-		performRemote key afile numcopies mincopies remote
+		performRemote pcc key afile numcopies mincopies remote
 
-performLocal :: Key -> AssociatedFile -> NumCopies -> MinCopies -> [VerifiedCopy] -> CommandPerform
-performLocal key afile numcopies mincopies preverified = lockContentForRemoval key fallback $ \contentlock -> do
+performLocal :: PreferredContentChecked -> Key -> AssociatedFile -> NumCopies -> MinCopies -> [VerifiedCopy] -> CommandPerform
+performLocal pcc key afile numcopies mincopies preverified = lockContentForRemoval key fallback $ \contentlock -> do
 	u <- getUUID
 	(tocheck, verified) <- verifiableCopies key [u]
-	doDrop u (Just contentlock) key afile numcopies mincopies [] (preverified ++ verified) tocheck
+	doDrop pcc u (Just contentlock) key afile numcopies mincopies [] (preverified ++ verified) tocheck
 		( \proof -> do
 			fastDebug "Command.Drop" $ unwords
 				[ "Dropping from here"
@@ -134,12 +133,12 @@ performLocal key afile numcopies mincopies preverified = lockContentForRemoval k
 	-- to be done except for cleaning up.
 	fallback = next $ cleanupLocal key
 
-performRemote :: Key -> AssociatedFile -> NumCopies -> MinCopies -> Remote -> CommandPerform
-performRemote key afile numcopies mincopies remote = do
+performRemote :: PreferredContentChecked -> Key -> AssociatedFile -> NumCopies -> MinCopies -> Remote -> CommandPerform
+performRemote pcc key afile numcopies mincopies remote = do
 	-- Filter the uuid it's being dropped from out of the lists of
 	-- places assumed to have the key, and places to check.
 	(tocheck, verified) <- verifiableCopies key [uuid]
-	doDrop uuid Nothing key afile numcopies mincopies [uuid] verified tocheck
+	doDrop pcc uuid Nothing key afile numcopies mincopies [uuid] verified tocheck
 		( \proof -> do 
 			fastDebug "Command.Drop" $ unwords
 				[ "Dropping from remote"
@@ -169,12 +168,11 @@ cleanupRemote key remote ok = do
  - verify that enough copies of a key exist to allow it to be
  - safely removed (with no data loss).
  -
- - Also checks if it's required content, and refuses to drop if so.
- -
  - --force overrides and always allows dropping.
  -}
 doDrop
-	:: UUID
+	:: PreferredContentChecked
+	-> UUID
 	-> Maybe ContentRemovalLock
 	-> Key
 	-> AssociatedFile
@@ -185,10 +183,10 @@ doDrop
 	-> [UnVerifiedCopy]
 	-> (Maybe SafeDropProof -> CommandPerform, CommandPerform)
 	-> CommandPerform
-doDrop dropfrom contentlock key afile numcopies mincopies skip preverified check (dropaction, nodropaction) = 
+doDrop pcc dropfrom contentlock key afile numcopies mincopies skip preverified check (dropaction, nodropaction) = 
 	ifM (Annex.getState Annex.force)
 		( dropaction Nothing
-		, ifM (checkRequiredContent dropfrom key afile)
+		, ifM (checkRequiredContent pcc dropfrom key afile)
 			( verifyEnoughCopiesToDrop nolocmsg key 
 				contentlock numcopies mincopies
 				skip preverified check
@@ -203,18 +201,27 @@ doDrop dropfrom contentlock key afile numcopies mincopies skip preverified check
 		showLongNote "(Use --force to override this check, or adjust numcopies.)"
 		a
 
-checkRequiredContent :: UUID -> Key -> AssociatedFile -> Annex Bool
-checkRequiredContent u k afile =
-	ifM (isRequiredContent (Just u) S.empty (Just k) afile False)
-		( requiredContent
-		, return True
-		)
+{- Checking preferred content also checks required content, so when
+ - auto mode causes preferred content to be checked, it's redundant
+ - for checkRequiredContent to separately check required content, and
+ - providing this avoids that extra work. -}
+newtype PreferredContentChecked = PreferredContentChecked Bool
 
