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

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am: 2dc25441e2

Change-Id: Ia97c9c1888bcb751de5b2fccde30e036c2618591
This commit is contained in:
Alessio Balsini 2019-09-04 14:35:35 -07:00 • committed by android-build-merger
commit ab92aeb61b
1 changed files with 21 additions and 2 deletions
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23
fs_mgr/libsnapshot/snapshot.cpp
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@ -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,