Realistically, there is no online method for table repait.

There are two techniques to repair mydb.mytable

TECHNIQUE #1 : Repair Online

REPAIR TABLE mydb.mytable;

This will perform the table repair with mysql runnng. This will perform a full table lock so no one can access the table.

TECHNIQUE #2 : Repair Offline

To repair a table offline, move the files making up the table to another folder and perform the repair there. For example, to repair mydb.mytable using the folder /var/lib/mysql

REPAIR_OPTION="-r"
DB_NAME=mydb
TABLE_NAME=mytable
FRM=${TABLE_NAME}.frm
MYD=${TABLE_NAME}.MYD
MYI=${TABLE_NAME}.MYI
cd /var/lib/mysql/${DB_NAME}
mv ${FRM} ..
mv ${MYD} ..
mv ${MYI} ..
cd ..
myisamchk -${REPAIR_OPTION} ${MYD}
myisamchk -${REPAIR_OPTION} ${MYI}
mv ${FRM} /var/lib/mysql/${DB_NAME}
mv ${MYD} /var/lib/mysql/${DB_NAME}
mv ${MYI} /var/lib/mysql/${DB_NAME}

If -r does not work, rerun these lines using REPAIR_OPTION="-o"

EPILOGUE

Neither of these techniques will allow REPAIR TABLE operations while the file is live.

According to the documentation, the tool accomplishes its magic of keeping the old and new tables in sync while the copy process is happening via the creation of its own triggers and is therefore not compatible with tables already using triggers no matter what kind of change you intend to make.

Any modifications to data in the original tables during the copy will be reflected in the new table, because the tool creates triggers on the original table to update the corresponding rows in the new table. The use of triggers means that the tool will not work if any triggers are already defined on the table.

^{— http://www.percona.com/doc/percona-toolkit/2.1/pt-online-schema-change.html}