lemen log permission denIEd

说明下，nginx+fpm， fpm是以nobody运行的，项目也给了nobody用户和组，775的权限，为什么还会有如下的错误呢
Fatal error: Uncaught exception 'UnexpectedValueException' with message 'The stream or file "/mnt/xvdb1/virtualhost/2015app/change/storage/logs/lumen.log" could not be opened: failed to open stream: Permission denied' in /mnt/xvdb1/virtualhost/2015app/change/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php:84 Stack trace: #0 /mnt/xvdb1/virtualhost/2015app/change/vendor/monolog/monolog/src/Monolog/Handler/AbstractProcessingHandler.php(37): Monolog\Handler\StreamHandler->write(Array) #1 /mnt/xvdb1/virtualhost/2015app/change/vendor/monolog/monolog/src/Monolog/Logger.php(265): Monolog\Handler\AbstractProcessingHandler->handle(Array) #2 /mnt/xvdb1/virtualhost/2015app/change/vendor/monolog/monolog/src/Monolog/Logger.php(543): Monolog\Logger->addRecord(400, 'exception 'Unex...', Array) #3 /mnt/xvdb1/virtualhost/2015app/change/vendor/laravel/lumen-framework/src/Exceptions/Handler.php(30): Monolog\Logger->error('exception 'Unex...') #4 /mnt/xvdb1/virtualhost/2015app/change/app/Exceptions/Handler.php(27): Laravel\Lumen\ in /mnt/xvdb1/virtualhost/2015app/change/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php on line 84