Hot backup implementation

This drafting is obsoleted.

Please check
http://www.percona.com/docs/wiki/percona-xtrabackup:start

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##### <Way1> has been selected #####
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Difficulty (size) to implement
Way1 : big
Way2-A : small
Way2-B : medium

Need crash recovery
Way1 : No
Way2-A : Yes
Way2-B : No (but recreates logfiles if no logfiles)

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Hot backuped file: We can startup InnoDB directly from the files (datafiles, logfiles).
        The normal crash recovery processing at the startup may be acceptable.
        (If the recovery always succeeds.)

-
We may have several way to implementation.

<Way1> As external executable

        - Its usage will be similar to InnoDBHotBackup.
        - The executable will be created with XtraDB.

        (implementation)
        copying all transaction log records from the last checkpoint at the start,
        during copying datafiles.
        applying the logs to the datafiles and generate proper logfiles.

<Way2> As internal XtraDB

        - will be implemented as like UDF. (We should patch only to InnoDB part)
                (e.g. SQL> select innodb_extracommand("START BACKUP");)

        (implementation)
        <Way2-A> "START BACKUP" ~ "END BACKUP"
                During the period, stop checkpointing, and we can copy or snapshot
                the datafiles and logfiles.

                Of course, we need crash recovery when startup from the files.

                If the log records reach to the (near of ?) limit during the period,
                naturally the system will stall.

                ##### NEED INVESTIGATION #####
                the safety of the copying logfiles.
                ##############################

        <Way2-B> "SNAPSHOT"
                The command creates or overwrites all datafiles (and logfiles ?) snapshot
                to another directory.

                We can startup from the copied datafiles without logfiles. (maybe)

                - special copying process.
                - applying transaction log to the copied files and closing correctly
                - finish.

                ##### NEED INVESTIGATION #####
                the necessity of the logfiles.
                ##############################