-requiredContent :: Annex Bool
-requiredContent = do
-	showLongNote "That file is required content, it cannot be dropped!"
-	showLongNote "(Use --force to override this check, or adjust required content configuration.)"
-	return False
+checkRequiredContent :: PreferredContentChecked -> UUID -> Key -> AssociatedFile -> Annex Bool
+checkRequiredContent (PreferredContentChecked True) _ _ _ = return True
+checkRequiredContent (PreferredContentChecked False) u k afile =
+	checkDrop isRequiredContent False (Just u) (Just k) afile Nothing >>= \case
+		Nothing -> return True
+		Just afile' -> do
+			if afile == afile'
+				then showLongNote "That file is required content. It cannot be dropped!"
+				else showLongNote $ "That file has the same content as another file"
+					++ case afile' of
+						AssociatedFile (Just f) -> " (" ++ fromRawFilePath f ++ "),"
+						AssociatedFile Nothing -> ""
+					++ " which is required content. It cannot be dropped!"
+			showLongNote "(Use --force to override this check, or adjust required content configuration.)"
+			return False
 
 {- In auto mode, only runs the action if there are enough
  - copies on other semitrusted repositories. -}
diff --git a/Command/DropUnused.hs b/Command/DropUnused.hs
index 6c7ca34d41..0b81a9e467 100644
--- a/Command/DropUnused.hs
+++ b/Command/DropUnused.hs
@@ -49,7 +49,7 @@ perform :: Maybe Remote -> NumCopies -> MinCopies -> Key -> CommandPerform
 perform from numcopies mincopies key = case from of
 	Just r -> do
 		showAction $ "from " ++ Remote.name r
-		Command.Drop.performRemote key (AssociatedFile Nothing) numcopies mincopies r
+		Command.Drop.performRemote pcc key (AssociatedFile Nothing) numcopies mincopies r
 	Nothing -> ifM (inAnnex key)
 		( droplocal
 		, ifM (objectFileExists key)
@@ -63,7 +63,8 @@ perform from numcopies mincopies key = case from of
 			)
 		)
   where
-	droplocal = Command.Drop.performLocal key (AssociatedFile Nothing) numcopies mincopies []
+	droplocal = Command.Drop.performLocal pcc key (AssociatedFile Nothing) numcopies mincopies []
+	pcc = Command.Drop.PreferredContentChecked False
 
 performOther :: (Key -> Git.Repo -> RawFilePath) -> Key -> CommandPerform
 performOther filespec key = do
diff --git a/Command/Lock.hs b/Command/Lock.hs
index e7af74ca9f..3d085393a8 100644
--- a/Command/Lock.hs
+++ b/Command/Lock.hs
@@ -62,7 +62,7 @@ perform file key = do
 	lockdown =<< calcRepo (gitAnnexLocation key)
 	addLink (CheckGitIgnore False) file key
 		=<< withTSDelta (liftIO . genInodeCache file)
-	next $ cleanup file key
+	next $ return True
   where
 	lockdown obj = do
 		ifM (isUnmodified key obj)
@@ -97,10 +97,5 @@ perform file key = do
 
