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Commit 2f9a4ff5 authored by Alessio Balsini's avatar Alessio Balsini
Browse files

COW file size: better explain the exception size 



The exception size was hardcoded and its explanation hidden in one of
the comments.
Move it to a separate constant e better explain why that is 64 * 2 / 8.

Bug: 176972301
Test: m
Signed-off-by: default avatarAlessio Balsini <balsini@google.com>
Change-Id: Ifcb527540882222916ada07dacf3f76f87609539
parent aab16613

fs_mgr/libsnapshot/dm_snapshot_internals.h

+14 −10
Original line number Diff line number Diff line
@@ -28,7 +28,7 @@ class DmSnapCowSizeCalculator { 
    DmSnapCowSizeCalculator(unsigned int sector_bytes, unsigned int chunk_sectors)

        : sector_bytes_(sector_bytes),

          chunk_sectors_(chunk_sectors),

          exceptions_per_chunk(chunk_sectors_ * sector_bytes_ / (64 * 2 / 8)) {}

          exceptions_per_chunk(chunk_sectors_ * sector_bytes_ / exception_size_bytes) {}



    void WriteByte(uint64_t address) { WriteSector(address / sector_bytes_); }

    void WriteSector(uint64_t sector) { WriteChunk(sector / chunk_sectors_); }
@@ -124,18 +124,22 @@ class DmSnapCowSizeCalculator { 
    const uint64_t chunk_sectors_;



    /*

     * The COW device stores tables to map the modified chunks. Each table

     * has the size of exactly 1 chunk.

     * Each row of the table (also called exception in the kernel) contains two

     * 64 bit indices to identify the corresponding chunk, and this 128 bit row

     * size is a constant.

     * The number of exceptions that each table can contain determines the

     * number of data chunks that separate two consecutive tables. This value

     * is then fundamental to compute the space overhead introduced by the

     * tables in COW devices.

     * The COW device stores tables to map the modified chunks. Each table has

     * the size of exactly 1 chunk.

     * Each entry of the table is called exception and the number of exceptions

     * that each table can contain determines the number of data chunks that

     * separate two consecutive tables. This value is then fundamental to

     * compute the space overhead introduced by the tables in COW devices.

     */

    const uint64_t exceptions_per_chunk;



    /*

     * Each row of the table (called exception in the kernel) contains two

     * 64 bit indices to identify the corresponding chunk, and this 128 bit

     * pair is constant in size.

     */

    static constexpr unsigned int exception_size_bytes = 64 * 2 / 8;



    /*

     * Validity check for the container.

     * It may happen that the caller attempts the write of an invalid chunk