Merge "CreateSnapshot: reduce zero-filling for CoW images"

    }

    }

    auto cow_name = GetCowName(name);

    auto cow_name = GetCowName(name);

    int cow_flags = IImageManager::CREATE_IMAGE_ZERO_FILL;

    int cow_flags = IImageManager::CREATE_IMAGE_DEFAULT;

    return images_->CreateBackingImage(cow_name, cow_size, cow_flags);

    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,

bool SnapshotManager::MapSnapshot(LockedFile* lock, const std::string& name,