 	lostcontent = logStatus key InfoMissing
 
-cleanup :: RawFilePath -> Key -> CommandCleanup
-cleanup file key = do
-	Database.Keys.removeAssociatedFile key =<< inRepo (toTopFilePath file)
-	return True
-
 errorModified :: a
 errorModified =  giveup "Locking this file would discard any changes you have made to it. Use 'git annex add' to stage your changes. (Or, use --force to override)"
diff --git a/Command/Migrate.hs b/Command/Migrate.hs
index 669be7f56c..9a0d69f35a 100644
--- a/Command/Migrate.hs
+++ b/Command/Migrate.hs
@@ -86,7 +86,7 @@ perform file oldkey oldbackend newbackend = go =<< genkey (fastMigrate oldbacken
 			urls <- getUrls oldkey
 			forM_ urls $ \url ->
 				setUrlPresent newkey url
-			next $ Command.ReKey.cleanup file oldkey newkey
+			next $ Command.ReKey.cleanup file newkey
 		, giveup "failed creating link from old to new key"
 		)
 	genkey Nothing = do
diff --git a/Command/Mirror.hs b/Command/Mirror.hs
index 61c9a1f888..b5f49b2373 100644
--- a/Command/Mirror.hs
+++ b/Command/Mirror.hs
@@ -69,7 +69,7 @@ startKey o afile (si, key, ai) = case fromToOptions o of
 		( Command.Move.toStart Command.Move.RemoveNever afile key ai si =<< getParsed r
 		, do
 			(numcopies, mincopies) <- getnummincopies
-			Command.Drop.startRemote afile ai si numcopies mincopies key =<< getParsed r
+			Command.Drop.startRemote pcc afile ai si numcopies mincopies key =<< getParsed r
 		)
 	FromRemote r -> checkFailedTransferDirection ai Download $ do
 		haskey <- flip Remote.hasKey key =<< getParsed r
@@ -82,10 +82,11 @@ startKey o afile (si, key, ai) = case fromToOptions o of
 			Right False -> ifM (inAnnex key)
 				( do
 					(numcopies, mincopies) <- getnummincopies
-					Command.Drop.startLocal afile ai si numcopies mincopies key []
+					Command.Drop.startLocal pcc afile ai si numcopies mincopies key []
 				, stop
 				)
   where
 	getnummincopies = case afile of
 		AssociatedFile Nothing -> (,) <$> getNumCopies <*> getMinCopies
 		AssociatedFile (Just af) -> getFileNumMinCopies af
+	pcc = Command.Drop.PreferredContentChecked False
diff --git a/Command/Move.hs b/Command/Move.hs
index fcb7390a6a..6d2cc50c30 100644
--- a/Command/Move.hs
+++ b/Command/Move.hs
@@ -293,7 +293,7 @@ toHereStart removewhen afile key ai si =
  - repository reduces the number of copies, and should fail if
  - that would violate numcopies settings.
  -
- - On the other hand, when the destiation repository does not already
+ - On the other hand, when the destination repository does not already
  - have a copy of a file, it can be dropped without making numcopies
  - worse, so the move is allowed even if numcopies is not met.
  -
@@ -311,7 +311,7 @@ toHereStart removewhen afile key ai si =
  -}
 willDropMakeItWorse :: UUID -> UUID -> DestStartedWithCopy -> Key -> AssociatedFile -> Annex DropCheck
 willDropMakeItWorse srcuuid destuuid (DestStartedWithCopy deststartedwithcopy) key afile =
-	ifM (Command.Drop.checkRequiredContent srcuuid key afile)
+	ifM (Command.Drop.checkRequiredContent (Command.Drop.PreferredContentChecked False) srcuuid key afile)
 		( if deststartedwithcopy
 			then unlessforced DropCheckNumCopies
 			else ifM checktrustlevel
diff --git a/Command/ReKey.hs b/Command/ReKey.hs
index 368fb42ef2..077dd5a628 100644
--- a/Command/ReKey.hs
+++ b/Command/ReKey.hs
@@ -15,8 +15,6 @@ import Annex.Link
 import Annex.Perms
 import Annex.ReplaceFile
 import Logs.Location
-import Git.FilePath
-import qualified Database.Keys
 import Annex.InodeSentinal
 import Utility.InodeCache
 import qualified Utility.RawFilePath as R
@@ -79,7 +77,7 @@ perform file oldkey newkey = do
 		, unlessM (Annex.getState Annex.force) $
 			giveup $ fromRawFilePath file ++ " is not available (use --force to override)"
 		)
-	next $ cleanup file oldkey newkey
+	next $ cleanup file newkey
 
 {- Make a hard link to the old key content (when supported),
  - to avoid wasting disk space. -}
@@ -119,8 +117,8 @@ linkKey file oldkey newkey = ifM (isJust <$> isAnnexLink file)
 					LinkAnnexNoop -> True
 	)
 
-cleanup :: RawFilePath -> Key -> Key -> CommandCleanup
-cleanup file oldkey newkey = do
+cleanup :: RawFilePath -> Key -> CommandCleanup
+cleanup file newkey = do
 	ifM (isJust <$> isAnnexLink file)
 		( do
 			-- Update symlink to use the new key.
@@ -131,8 +129,6 @@ cleanup file oldkey newkey = do
 			liftIO $ whenM (isJust <$> isPointerFile file) $
 				writePointerFile file newkey mode
 			stagePointerFile file mode =<< hashPointerFile newkey
-			Database.Keys.removeAssociatedFile oldkey 
-				=<< inRepo (toTopFilePath file)
 		)
 	whenM (inAnnex newkey) $
 		logStatus newkey InfoPresent
diff --git a/Database/Keys.hs b/Database/Keys.hs
index c050002617..842b53f1d1 100644
--- a/Database/Keys.hs
+++ b/Database/Keys.hs
@@ -1,6 +1,6 @@
 {- Sqlite database of information about Keys
  -
- - Copyright 2015-2019 Joey Hess <id@joeyh.name>
+ - Copyright 2015-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -44,6 +44,9 @@ import Git.FilePath
 import Git.Command
 import Git.Types
 import Git.Index
+import Git.Sha
+import Git.Branch (writeTreeQuiet, update')
+import qualified Git.Ref
 import Config.Smudge
 import qualified Utility.RawFilePath as R
 
@@ -52,10 +55,6 @@ import qualified Data.ByteString.Char8 as S8
 import qualified System.FilePath.ByteString as P
 
 {- Runs an action that reads from the database.
- -
- - If the database doesn't already exist, it's not created; mempty is
- - returned instead. This way, when the keys database is not in use,
- - there's minimal overhead in checking it.
  -
  - If the database is already open, any writes are flushed to it, to ensure
  - consistency.
@@ -73,7 +72,7 @@ runReader a = do
 		v <- a (SQL.ReadHandle qh)
 		return (v, st)
 	go DbClosed = do
-		st' <- openDb False DbClosed
+		st' <- openDb True DbClosed
 		v <- case st' of
 			(DbOpen qh) -> a (SQL.ReadHandle qh)
 			_ -> return mempty
@@ -95,7 +94,7 @@ runWriter a = do
 		v <- a (SQL.WriteHandle qh)
 		return (v, st)
 	go st = do
-		st' <- openDb True st
+		st' <- openDb False st
 		v <- case st' of
 			DbOpen qh -> a (SQL.WriteHandle qh)
 			_ -> error "internal"
@@ -104,7 +103,7 @@ runWriter a = do
 runWriterIO :: (SQL.WriteHandle -> IO ()) -> Annex ()
 runWriterIO a = runWriter (liftIO . a)
 
-{- Opens the database, perhaps creating it if it doesn't exist yet.
+{- Opens the database, creating it if it doesn't exist yet.
  -
  - Multiple readers and writers can have the database open at the same
  - time. Database.Handle deals with the concurrency issues.
@@ -115,22 +114,21 @@ runWriterIO a = runWriter (liftIO . a)
 openDb :: Bool -> DbState -> Annex DbState
 openDb _ st@(DbOpen _) = return st
 openDb False DbUnavailable = return DbUnavailable
-openDb createdb _ = catchPermissionDenied permerr $ withExclusiveLock gitAnnexKeysDbLock $ do
+openDb forwrite _ = catchPermissionDenied permerr $ withExclusiveLock gitAnnexKeysDbLock $ do
 	dbdir <- fromRepo gitAnnexKeysDb
 	let db = dbdir P.</> "db"
 	dbexists <- liftIO $ R.doesPathExist db
-	case (dbexists, createdb) of
-		(True, _) -> open db
-		(False, True) -> do
+	case dbexists of
+		True -> open db
+		False -> do
 			initDb db SQL.createTables
 			open db
-		(False, False) -> return DbUnavailable
   where
-	-- If permissions don't allow opening the database, treat it as if
-	-- it does not exist.
-	permerr e = case createdb of
-		False -> return DbUnavailable
-		True -> throwM e
+	-- If permissions don't allow opening the database, and it's being
+	-- opened for read, treat it as if it does not exist.
+	permerr e
+		| forwrite = throwM e
+		| otherwise = return DbUnavailable
 	
 	open db = do
 		qh <- liftIO $ H.openDbQueue H.MultiWriter db SQL.containedTable
@@ -191,20 +189,17 @@ removeInodeCache = runWriterIO . SQL.removeInodeCache
 isInodeKnown :: InodeCache -> SentinalStatus -> Annex Bool
 isInodeKnown i s = or <$> runReaderIO ((:[]) <$$> SQL.isInodeKnown i s)
 
