Filesystems, mount options and others

As you probably know, mount options can significantly impact filesystem speed, features and reliability. Moreover, the existence of filesystem-specific options mean that it is often quite hard to 100% match them across the various filesystems.

Fortunately, the vast majority of FS-specific options have very reasonable and reliability-focused predefined values, so we can generally use the defaults with no problem. However, If you want to do a significant comparison, one option should absolutely be the same between the different setups: the write barrier option.

Write barriers are a synchronization method that enable OS to safety flush the on-disk cache content to the physical disk platters. Without write barriers, a fsync() call will flush the main memory disk cache, but it will not flush the disk/controller cache. While disabling barrier can sometime speed up the filesystem/disks combo considerably, it can also lead to data loss, even when the OS assumes that all data were safety written to disk. For example, a power outage will cause the loss of any data in the disk cache that were not written to the disk platters.

However, there are circumstances when write barriers can be disabled without problems: think to a UPS-protected server with a battery backed disk cache, or simply to a controller/disk combo with no DRAM cache at all. In this case, any power outage will not imply a cache data loss, so barriers can be safety disabled.

The Dell R510 server system used for this benchmark round is equipped with a PERC H200 disk controller with no DRAM cache. Moreover this controller disable any disk level cache found on the attacked disks, so I disabled write barriers with the “nobarrier” mount option.

Please keep in mind that enabling write barriers can cause a different, FS-specific performance drop. For example, generally XFS incur into a greater drop than EXT4. So, while the relative standing should remain more-or-less similar, the following results should be considered valid only for installations with write barriers disabled.

UPDATE 05/04/2011:

For more informations about EXT4 and XFS history, mount options and other things, you can visit the following wikipedia pages: