Merge "CreateSnapshot: reduce zero-filling for CoW images"
am: 2dc25441e2
Change-Id: Ia97c9c1888bcb751de5b2fccde30e036c2618591
diff --git a/fs_mgr/libsnapshot/snapshot.cpp b/fs_mgr/libsnapshot/snapshot.cpp
index 3820523..bea6739 100644
--- a/fs_mgr/libsnapshot/snapshot.cpp
+++ b/fs_mgr/libsnapshot/snapshot.cpp
@@ -217,8 +217,27 @@
}
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,