-{- Looks at staged changes to find when unlocked files are copied/moved,
- - and updates associated files in the keys database.
+{- Looks at staged changes to annexed files, and updates the keys database,
+ - so that its information is consistent with the state of the repository.
  -
- - Since staged changes can be dropped later, does not remove any
- - associated files; only adds new associated files.
- -
- - This needs to be run before querying the keys database so that
- - information is consistent with the state of the repository.
+ - This is run with a lock held, so only one process can be running this at
+ - a time.
  -
  - To avoid unncessary work, the index file is statted, and if it's not
  - changed since last time this was run, nothing is done.
  -
- - Note that this is run with a lock held, so only one process can be
- - running this at a time.
+ - A tree is generated from the index, and the diff between that tree
+ - and the last processed tree is examined for changes.
  -
  - This also cleans up after a race between eg a git mv and git-annex
  - get/drop/similar. If git moves the file between this being run and the
@@ -218,34 +213,74 @@ isInodeKnown i s = or <$> runReaderIO ((:[]) <$$> SQL.isInodeKnown i s)
  - filter. If a drop missed the file then the file is added back into the
  - annex. If a get missed the file then the clean filter populates the
  - file.
+ -
+ - There is a situation where, after this has run, the database can still
+ - contain associated files that have been deleted from the index.
+ - That happens when addAssociatedFile is used to record a newly
+ - added file, but that file then gets removed from the index before
+ - this is run. Eg, "git-annex add foo; git rm foo"
+ - So when using getAssociatedFiles, have to make sure the file still
+ - is an associated file.
  -}
 reconcileStaged :: H.DbQueue -> Annex ()
 reconcileStaged qh = do
 	gitindex <- inRepo currentIndexFile
 	indexcache <- fromRawFilePath <$> fromRepo gitAnnexKeysDbIndexCache
 	withTSDelta (liftIO . genInodeCache gitindex) >>= \case
-		Just cur -> 
-			liftIO (maybe Nothing readInodeCache <$> catchMaybeIO (readFile indexcache)) >>= \case
-				Nothing -> go cur indexcache
-				Just prev -> ifM (compareInodeCaches prev cur)
-					( noop
-					, go cur indexcache
-					)
+		Just cur -> readindexcache indexcache >>= \case
+			Nothing -> go cur indexcache =<< getindextree
+			Just prev -> ifM (compareInodeCaches prev cur)
+				( noop
+				, go cur indexcache =<< getindextree
+				)
 		Nothing -> noop
   where
-	go cur indexcache = do
-		(l, cleanup) <- inRepo $ pipeNullSplit' diff
+	lastindexref = Ref "refs/annex/last-index"
+
+	readindexcache indexcache = liftIO $ maybe Nothing readInodeCache
+		<$> catchMaybeIO (readFile indexcache)
+
+	getoldtree = fromMaybe emptyTree <$> inRepo (Git.Ref.sha lastindexref)
+
+	go cur indexcache (Just newtree) = do
+		oldtree <- getoldtree
+		when (oldtree /= newtree) $ do
+			updatetodiff (fromRef oldtree) (fromRef newtree)
+			liftIO $ writeFile indexcache $ showInodeCache cur
+			-- Storing the tree in a ref makes sure it does not
+			-- get garbage collected, and is available to diff
+			-- against next time.
+			inRepo $ update' lastindexref newtree
+	-- git write-tree will fail if the index is locked or when there is
+	-- a merge conflict. To get up-to-date with the current index, 
+	-- diff --cached with the old index tree. The current index tree
+	-- is not known, so not recorded, and the inode cache is not updated,
+	-- so the next time git-annex runs, it will diff again, even
+	-- if the index is unchanged.
+	go _ _ Nothing = do
+		oldtree <- getoldtree
+		updatetodiff (fromRef oldtree) "--cached"
+		
+	updatetodiff old new = do
+		(l, cleanup) <- inRepo $ pipeNullSplit' $ diff old new
 		changed <- procdiff l False
 		void $ liftIO cleanup
 		-- Flush database changes immediately
 		-- so other processes can see them.
 		when changed $
 			liftIO $ H.flushDbQueue qh
-		liftIO $ writeFile indexcache $ showInodeCache cur
 	
