diff --git a/fs_mgr/libsnapshot/snapshot.cpp b/fs_mgr/libsnapshot/snapshot.cpp index 382052312..bea6739b6 100644 --- a/fs_mgr/libsnapshot/snapshot.cpp +++ b/fs_mgr/libsnapshot/snapshot.cpp @@ -217,8 +217,27 @@ bool SnapshotManager::CreateSnapshot(LockedFile* lock, const std::string& name, } auto cow_name = GetCowName(name); - int cow_flags = IImageManager::CREATE_IMAGE_ZERO_FILL; - return images_->CreateBackingImage(cow_name, cow_size, cow_flags); + int cow_flags = IImageManager::CREATE_IMAGE_DEFAULT; + if (!images_->CreateBackingImage(cow_name, cow_size, cow_flags)) { + return false; + } + + // when the kernel creates a persistent dm-snapshot, it requires a CoW file + // to store the modifications. The kernel interface does not specify how + // the CoW is used, and there is no standard associated. + // By looking at the current implementation, the CoW file is treated as: + // - a _NEW_ snapshot if its first 32 bits are zero, so the newly created + // dm-snapshot device will look like a perfect copy of the origin device; + // - an _EXISTING_ snapshot if the first 32 bits are equal to a + // kernel-specified magic number and the CoW file metadata is set as valid, + // so it can be used to resume the last state of a snapshot device; + // - an _INVALID_ snapshot otherwise. + // To avoid zero-filling the whole CoW file when a new dm-snapshot is + // created, here we zero-fill only the first 32 bits. This is a temporary + // workaround that will be discussed again when the kernel API gets + // consolidated. + ssize_t dm_snap_magic_size = 4; // 32 bit + return images_->ZeroFillNewImage(cow_name, dm_snap_magic_size); } bool SnapshotManager::MapSnapshot(LockedFile* lock, const std::string& name,