Dumping a svn repo is so easy… I made a windows batch(watch out the disk space ):

@echo off  

call :svndump %DATE:~0,10% reponame
goto :EOF  

:svndump
setlocal
echo [%1%] >> log_backup.txt
echo %TIME% dumping [%2%] start >> log_backup.txt

if not exist %1 mkdir %1
"svnadmin.exe" dump Path\To\Svnrep\%2 > %1\%2.dmp
echo dump success, start compressing... >> log_backup.txt
"7z.exe" a %1\%2.dmp.7z %1\%2.dmp
del /F %1\%2.dmp >> log_backup.txt

echo %TIME% dumping [%2%] finished >> log_backup.txt
echo. >> log_backup.txt
endlocal
goto :EOF