-	diff =
-		-- Avoid running smudge or clean filters, since we want the
-		-- raw output, and they would block trying to access the
+	-- Avoid running smudge clean filter, which would block trying to
+	-- access the locked database. git write-tree sometimes calls it,
+	-- even though it is not adding work tree files to the index,
+	-- and so the filter cannot have an effect on the contents of the
+	-- index or on the tree that gets written from it.
+	getindextree = inRepo $ \r -> writeTreeQuiet $ r
+		{ gitGlobalOpts = gitGlobalOpts r ++ bypassSmudgeConfig }
+	
+	diff old new =
+		-- Avoid running smudge clean filter, since we want the
+		-- raw output, and it would block trying to access the
 		-- locked database. The --raw normally avoids git diff
 		-- running them, but older versions of git need this.
 		bypassSmudgeConfig ++
@@ -253,20 +288,18 @@ reconcileStaged qh = do
 		-- (The -G option may make it be used otherwise.)
 		[ Param "-c", Param "diff.external="
 		, Param "diff"
-		, Param "--cached"
+		, Param old
+		, Param new
 		, Param "--raw"
 		, Param "-z"
 		, Param "--no-abbrev"
-		-- Optimization: Only find pointer files. This is not
-		-- perfect. A file could start with this and not be a
-		-- pointer file. And a pointer file that is replaced with
-		-- a non-pointer file will match this.
-		, Param $ "-G^" ++ fromRawFilePath (toInternalGitPath $
+		-- Optimization: Limit to pointer files and annex symlinks.
+		-- This is not perfect. A file could contain with this and not
+		-- be a pointer file. And a pointer file that is replaced with
+		-- a non-pointer file will match this. This is only a
+		-- prefilter so that's ok.
+		, Param $ "-G" ++ fromRawFilePath (toInternalGitPath $
 			P.pathSeparator `S.cons` objectDir')
-		-- Don't include files that were deleted, because this only
-		-- wants to update information for files that are present
-		-- in the index.
-		, Param "--diff-filter=AMUT"
 		-- Disable rename detection.
 		, Param "--no-renames"
 		-- Avoid other complications.
@@ -276,20 +309,28 @@ reconcileStaged qh = do
 	
 	procdiff (info:file:rest) changed
 		| ":" `S.isPrefixOf` info = case S8.words info of
-			(_colonsrcmode:dstmode:_srcsha:dstsha:_change:[])
-				-- Only want files, not symlinks
-				| dstmode /= fmtTreeItemType TreeSymlink -> do
-					maybe noop (reconcile (asTopFilePath file))
-						=<< catKey (Ref dstsha)
-					procdiff rest True
-				| otherwise -> procdiff rest changed
+			(_colonsrcmode:dstmode:srcsha:dstsha:_change:[]) -> do
+				removed <- catKey (Ref srcsha) >>= \case
+					Just oldkey -> do
+						liftIO $ SQL.removeAssociatedFile oldkey
+							(asTopFilePath file)
+							(SQL.WriteHandle qh)
+						return True
+					Nothing -> return False
+				added <- catKey (Ref dstsha) >>= \case
+					Just key -> do
+						liftIO $ SQL.addAssociatedFile key
+							(asTopFilePath file)
+							(SQL.WriteHandle qh)
+						when (dstmode /= fmtTreeItemType TreeSymlink) $
+							reconcilerace (asTopFilePath file) key
+						return True
+					Nothing -> return False
+				procdiff rest (changed || removed || added)
 			_ -> return changed -- parse failed
 	procdiff _ changed = return changed
 
-	-- Note that database writes done in here will not necessarily 
-	-- be visible to database reads also done in here.
-	reconcile file key = do
-		liftIO $ SQL.addAssociatedFileFast key file (SQL.WriteHandle qh)
+	reconcilerace file key = do
 		caches <- liftIO $ SQL.getInodeCaches key (SQL.ReadHandle qh)
 		keyloc <- calcRepo (gitAnnexLocation key)
 		keypopulated <- sameInodeCache keyloc caches
diff --git a/Git/Branch.hs b/Git/Branch.hs
index 70faca335f..54af1018ba 100644
--- a/Git/Branch.hs
+++ b/Git/Branch.hs
@@ -185,8 +185,18 @@ commitAlways :: CommitMode -> String -> Branch -> [Ref] -> Repo -> IO Sha
 commitAlways commitmode message branch parentrefs repo = fromJust
 	<$> commit commitmode True message branch parentrefs repo
 
