• istruewing@chilla.local's avatar
    Bug#8283 - OPTIMIZE TABLE causes data loss · 5f08a831
    istruewing@chilla.local authored
    OPTIMIZE TABLE with myisam_repair_threads > 1 performs a non-quick 
    parallel repair. This means that it does not only rebuild all 
    indexes, but also the data file.
    
    Non-quick parallel repair works so that there is one thread per 
    index. The first of the threads rebuilds also the new data file.
    
    The problem was that all threads shared the read io cache on the
    old data file. If there were holes (deleted records) in the table,
    the first thread skipped them, writing only contiguous, non-deleted
    records to the new data file. Then it built the new index so that
    its entries pointed to the correct record positions. But the other
    threads didn't know the new record positions, but put the positions
    from the old data file into the index.
    
    The new design is so that there is a shared io cache which is filled
    by the first thread (the data file writer) with the new contiguous
    records and read by the other threads. Now they know the new record
    positions.
    
    Another problem was that for the parallel repair of compressed
    tables a common bit_buff and rec_buff was used. I changed it so
    that thread specific buffers are used for parallel repair.
    
    A similar problem existed for checksum calculation. I made this
    multi-thread safe too.
    5f08a831
mi_open.c 38.7 KB