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Commit 396a4f27 authored by Alessio Balsini's avatar Alessio Balsini
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CreateSnapshot: reduce zero-filling for CoW images 



Instead of zero-filling the whole image, only clean the dm-snapshot
metadata header of the CoW file.

Zero-filling a large image may take a long time, and this is a safe, but
not necessary operation if the operation is intended to prepare an
initial CoW file that will be used with a dm-snapshot device.
According to the Linux kernel code, only the first 32 bits of the CoW
file is used to detect if the dm-snapshot device under creation will be
at an initial state or the continuation of a previous dm-snapshot.
CREATE_IMAGE_RESET_SNAP is a flag that reduces the zeroing of the image
file to the first file chunk.

Change-Id: Ibc9fb3b6d19666a92125c988687ff8dacfad47be
Depends-On: I242e57a9a622fbb738bf82f8a260af1b13810069
Bug: 139378014
Test: manual, snapshot_test
Signed-off-by: default avatarAlessio Balsini <balsini@google.com>
parent 0c100685

fs_mgr/libsnapshot/snapshot.cpp

+21 −2
Original line number Diff line number Diff line
@@ -210,8 +210,27 @@ 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,