+-- Throws exception if the index is locked, with an error message output by
+-- git on stderr.
 writeTree :: Repo -> IO Sha
-writeTree repo = getSha "write-tree" $ pipeReadStrict [Param "write-tree"] repo
+writeTree repo = getSha "write-tree" $
+	pipeReadStrict [Param "write-tree"] repo
+
+-- Avoids error output if the command fails due to eg, the index being locked.
+writeTreeQuiet :: Repo -> IO (Maybe Sha)
+writeTreeQuiet repo = extractSha <$> withNullHandle go
+  where
+	go nullh = pipeReadStrict' (\p -> p { std_err = UseHandle nullh }) 
+		[Param "write-tree"] repo
 
 commitTree :: CommitMode -> String -> [Ref] -> Ref -> Repo -> IO Sha
 commitTree commitmode message parentrefs tree repo =
diff --git a/Git/Command.hs b/Git/Command.hs
index fef7eb91ad..2358b17361 100644
--- a/Git/Command.hs
+++ b/Git/Command.hs
@@ -70,17 +70,15 @@ pipeReadLazy params repo = assertLocal repo $ do
  - Nonzero exit status is ignored.
  -}
 pipeReadStrict :: [CommandParam] -> Repo -> IO S.ByteString
-pipeReadStrict = pipeReadStrict' S.hGetContents
+pipeReadStrict = pipeReadStrict' id
 
-{- The reader action must be strict. -}
-pipeReadStrict' :: (Handle -> IO a) -> [CommandParam] -> Repo -> IO a
-pipeReadStrict' reader params repo = assertLocal repo $ withCreateProcess p go
+pipeReadStrict' :: (CreateProcess -> CreateProcess) -> [CommandParam] -> Repo -> IO S.ByteString
+pipeReadStrict' fp params repo = assertLocal repo $ withCreateProcess p go
   where
-	p  = (gitCreateProcess params repo)
-		{ std_out = CreatePipe }
+	p = fp (gitCreateProcess params repo) { std_out = CreatePipe }
 
 	go _ (Just outh) _ pid = do
-		output <- reader outh
+		output <- S.hGetContents outh
 		hClose outh
 		void $ waitForProcess pid
 		return output
diff --git a/Limit/Wanted.hs b/Limit/Wanted.hs
index 5518fa0bc9..9188594b90 100644
--- a/Limit/Wanted.hs
+++ b/Limit/Wanted.hs
@@ -19,7 +19,7 @@ addWantGet = addPreferredContentLimit $
 
 addWantDrop :: Annex ()
 addWantDrop = addPreferredContentLimit $
-	checkWant $ wantDrop False Nothing Nothing
+	checkWant $ \af -> wantDrop False Nothing Nothing af (Just [])
 
 addPreferredContentLimit :: (MatchInfo -> Annex Bool) -> Annex ()
 addPreferredContentLimit a = do
diff --git a/Logs/PreferredContent.hs b/Logs/PreferredContent.hs
index adb0189ec3..1391bde4ce 100644
--- a/Logs/PreferredContent.hs
+++ b/Logs/PreferredContent.hs
@@ -46,8 +46,8 @@ import Logs.Remote
 import Types.StandardGroups
 import Limit
 
-{- Checks if a file is preferred content for the specified repository
- - (or the current repository if none is specified). -}
+{- Checks if a file is preferred content (or required content) for the
+ - specified repository (or the current repository if none is specified). -}
 isPreferredContent :: Maybe UUID -> AssumeNotPresent -> Maybe Key -> AssociatedFile -> Bool -> Annex Bool
 isPreferredContent = checkMap preferredContentMap
 
diff --git a/Test.hs b/Test.hs
index 300ec157ff..c5d419074a 100644
--- a/Test.hs
+++ b/Test.hs
@@ -376,6 +376,7 @@ unitTests note = testGroup ("Unit Tests " ++ note)
 	, testCase "bup remote" test_bup_remote
 	, testCase "crypto" test_crypto
 	, testCase "preferred content" test_preferred_content
+	, testCase "required_content" test_required_content
 	, testCase "add subdirs" test_add_subdirs
 	, testCase "addurl" test_addurl
 	]
@@ -749,6 +750,27 @@ test_preferred_content = intmpclonerepo $ do
 	git_annex "get" ["--auto", annexedfile] "get --auto of file with exclude=*"
 	annexed_notpresent annexedfile
 
