Repair MySQL table without downtime
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=&quot;-r&quot;
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=&quot;-o&quot;
EPILOGUE
Neither of these techniques will allow REPAIR TABLE operations while the file is live.