Prefer CoW over hard link

osu.Game/Database/ImportParameters.cs

@@ -17,6 +17,11 @@ public struct ImportParameters
         /// </remarks>
         public bool Batch { get; set; }
 
+        /// <summary>
+        /// Whether this import should use copy on write cloning if available.
+        /// </summary>
+        public bool PreferCopyOnWrite { get; set; }
+
         /// <summary>
         /// Whether this import should use hard links rather than file copy operations if available.
         /// </summary>

osu.Game/Database/LegacyModelImporter.cs

@@ -61,7 +61,7 @@ public Task ImportFromStableAsync(StableStorage stableStorage)
             {
                 var tasks = GetStableImportPaths(storage).Select(p => new ImportTask(p)).ToArray();
 
-                await Importer.Import(tasks, new ImportParameters { Batch = true, PreferHardLinks = true }).ConfigureAwait(false);
+                await Importer.Import(tasks, new ImportParameters { Batch = true, PreferHardLinks = true, PreferCopyOnWrite = true }).ConfigureAwait(false);
             });
         }
 

osu.Game/Database/RealmArchiveModelImporter.cs

@@ -336,7 +336,7 @@ public async Task<ExternalEditOperation<TModel>> BeginExternalEditing(TModel mod
                     foreach (var filenames in getShortenedFilenames(archive))
                     {
                         using (Stream s = archive.GetStream(filenames.original))
-                            files.Add(new RealmNamedFileUsage(Files.Add(s, realm, false, parameters.PreferHardLinks), filenames.shortened));
+                            files.Add(new RealmNamedFileUsage(Files.Add(s, realm, false, parameters.PreferHardLinks, parameters.PreferCopyOnWrite), filenames.shortened));
                     }
                 }
 

osu.Game/Database/RealmFileStore.cs

@@ -41,7 +41,7 @@ public RealmFileStore(RealmAccess realm, Storage storage)
         /// <param name="realm">The realm instance to add to. Should already be in a transaction.</param>
         /// <param name="addToRealm">Whether the <see cref="RealmFile"/> should immediately be added to the underlying realm. If <c>false</c> is provided here, the instance must be manually added.</param>
         /// <param name="preferHardLinks">Whether this import should use hard links rather than file copy operations if available.</param>
-        public RealmFile Add(Stream data, Realm realm, bool addToRealm = true, bool preferHardLinks = false)
+        public RealmFile Add(Stream data, Realm realm, bool addToRealm = true, bool preferHardLinks = false, bool preferCopyOnWrite = false)
         {
             string hash = data.ComputeSHA2Hash();
 
@@ -50,20 +50,24 @@ public RealmFile Add(Stream data, Realm realm, bool addToRealm = true, bool pref
             var file = existing ?? new RealmFile { Hash = hash };
 
             if (!checkFileExistsAndMatchesHash(file))
-                copyToStore(file, data, preferHardLinks);
+                copyToStore(file, data, preferHardLinks, preferCopyOnWrite);
 
             if (addToRealm && !file.IsManaged)
                 realm.Add(file);
 
             return file;
         }
 
-        private void copyToStore(RealmFile file, Stream data, bool preferHardLinks)
+        private void copyToStore(RealmFile file, Stream data, bool preferHardLinks, bool preferCopyOnWrite)
         {
-            if (data is FileStream fs && preferHardLinks)
+            if (data is FileStream fs && preferCopyOnWrite)
             {
+                // Prefer CoW over hard link because it provides better isolation.
+                if (preferCopyOnWrite && HardLinkHelper.TryCreateHardLink(Storage.GetFullPath(file.GetStoragePath(), true), fs.Name))
+                    return;
+
                 // attempt to do a fast hard link rather than copy.
-                if (HardLinkHelper.TryCreateHardLink(Storage.GetFullPath(file.GetStoragePath(), true), fs.Name))
+                if (preferHardLinks && HardLinkHelper.TryCreateHardLink(Storage.GetFullPath(file.GetStoragePath(), true), fs.Name))
                     return;
             }
 

osu.Game/Overlays/FirstRunSetup/ScreenImportFromStable.cs

@@ -122,10 +122,19 @@ private void updateStablePath()
             stableLocatorTextBox.Current.Value = storage.GetFullPath(string.Empty);
             importButton.Enabled.Value = true;
 
-            bool available = legacyImportManager.CheckSongsFolderHardLinkAvailability();
-            Logger.Log($"Hard link support for beatmaps is {available}");
+            bool cowAvailable = legacyImportManager.CheckSongsFolderCoWAvailability();
+            Logger.Log($"CoW support for beatmaps is {cowAvailable}");
 
-            if (available)
+            bool hardLinkAvailable = legacyImportManager.CheckSongsFolderHardLinkAvailability();
+            Logger.Log($"Hard link support for beatmaps is {hardLinkAvailable}");
+
+            if (cowAvailable)
+            {
+                copyInformation.Text = FirstRunOverlayImportFromStableScreenStrings.DataMigrationNoExtraSpaceWithCoW;
+                copyInformation.AddText(@" "); // just to ensure correct spacing
+                copyInformation.AddLink(FirstRunOverlayImportFromStableScreenStrings.LearnMoreAboutCoW, LinkAction.OpenWiki, @"Client/Release_stream/Lazer/File_storage#via-copy-on-write");
+            }
+            else if (hardLinkAvailable)
             {
                 copyInformation.Text = FirstRunOverlayImportFromStableScreenStrings.DataMigrationNoExtraSpace;
                 copyInformation.AddText(@" "); // just to ensure correct spacing