+test_required_content :: Assertion
+test_required_content = intmpclonerepo $ do
+	git_annex "get" [annexedfile] "get"
+	annexed_present annexedfile
+	git_annex "required" [".", "include=" ++ annexedfile] "annexedfile required"
+
+	git_annex_shouldfail "drop" [annexedfile] "drop of required content should fail"
+	annexed_present annexedfile
+
+	git_annex "drop" ["--auto", annexedfile] "drop --auto of required content skips it"
+	annexed_present annexedfile
+	
+	writecontent annexedfiledup $ content annexedfiledup
+	add_annex annexedfiledup "add of second file with same content failed"
+	annexed_present annexedfiledup
+	annexed_present annexedfile
+
+	git_annex_shouldfail "drop" [annexedfiledup] "drop of file sharing required content should fail"
+	annexed_present annexedfiledup
+	annexed_present annexedfile
+
 test_lock :: Assertion
 test_lock = intmpclonerepo $ do
 	annexed_notpresent annexedfile
diff --git a/Upgrade/V5.hs b/Upgrade/V5.hs
index bed8a6d801..2db92d57f9 100644
--- a/Upgrade/V5.hs
+++ b/Upgrade/V5.hs
@@ -47,7 +47,7 @@ upgrade automatic = flip catchNonAsync onexception $ do
 		, do
 			checkGitVersionForIndirectUpgrade
 		)
-	scanUnlockedFiles
+	scanAnnexedFiles
 	configureSmudgeFilter
 	-- Inode sentinal file was only used in direct mode and when
 	-- locking down files as they were added. In v6, it's used more
diff --git a/doc/bugs/indeterminite_preferred_content_state_for_duplicated_file.mdwn b/doc/bugs/indeterminite_preferred_content_state_for_duplicated_file.mdwn
index 0ca978dd3b..ee0d5a3930 100644
--- a/doc/bugs/indeterminite_preferred_content_state_for_duplicated_file.mdwn
+++ b/doc/bugs/indeterminite_preferred_content_state_for_duplicated_file.mdwn
@@ -19,3 +19,5 @@ So, this seems solvable in v7 repositories, but not in v5.
 Also, the associated files map may not be accurate at all times, so that's
 a wrinkle to using it for this. Also, only unlocked files get into the
 associated files map. --[[Joey]]
+
+> [[fixed|done]] --[[Joey]]
diff --git a/doc/git-annex-matching-options.mdwn b/doc/git-annex-matching-options.mdwn
index bc8cad5d93..fc7b794f4c 100644
--- a/doc/git-annex-matching-options.mdwn
+++ b/doc/git-annex-matching-options.mdwn
@@ -134,16 +134,17 @@ in either of two repositories.
   Matches files that the preferred content settings for the repository
   make it want to get. Note that this will match even files that are
   already present, unless limited with e.g., `--not --in .`
-  
-  Note that this will not match anything when using --all or --unused.
 
 * `--want-drop`
 
   Matches files that the preferred content settings for the repository
   make it want to drop. Note that this will match even files that have
   already been dropped, unless limited with e.g., `--in .`
-  
-  Note that this will not match anything when using --all or --unused.
+
+  Files that this matches will not necessarily be dropped by
+  `git-annex drop --auto`. This does not check that there are enough copies
+  to drop. Also the same content may be used by a file that is not wanted
+  to be dropped.
 
 * `--accessedwithin=interval`
 
diff --git a/doc/todo/Avoid_lengthy___34__Scanning_for_unlocked_files_...__34__/comment_4_2cb31617bb7003c5bf0e5def358da0e4._comment b/doc/todo/Avoid_lengthy___34__Scanning_for_unlocked_files_...__34__/comment_4_2cb31617bb7003c5bf0e5def358da0e4._comment
index b37477e6e4..b525275114 100644
--- a/doc/todo/Avoid_lengthy___34__Scanning_for_unlocked_files_...__34__/comment_4_2cb31617bb7003c5bf0e5def358da0e4._comment
+++ b/doc/todo/Avoid_lengthy___34__Scanning_for_unlocked_files_...__34__/comment_4_2cb31617bb7003c5bf0e5def358da0e4._comment
@@ -9,7 +9,12 @@ If most of the files are locked, that would actually make the scan
 somewhere around twice as slow as it currently is. So not a worthwhile
 optimisation.
 
-And I don't see much else there that could be optimised. Possibly the
+Update: Now that the scan also scans for locked files to make the
+associated files include information about them, the catKey optimisation
+did make sense. Unfortunately, that does mean this scan got a little bit
+slower still, since it has to use git ls-tree --long.
+
+I don't see much else there that could be optimised. Possibly the
 ls-tree parser could be made faster but it's already using attoparsec
 so unlikely to be many gains.
 """]]